idea of time travel

Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

Wormholes have been proposed as one possible means of traveling through time.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna.

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

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Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida .

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London.

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine .

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said.

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean .

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery.

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid."

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us."

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings.

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

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Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike.

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Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

Image of galaxies, taken by the Hubble Space Telescope.

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

Illustration of GPS satellites orbiting around Earth

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

Illustration of a hand holding a phone with a maps application active.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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The Magazine

Is time travel possible? An astrophysicist explains

Time travel is one of the most intriguing topics in science.

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

Related: The speed of light, explained

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical : Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Time travel paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

This article first appeared on the Conversation. You can read the original here .

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Is time travel even possible? An astrophysicist explains the science behind the science fiction

Published: Nov 13, 2023

By: Magazine Editor

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Written by Adi Foord , assistant professor of physics , UMBC

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

But is this just a fun idea for movies, or could it really happen?

Time is relative

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

This article is republished from The Conversation under a Creative Commons license. Read the original article and see more than 250 UMBC articles available in The Conversation.

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A brief history of time travel

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Of all time travel's paradoxes, here's the strangest of them all: hop on a TARDIS back to 1894 and the concept didn't even exist. "Time travel is a new idea," explains New York-based author James Gleick, 62. "It's a very modern myth." Gleick's entertaining Time Travel: A History , out in hardback in February, quantum leaps from HG Wells's The Time Machine - the original - via Proust and alt-history right up to your Twitter timeline. Until we get the DeLorean working for real, fellow travellers, consider it the next best thing.

The Mahabharata

Time travel appears in Hindu text The Mahabharata, and in stories such as Washington Irving's Rip Van Winkle (1819) - but it usually only involved a one-way trip. "People fell asleep, and woke  up in the future," says Gleick.

HG Wells's The Time Machine

"The idea of time travel with volition, in either direction, didn't arrive until Wells," says Gleick. It explains that time is a dimension - something not widely accepted until Einstein's theories in 1905.

Henri Bergson's Time And Free Will

Bergson's thesis is published soon after Wells's novel. "Bergson is a friend of Marcel Proust," says Gleick. Soon Proust et al are jumping on the idea of time travel to explore free will - and influencing new sci-fi in return.

Time Capsules

The idea of preserving a time stamp only arose in the 1930s in Scientific American. "It's the most pedestrian form of time travel: sending something into the future at a rate of one minute per minute."

Robert A Heinlein's By His Bootstraps

Heinlein's short story, published in Astounding Science Fiction, introduces the idea of a character appearing in multiple timelines, meeting themselves amid complex - and funny - paradoxes.

William Gibson's The Peripheral

Gleick cites Gibson's unique twist on the genre: "We can't send people, but what if you could send information back to the past?" It's a chilling new take. "It shows how our cultural conception of time is changing."

This article was originally published by WIRED UK

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March 18, 2024

The Great Debate: Could We Ever Travel through Time?

Our space and physics editors go head-to-head over a classic mind-bending question.

By Clara Moskowitz & Lee Billings

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Illustration of a Bohr atom model spinning around the words Science Quickly with various science and medicine related icons around the text

Clara Moskowitz: Hi, I’m Clara Moskowitz, a space editor here at Scientific American. We’re taking a break this week to look back at some of our favorite podcast episodes. I chose this one about the physics of time travel, because I’m a big sci-fi geek, so I’m fascinated by the topic. But also, it was such a fun debate to have with my colleague and friend, Lee Billings, another space editor here. We each picked a side – I was pro time travel, he was con—and dug our heels in. Check it out!

[Clip: Show theme music]

Moskowitz: We’re here today to talk about time travel. A perennial – dare I say, timeless–topic of science fiction, but is it possible? Is there any chance at all that it could actually happen?

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Lee Billings: No. No, no no no no. (laughs). Well, kinda. Not really. ARGH. I’m Lee Billings.

Moskowitz: I’m Clara Moskowitz, and this is Cosmos, Quickly , the biweekly space podcast from Scientific American .

Moskowitz: We’re going to have a little friendly debate.

Billings: Really? I came for a throwdown.

Moskowitz: Well, a wrangle. A parley. A confab. Lee, what do you have against time travel?

Billings: So I love the idea of time travel! And in fact I do it all the time—like most everyone else I’m traveling into the future at one second per second. I’m less of a fan, though, of more speculative time travel, which is good fodder for goofy sci-fi stories, but in the real world it’s an implausible distraction.

Moskowitz: But really, we can stay within plausible physics and still see how more extreme versions of time travel are possible. See, Einstein’s special theory of relativity shows that the rate time flows at depends on how fast you’re moving.

Billings: Einstein strikes again, what a rascal.

Moskowitz: If you’re traveling in a starship at close to the speed of light, you’ll still experience the familiar one-second-per-second ticking of a clock– but an observer back on Earth would see your clock moving glacially slow. To them, you’d be moving through time at a snail’s pace. That means that when you finally got back, maybe only a year would have passed for you, but a century could have gone by for your friends on Earth. Ergo, you just traveled to the future!

Billings: Right, right, no one’s disputing any of that! We can even measure this sort of “time dilation” right now on Earth, not with starships, but with subatomic particles. Some of those particles have very short lifetimes, decaying almost instantaneously. But if we drastically speed them up, like in a particle accelerator, we find they endure longer in proportion to how fast they’re going. So riddle me this, though, Clara: How can we travel into the past? That’s something so hard to do–effectively impossible, almost–that it’s scarcely worth thinking about.

[Clip: Back to the Future : “This is what makes time travel possible. The flux capacitor!”]

Moskowitz: I get it—no one has yet conceived of a way to journey to the past. But the crazy thing is it’s not impossible. Time is one of the four dimensions in the universe, along with three dimensions of space. And we move through space in all directions just fine, and according to physics, travel through time should be just as possible.

One way that people have looked into is via a wormhole—a shortcut bridge through spacetime that was predicted by general relativity. Wormholes can connect distant points in spacetime, meaning you could conceivably use one to bridge not just the gap between here and a distant galaxy, but the span between 2023 and 1923.

[CLIP: Interstellar : “That’s the wormhole.”]

Billings : Ah yes, wormholes—the last refuge of scoundrels and desperate physicists. The trouble with wormholes Clara, is that, unlike a DeLorean, we have no evidence they actually exist—and, even if they did, it seems the only ways to make them traversable and stable involves using negative energy or negative mass to prop them open. And, guess what, just like wormholes themselves, we have no evidence these weird forms of matter and energy actually exist, either. And let’s just beat this dead horse one more time—even if wormholes exist, as well as the means to make them traversable, to go back in time seems to require anchoring one end in a region of very warped spacetime, like around a black hole, or accelerating it to nearly lightspeed. Are you sensing a theme here, Clara?

Moskowitz: Yeah, yeah. All I can say is that just because there’s no evidence any of these things exist, there’s also no evidence they don’t or can’t exist. Wormholes are real solutions to the equations of general relativity, and even negative energy and mass are concepts that come up in the math and aren’t prohibited.

Billings: Well how about some more practical arguments, then? If time travel were possible, wouldn’t we have met some time travelers by now? Wouldn’t someone have gone back and killed Hitler—or at least prevented me from wearing that ridiculous outfit to my high school prom? You know there’s a famous story about physicist Stephen Hawking, who invited time travelers to come to a party he was holding. The trick was the the party happened in 2009, but the invitation came out in a miniseries that was broadcast in 2010—thus, only time travelers would have been able to attend.

[CLIP: Stephen Hawking Time Travel Party: “Here is the invitation, giving the exact coordinates in time and space. I am hoping in one form or another it will survive for many thousands of years.”]

Billings: Sadly, the hors d'oeuvres went uneaten and the champagne sat unopened, because, clearly, time travel to the past is impossible!

Moskowitz: I admit a party with Stephen Hawking should have been pretty alluring to time travelers, if they were out there. But you’re forgetting about the International Clause of Secrecy that all time travelers probably have to swear to, making sure to hide their identities and abilities from those in earlier eras.

Billings: Hmm, yes the clause of secrecy here. Feels like we’re really veering into science fiction territory special pleading here. And don’t forget all the paradoxes that we have to worry about too. There are lots of good reasons to think time travel might introduce insurmountable paradoxes in physics. The most famous being the grandfather—or grandmother—paradox. If time travel were possible into the past, so the thinking goes, then a person could go back in time and kill their own grandparents, thus making it impossible for them to be born and impossible for them to travel back in time to ever commit the murder, and so on and so on.

Moskowitz: I wonder if it could be like a many-worlds scenario, where each change a time traveler makes to the past spawns a whole new universe that carries on from that point. So if I went back in time and killed one of my forebears, then a new branch universe would begin where that whole line of descendents, including me, never existed. I mean, it sounds crazy, but then again, physics is pretty enamored with multiverses, and they seem to pop up for lots of reasons already. Maybe it’s not impossible?

Billings: If not impossible, then I’d say, implausible.

Moskowitz: Well, I’m forever an optimist, Lee! Thanks for listening to the Cosmos, Quickly .

Billings: Our show is produced by Jeff DelViscio, Tulika Bose and Kelso Harper. Our music was composed by Dominic Smith.

Moskowtiz: If you like the show, please consider rating or leaving a review. You can also email feedback, questions, and tips to [email protected] .

Billings: For more spacetime hijinks and all your science news, head to SciAm.com. This has been Cosmos, Quickly . I’m Lee Billings.

Moskowitz: I’m Clara Moskowitz.

Billings: And we’ll see you next time, in the future!

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Exploring the Reality of Time Travel: Science Fact vs. Science Fiction

By Adi Foord, University of Maryland, Baltimore County November 16, 2023

Time travel, a longstanding fascination in science fiction, remains a complex and unresolved concept in science. The second law of thermodynamics suggests time can only move forward, while Einstein’s theory of relativity shows time’s relativity to speed. Theoretical ideas like wormholes offer potential methods, but practical challenges and paradoxes, such as the “grandfather paradox,” complicate the feasibility of actual time travel.

Will it ever be possible for time travel to occur?

But is this just a fun idea for movies, or could it really happen?

The Science Behind Time Travel

Time Is Relative

Theoretical Possibilities and Challenges

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes, or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and Failed Dinner Parties

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time, and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

James Webb Space Telescope Artist Conception

Artist’s rendering of the James Webb Space Telescope. Credit: Northrop Grumman

Telescopes Are Time Machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel . As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope , the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang , about 13.7 billion years ago.

Written by Adi Foord, Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County.

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8 comments on "exploring the reality of time travel: science fact vs. science fiction".

Until the problem of the second law of thermodynamics(entropy) is solved, the concept of time travel will remain the subject of science fiction. Since this is a basic law of our universe, there is no conceivable way that we know of to do this. The great thing about our knowledge of the universe is that it continues to grow and with that our view of what is possible continues to change. After all, at one time it was believed we could never leave earth!

The 7 planets are soul pollen in the space @ life has been the world has been prepared @ The pollen of the universe is hidden @ Around the axis of the galaxy, the universe is hidden@@ The pollen of the galaxies is a hidden cluster Itis made that we thought about what wisdom is in God’s work and how it is arranged in the form of words.The verse that is made is as follows @@ John, you are in time @ worlds, planets around the axis of the branches of galaxies @@ 8 Prophets, divine prophets, God-aware witnesses @ God’s words of revelation, they are aware @@ 48 What the words of revelation that every prophet has about @ sometimes Sometimes the message of God has become a verbal cliché in the head @@ 62 The message of God was given to every prophet @ The message was made around the power of God @@ 46 The truth of the religions of the cradle of time is said in the world @ Prophets always came for justice in time @@ 62 A warrior became brave in time @ Delaver Ghahrmani Boud Taarani@@ 43 Omar Noah never died, who knows!@ Imransan Is there an unseen world, immortality!?@@ 49 Men’s rights in the sign @ Human rights, the observance of world justice @@ 40 We have God’s love @ A love that is not patched, separation!!!: @@ 39 Take one word from the end of the first and second stanzas to the bottom of these eight verbal verses, and this sentence is made @@ God-aware, the world is born, you know the sign of God @ Agah,the beginning of time, the world, eternity, the world of separation @@ The meaning of this sentence is That God, who before us humans lived on the earth, formed the earth’s crusts with full knowledge, and we humans know the sign of God, which is on the earth, on the continents and countries, and some names have been shown by God for our knowledge since the end of time.In a later video, if I have a lifetime left, I will explain exactly about these poems, God willing @@ The word of the Prophet 17 is the 17th and Muhammad (PBUH) said that the Bedouin Arabs should pray 17 rakats so that theyperform ablution and be clean and not kill and loot.From the caravans, these were all God’s will, and he is good everywhere, in every nation, God does not like evildoers, sinners, and oppressors.Muhammad (PBUH) was God’s last messenger to the Arab people, he was God’s best prophet, and the third verse is because they do not accept Muhammad (PBUH).Some Iranians who were in contact with me, that’s why in the third verse of this surah, he said that his message was given to every prophet, the message is based on divine power, and the word truth, the first two letters of which istruth, is truth, and truth is the 43rd and forty-third word, and the word is time.It is exactly 46, and this song of the Prophets was revealed at the age of 46 ببخشید اگر قبلآ مطالبی فرستادم که به مذاهب مذاهم ارتباط داد شاید به درد شما نمیخورد من در کامنت های بعدی سعی میکنم از نجوم و حیات زمین سخن بگویم این چند بیت را به خارجی انگلیسی که تبدیل کردم معنی آن حیرت انگیز تعغیر کرد گفتم برای شما بفرستم و بداند که این کلام من نبوده کلام خداوند بوده شما نظرتان در مورد خداوند چیست من میگویم خداوند که پدر ومادر انسان بودند قادر به ساختن ما بودند اما آیا آنها قادر به ساختن ستاره ها هم بودند

Your comment has validity to God. But it surely has no place here, it is only fare if the hole comment were in english and has less of a convincing push in a belief a person either believes or not.

It’s too bad physics can’t come to a complete consensus about time, I would like to add some thought about discoveries it has been proven that time travels in only one direction forward, the experiment dealt with light thru glass and how it reacts in the middle and what change happens after light exits the other side, a simple explanation of this experiment. Brings me to theorize and start that time existed before the big bang and is outside of our universes influence, when time is acted on by gravity the ( Form ) of time is changed until the influence no longer has effect, this could go hand in hand with light photons the photon has a influence in the Form that time has. This can not be a observed difference unless we were to see beyond the speed of light. We do agree that physics changes at a subatomic state and also does some strange changing once the speed of light has been exceeded.

The experiment I referred to was posted on IFL in October 2023 headlined ( Solution to complex light problem shows that time can only go Forward ).

One of the problem with travel time is the one people keep forever. And, that is that the earth is always move through space. Matter of fact, the earth is not in the same place that it was 50 years ago. So you will have to move through space as well as time.

Ironically, the only science fiction that seems to handle this is the original story “The Time Machine”.

Time travel is happening now. It has been done since the 1950s. The method satisfies all the requirements. Traveler can’t change the past, but only observe. You can’t go farther back than when the machine was first invented (1950s). There’s one more limitation, you can only observe what the machine was directed. The time machine, the common video camera, and video tape recorder. Now it’s the camera, and file capture computer. Yes, viewing a video tape is effectively going back in time. It’s more than the video, it’s the sound too. There are working versions of smell, and touch which can be recorded too. If you record, and replicate all the senses, you have effectively complete time travel. The most primitive form is the picture. This technology has been around for thousands of years, and is manually intensive. Later many pictures were strung together to make a film. Using a camera to record film was the first example of complete visual time travel (back to when the film was made). Later sound was added, and we have the movies. A way of going back in time that included sight, and sound. Now we have video systems (YouTub) that can play back past events selected by you. Yes, video systems are virtual time travel machines we have now.

How Stellar Cannibalism Illuminates Cosmic Evolution

جزایر فیلیپین دایناسوری که توسط انسان خلق شده در بیش ده ها میلیون سال و بخاطر ریخته شدن دوهزار متر خاک غرق شده بخاطر بالا آمدن آب اقیانوس اما جزایر فیلیپین شبیه دایناسوریست

The address of the above comment on the site about a thief who was trained by a dinosaur bird who was trained by humans tens of millions of years ago and who arrested murderers and robbers.The police were arrested by big birds.don’t the scientists of the world think about this?They were buried in the bed of important cities, they were buried with all the tools and machines, the traces of humans tens of millions of years ago, they had a civilization and a history of hundreds of thousands of years, they built a base underthe earth, from the meteorites that explode from the planets when they hit the sun, and they knew that several thousand meters of soil is poured on the surface of the seas and islands, and they knew that the shapes they made of the islands in thecountry of Papua may go under the ocean, of course, the Philippine islands.The picture is of a baby dinosaur that went under the ocean, but the northern island of Australia, which is Papua, is quite clear.It is a big dinosaur whose tail is towards the east and its mouth is open towards the west.There is the Philippines, but it was more difficult to take the soil to the Philippine islands to create a dinosaur than the island of Papua, that is why the height of the soil in the Philippines is lower, and when the meteorites fell a few kilometerson the surface of the ocean, the image created by humans under the ocean in the shape of a dinosaur is hidden in the American continent The picture is of a bird in the shape of a dinosaur that is flying, and this bird was made to flyby the Indians of the tribe, that bird was talking to people, but its spirit might have heard something from the police because a thief while in the bird’s mouth He was handcuffed by the police and the weapon, which is a machine gun, is fromthe east of the American continent on the coast

The country of Florida is a machine gun.When you continue to New York City, you will reach the mouth of the dinosaur, where a thief is trapped in the mouth of a bird, and the little finger of the police handcuffed the thief’s hand, and a small colt is in the hand ofthe thief, who the police caught in the mouth of the dinosaur.put in the mouth of the bird dinosaur, you can clearly see that the thief fell on the ground and was shot in the head, and it is clear that his forehead was pierced, the bird’s mouth is open in flight, the head of the birdis from the east of the American continent, and a fish is placed in the bird’s mouth in the water of the ocean.The stretched glove of the police, which is in the form of a fist, with a handcuff attached to the left hand of the thief who fell on the ground in the sea and the mouth of the bird, the head of the thief and the killer, is located towards the southwest and west coast of America.All these images were created from the American continent and islands by Humans were created, but they didn’t have enough fuel and time to create more accurate images and meteorites ruined the beauty of the images, but it is clear that all these changes were createdby humans, but you have to consider that two thousand meters of soil from meteorites are fish.And they buried the whales under the beaches, and after a very long time, the bodies of the whales turned into oil under the two thousand meters of soil on the beaches, and on the other hand, the presence of two thousand meters of soil onthe surface of the seas and droughts could not make the created images disappear.

Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

a woman stands among a crowd of people moving around her

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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What Einstein and Bill Gates Teach Us About Time Travel

Time travel has long been a staple of science fiction books and movies. But will we ever be able to build a time machine and beam ourselves backward and forward in time in real life?

In some sense, of course, we’re all time travelers: We move forward in time from one minute to the next. But going back in time, whether to avoid some mistake or perhaps to repeat it, is something far more elusive. And for those who yearn to see what the world will look like a century or a millennium from now, the slow tick of your watch as time passes just doesn’t cut it.

The theoretical underpinnings of time travel date back to 1905, when Albert Einstein wrote down his special theory of relativity that showed space and time are intimately linked, and to 1916, when Einstein’s general theory of relativity showed that space and time are malleable — that is, they respond to the presence of matter or energy by warping, bending, expanding, and contracting. By extension, this means if one can imagine space being filled with some exotic form of energy, then space and time could warp in a way so that time, as well as space, could bend back upon themselves like circles, allowing one to move forward in a straight line and still return to one’s starting point in both space and time.

Related: Waves May Let Us See the Big Bang's Earliest Moments

But even after thinking about the time travel problem for more than a century, physicists haven’t advanced the ball very far. We recognize that while Einstein’s equations do allow for round-trip time travel (at least in principle), other physics considerations probably rule out creating the exotic forms of energy that would make it possible.

The first person to write down a mathematical solution of the general relativity equations that described an exotic type of space-time that might permit time travel was mathematician Kurt Gödel, a close colleague of Einstein’s at the Institute for Advanced Study in Princeton, N.J. He presented his result in a scientific paper that he gave Einstein as a birthday present on his 70th birthday in 1949.

But just being able to determine that general relativity allows configurations of space and time in which forward or backward time travel is possible doesn’t mean those configurations can actually be created. Since general relativity implies that the configuration of space-time is determined by the nature of the matter and energy within it, one needs to determine whether it’s possible to create the appropriate type of matter and energy in the laboratory.

Is it possible? Probably not, but we don’t know for sure.

Perhaps the easiest way to understand the problem is to examine the simplest example of a theoretical time machine: a “wormhole” — essentially a shortcut through a curved space, like a tunnel under a mountain (or for our purposes here a tunnel connecting two distant points in space). That’s why I chose this example when discussing time travel two decades ago in my book "The Physics of Star Trek."

So imagine a wormhole one of its mouths moving through space in a big circle, at, say, 95 percent of the speed of light. Now, special relativity tells us that observers in relative motion experience time differently, such that — to a ground-based observer — clocks on a fast-moving rocket ship would tick more slowly than clocks on the ground.

Thus an observer riding on the wormhole’s mouth as it zooms through space might determine from his or her clock that the round-trip took a week. But an observer at the other end of the wormhole, at rest in the background space, would look at his clock and determine that the trip took, say, three years. If the second observer then moves through the wormhole and comes out the other end, he or she will arrive to meet his or her colleague at the other end — and discover that the time is now three years before he entered the wormhole in the first place!

Are wormholes possible in real life? The answer is … we don’t know! We do know that no stable wormholes can exist if the only forms of matter and energy are the ones we’ve been able to create in laboratories: In that case, each mouth of the wormhole would collapse to form a black hole in a time shorter than it would take to traverse the wormhole. But if it were possible to create some material with very peculiar characteristics — namely a material that was gravitationally repulsive — it might be able to hold a wormhole open against gravitational collapse. And that would bring time travel a step closer to reality.

All the evidence so far suggests that it is probably impossible for us to create such a material. Yet ultimately, it is the limitation of our understanding of general relativity, especially in the domain where quantum mechanical effects might be significant, that currently prohibits us from proving the impossibility of building a time machine.

In other words, time travel is not yet ruled out in our universe.

Even so, most physicists now working would bet against the possibility of time travel, not merely because of the practical difficulties of generating the necessary conditions to allow it but also because of the implications of time travel if it becomes possible.

Related: These ER Docs Invented a Real Star Trek Tricorder

For example, if were to go back in time and change the past, we would also change the future. This situation is a common plot twist of modern science fiction (including “Star Trek” and the film series “Back to the Future”). And it leads to a host of possible paradoxes, including what would happen if you went back in time and killed your grandmother before she gave birth to your mother. If your mother was never born, of course, then you would never have been born. But in that case, how did you go back in time and kill your grandmother in the first place?

One possible resolution of this paradox would be that the only kind of time travel allowed by the laws of physics is travel in which you are doomed to repeat the same sequence of events again and again, no matter how much you’d like to alter things. Time would travel in a circle instead of a straight line. As in the movie “Groundhog Day,” you would be doomed to repeat events for all eternity.

Traveling forward in time and returning also produces problems of the sort seen in “Back to the Future.” As I have pointed out previously, the fact that Bill Gates remains the richest person in the world argues against the existence of a forward-and-back time machine. For if you could jump forward even a single day and then return to the present time, within a year you could make investments in the stock market that would turn even a small sum into an astronomically large one. Gates’s mere $80 billion fortune would seem minuscule.

Related: Doomsday Clock Insider on How Worried We Should Be

My colleague Stephen Hawking once presented another interesting argument against the possibility of time travel. He said that if it were possible, then we would forever be inundated with tourists from the future. (I countered by saying maybe they all went back to the 1960s and no one noticed!)

As much as the idea of time travel runs counter to our common sense understanding of reality, the universe is the way it is, whether we like it or not. Even if time travel appears to present difficulties for such notions as causality, the history of physics has taught us that new discoveries force us to periodically modify our understanding of cherished notions.

The bottom line? While the possibility of time travel continues to tantalize physicists and laypeople alike, the odds are against it. And if time travel were shown to possible in principle, the amount of energy required to create the conditions for time travel would likely be greater than the total energy available on Earth. At least for the foreseeable future, time travel will remain the stuff of science fiction.

Lawrence M. Krauss is Director of the Origins Project at Arizona State University. His most recent book is “The Greatest Story Ever Told. So Far: Why are We Here?” (Atria Books, 2017)

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Lawrence M. Krauss is Director of the Origins Project at Arizona State University, and the author, most recently, of The Greatest Story Ever Told… So Far (Atria Books, 2017)

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Time travel: five ways that we could do it

Cathal O’Connell

Cathal O'Connell is a science writer based in Melbourne.

In 2009 the British physicist Stephen Hawking held a party for time travellers – the twist was he sent out the invites a year later (No guests showed up). Time travel is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built.

But travel to the future? That’s a different story.

Of course, we are all time travellers as we are swept along in the current of time, from past to future, at a rate of one hour per hour.

But, as with a river, the current flows at different speeds in different places. Science as we know it allows for several methods to take the fast-track into the future. Here’s a rundown.

1. Time travel via speed

This is the easiest and most practical way to time travel into the far future – go really fast.

According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.

This is not a just a conjecture or thought experiment – it’s been measured. Using twin atomic clocks (one flown in a jet aircraft, the other stationary on Earth) physicists have shown that a flying clock ticks slower, because of its speed.

In the case of the aircraft, the effect is minuscule. But If you were in a spaceship travelling at 90% of the speed of light, you’d experience time passing about 2.6 times slower than it was back on Earth.

And the closer you get to the speed of light, the more extreme the time-travel.

Computer solves a major time travel problem

The highest speeds achieved through any human technology are probably the protons whizzing around the Large Hadron Collider at 99.9999991% of the speed of light. Using special relativity we can calculate one second for the proton is equivalent to 27,777,778 seconds, or about 11 months , for us.

Amazingly, particle physicists have to take this time dilation into account when they are dealing with particles that decay. In the lab, muon particles typically decay in 2.2 microseconds. But fast moving muons, such as those created when cosmic rays strike the upper atmosphere, take 10 times longer to disintegrate.

2. Time travel via gravity

The next method of time travel is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.

As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.

Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimetres above the other, and measured the difference in their rate of ticking. The lower one ticked slower because it feels a slightly stronger gravity.

To travel to the far future, all we need is a region of extremely strong gravity, such as a black hole. The closer you get to the event horizon, the slower time moves – but it’s risky business, cross the boundary and you can never escape.

And anyway, the effect is not that strong so it’s probably not worth the trip.

Assuming you had the technology to travel the vast distances to reach a black hole (the nearest is about 3,000 light years away), the time dilation through travelling would be far greater than any time dilation through orbiting the black hole itself.

(The situation described in the movie Interstellar , where one hour on a planet near a black hole is the equivalent of seven years back on Earth, is so extreme as to be impossible in our Universe, according to Kip Thorne, the movie’s scientific advisor.)

The most mindblowing thing, perhaps, is that GPS systems have to account for time dilation effects (due to both the speed of the satellites and gravity they feel) in order to work. Without these corrections, your phones GPS capability wouldn’t be able to pinpoint your location on Earth to within even a few kilometres.

3. Time travel via suspended animation

Another way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.

Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.

Could humans ever do the same?

Though completely stopping your metabolism is probably far beyond our current technology, some scientists are working towards achieving inducing a short-term hibernation state lasting at least a few hours. This might be just enough time to get a person through a medical emergency, such as a cardiac arrest, before they can reach the hospital.

In 2005, American scientists demonstrated a way to slow the metabolism of mice (which do not hibernate) by exposing them to minute doses of hydrogen sulphide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13 °C and metabolism decreased 10-fold. After six hours the mice could be reanimated without ill effects.

Unfortunately, similar experiments on sheep and pigs were not successful, suggesting the method might not work for larger animals.

Another method, which induces a hypothermic hibernation by replacing the blood with a cold saline solution, has worked on pigs and is currently undergoing human clinical trials in Pittsburgh.

4. Time travel via wormholes

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.

Attempts to prove this either way have failed, ultimately because of the incompatibility between general relativity and quantum mechanics.

5. Time travel using light

Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.

According to Mallet, the right geometry could lead to time travel into either the past and the future.

Since publishing his theory in 2000, Mallet has been trying to raise the funds to pay for a proof of concept experiment, which involves dropping neutrons through a circular arrangement of spinning lasers.

His ideas have not grabbed the rest of the physics community however, with others arguing that one of the assumptions of his basic model is plagued by a singularity, which is physics-speak for “it’s impossible”.

The Royal Institution of Australia has an Education resource based on this article. You can access it here .

Related Reading: Computer solves a major time travel problem

Originally published by Cosmos as Time travel: five ways that we could do it

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How would time travel affect life as we know it?

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Key Takeaways

If unrestricted time travel were possible, it would lead to a complete breakdown of the rational order of things.
The ability to travel to both the past and future would upend our understanding of time.
Stephen Hawking's "chronology protection hypothesis" suggests there might be natural laws preventing unrestricted time travel.

Science fiction has thoroughly covered the topic of time travel, starting with H.G. Wells' "The Time Machine" in 1895 and continuing right up to modern movies like " Déjà Vu " starring Denzel Washington. But physicists have also explored the nature of time and the plausibility of time travel for more than century, beginning with Albert Einstein's theories of relativity. Thanks to Einstein, scientists know that time slows as moving objects approach the speed of light. Gravity also slows time. This means that, in one sense, all of us can already consider ourselves time travelers in a limited way because we experience a tiny time warp (a difference of only nanoseconds) when we, for example, take a flight on an airplane. But physicists who study time travel today search for plausible ways to create a time warp large enough to allow noticeable travel into the past or future.

In his book "How to Build a Time Machine," physicist Paul Davies writes, "The theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel -- to any epoch, past or future -- might just be possible, too." This astonishing statement begs an important question: If time travel did indeed become a reality, how would it affect our world as we currently experience it?

First, it's important to realize that building a time machine would likely involve enormous expense, and the sheer complexity of such an apparatus would mean only a limited group of time travelers would have access to it. But even a small group of "astronauts" traveling through time and space could conceivably have a tremendous impact on life as we know it today. The possibilities, in fact, seem almost infinite.

Let's begin by assuming that it's possible to create a complete loop in time travel -- that time travelers could travel back into the past and then return to the future (or vice versa). Although scientists view traveling to the future as a much less problematic proposition than traveling to the past, our daily lives wouldn't change much if we could only send time travelers backward or forward in time, unable to recall them to the present. If we could, in fact, complete this loop of time travel, we can conjure up an incredible array of possible effects.

Possibilities and Paradoxes of Time Travel

Time travel turned total mayhem.

Imagine sending a time traveling astronaut 100 years into the future. The time traveler could witness technological advancements that we can only dream of today, much as people at the turn of the 20th century likely couldn't imagine the items we take for granted in 2010, such as iPods or laptop computers. The time traveler could also gain insight into medical advancements, such as new medicines, treatments and surgical techniques. If the time traveler could bring this knowledge backward in time to the present, the time from which he or she came, society could effectively leap forward in terms of its technical and scientific knowledge.

The futuristic time traveler could also bring back knowledge of what lay ahead for the world. He or she could warn of natural disasters, geopolitical conflicts, epidemics and other events of worldwide importance. This knowledge could potentially change the very way we operate. For example, what if a time traveler journeyed into the future and literally saw the effects that automobiles would eventually have on our planet? What if the time traveler witnessed an environment so polluted and damaged that it's unrecognizable? How might that change our willingness to use alternative forms of transportation?

Imagine that time travel became less restricted and more available to a larger population. Perhaps travel into the future would be exploited for personal gain. A futuristic time traveler could draw on knowledge of the stock market to guide his or her investment decisions, effectively using the granddaddy of all insider information to amass a fortune. Militaries might rely on time travel to gain valuable knowledge about the enemy's positioning and resources in future battles. Terrorists could use time travel to scout out the scenes of future attacks, allowing them to carefully plan with precise knowledge of future conditions.

The potential effects seem equally limitless in terms of the less likely possibility of time travel into the past. History books would no longer be based solely on exhaustive research and interpretation of ancient materials. Time travelers could resolve historical debates and verify how things did or didn't happen in the past. Imagine how different our understanding of the world might be if we could say definitively, for example, whether Moses actually parted the Red Sea or whether Lee Harvey Oswald acted alone in killing John F. Kennedy. A journey into the past could prove or disprove religious beliefs or result in face-to-face encounters with people such as Jesus, Buddha, Napoleon or Cleopatra -- or even the time traveler's former self. Perhaps time travelers could even bring back from the past things that had been lost, such as extinct species or dead and long-forgotten languages.

But here it's very important to raise the issue of self-consistent narratives and paradoxes. The concept of self-consistent narratives tells us that anything a time traveler would alter or affect in the past would have to remain consistent with the future from which he or she journeyed. Changing the past would effectively change the future, creating a causal loop. But such causal loops would only pose inherent problems if changes to the past resulted in a future different from the one the time traveler came from.

But perhaps the question of how time travel would affect life as we know it goes deeper than even a discussion of potential paradoxes and causal loops. Perhaps a discussion of specific effects of consequences on life as we know it makes little sense when faced with something that could change everything about the way in which we perceive our world.

Physicist Paul Davies gives a good example of a consistent causal loop in his book "How to Build a Time Machine." A mathematics professor uses a time machine to travel forward in time, where he discovers a new theorem. He returns back to the time he came from and gives one of his particularly gifted students the idea for that theorem. The student goes on to publish the theorem, and it turns out that it was this very student's work that the professor perused during his journey to the future. The narrative here is consistent.

On the other hand, with the grandfather paradox, a time traveler goes back in time and kills his grandfather. But if the time traveler's grandfather dies before the time traveler is born, how can he or she exist at all? And if the time traveler doesn't exist, how could he or she travel back in time to kill granddad?

As physicist Paul Davies describes it, unrestricted time travel -- meaning time travel that could form a complete loop to both the past and future -- would ultimately lead to total mayhem. In his words, "Time travel opens a view of the world that is a sort of madhouse where the rational order of things would no longer work. Under those circumstances, it's very hard to see how ordinary human life could continue."

In a world where the relationship between past, present and future is turned on its head, we would transcend the things that define our lives today. We would lose our notion of how time works, which could be so fundamentally damaging to our worldview that we would no longer care as much about the things that matter to us today: work, finances, making plans with friends and family, shopping -- you name it. These things just wouldn't be relevant in this crazy new world because we'd have a newfound preoccupation with simply making sense of a world without a set chronology -- we wouldn't know the order in which things occur.

It may be beside the point, then, to talk about resolving historical debates, saving endangered species or gaining technological, financial or military insight because those things might very likely fall by the wayside in the strange world that would follow the advent of unrestricted time travel.

As Davies makes clear, none of this fallout would occur from one-way travel. Hitching a one-way ride to the future or even the past (assuming we stick with self-consistent narratives) wouldn't cause this kind of profound reordering of the world as we currently experience it. But closing that loop of travel could be, in a word, disastrous.

Davies points out that science fiction normally focuses on the novelty aspect of time travel. But according to him, "It's not a novelty or a curiosity, it's something that strikes at the very rational basis of how we live and function. It's really hard to imagine that anything could be the same again." In his view, unrestricted time travel could change life as we know it so dramatically that we wouldn't even recognize it. Because chronology would have no meaning, we couldn't easily tell if something happened before or after, was a cause or an effect, and we would lose the ability to predict rationally the outcomes of our actions. In essence, it would be as though we had all gone insane.

These sobering potential effects of time travel have caused some scientists to wonder whether a principle exists in nature that would actually prevent unrestricted time travel, such as Stephen Hawking's "chronology protection hypothesis." This type of "theory of everything" might provide a scientific explanation as to why we could never unhinge the universe as we know it by making unrestricted time travel a reality. Scientists have yet to discover such a theory, but hearing Davies' take on the frightening effects of time travel makes one hope that they find it soon -- even if it means that we won't ever know for sure who killed JFK.

Frequently Asked Questions

How could time travel impact our understanding of history, what are the ethical implications of time travel, lots more information, related articles.

What is the fourth dimension?
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How Special Relativity Works
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Theory of Relativity

Time travel for travelers? It’s tricky.

Scientific theories suggest it’s possible to travel through time. But the reality isn’t so clear.

A photo illustration of Robot Restaurant in Tokyo.

Time travel has fascinated scientists and writers for at least 125 years. The concept feels especially intriguing now, when physical travel is limited. Here, a photo illustration of Tokyo’s Robot Restaurant captures the idea of speeding through time.

I’m stuck at home, you’re stuck at home, we’re all stuck at home. Jetting off to some fun-filled destination like we used to might not be in the cards for a little while yet. But what about travelling through time? And not just the boring way, where we wait for the future to arrive one second at a time. What if you could zip through time at will, travelling forward to the future or backward to the past as easily as pushing buttons on the dashboard of a souped-up DeLorean, just like in the movie Back to the Future ?

Time travel has been a fantasy for at least 125 years. H.G. Wells penned his groundbreaking novel, The Time Machine , in 1895, and it’s something that physicists and philosophers have been writing serious papers about for almost a century.

What really kick-started scientific investigations into time travel was the notion, dating to the closing years of the 19th century, that time could be envisioned as a dimension, just like space. We can move easily enough through space, so why not time?

At the end of the 19th century, scientists thought of time as a dimension like space, where travelers can go anywhere they want. This photo illustration of Tokyu Plaza in Tokyo’s Omotesando Harajuku evokes the feeling of visiting endless destinations.

“In space, you can go wherever you want, so maybe in time you can similarly go anywhere you want,” says Nikk Effingham, a philosopher at the University of Birmingham in the United Kingdom . “From there, it’s a short step to time machines.”

( Why are people obsessed with time travel? Best-selling author James Gleick has some ideas .)

Dueling theories

Wells was a novelist, not a physicist, but physics would soon catch up. In 1905, Albert Einstein published the first part of his relativity theory, known as special relativity . In it, space and time are malleable; measurements of both space and time depend on the relative speed of the person doing the measuring.

A few years later, the German mathematician Hermann Minkowski showed that, in Einstein’s theory, space and time could be thought of as two aspects of a single four-dimensional entity known as space-time . Then, in 1915, Einstein came up with the second part of his theory, known as general relativity . General relativity renders gravity in a new light: Instead of thinking of it as a force, general relativity describes gravity as a bending or warping of space-time.

But special relativity is enough to get us started in terms of moving through time. The theory “establishes that time is much more similar to space than we had previously thought,” says Clifford Johnson, a physicist at the University of Southern California. “So maybe everything we can do with space, we can do with time.”

Well, almost everything. Special relativity doesn’t give us a way of going back in time, but it does give us a way of going forward— and at a rate that you can actually control. In fact, thanks to special relativity, you can end up with two twins having different ages, the famous “twin paradox.”

Suppose you head off to the Alpha Centauri star system in your spaceship at a really high speed (something close to the speed of light), while your twin remains on Earth. When you come back home, you’ll find you’re now much younger than your twin. It’s counterintuitive, to say the least, but the physics, after more than a century, is rock solid.

“It is absolutely provable in special relativity that the astronaut who makes the journey, if they travel at very nearly the speed of light, will be much younger than their twin when they come back,” says Janna Levin, a physicist at Barnard College in New York . Interestingly, time appears to pass just as it always does for both twins; it’s only when they’re reunited that the difference reveals itself.

Maybe you were both in your 20s when the voyage began. When you come back, you look just a few years older than when you left, while your twin is perhaps now a grandparent. “My experience of the passage of time is utterly normal for me. My clocks tick at the normal rate, I age normally, movies run at the right pace,” says Levin. “I’m no further into my future than normal. But I’ve travelled into my twin’s future.”

( To study aging, scientist are looking to outer space .)

With general relativity, things really start to get interesting. In this theory, a massive object warps or distorts space and time. Perhaps you’ve seen diagrams or videos comparing this to the way a ball distorts a rubber sheet . One result is that, just as travelling at a high speed affects the rate at which time passes, simply being near a really heavy object—like a black hole —will affect one’s experience of time. (This trick was central to the plot of the 2014 film, Interstellar , in which Matthew McConaughey’s character spends time in the vicinity of a massive black hole. When he returns home, he finds that his young daughter is now elderly.)

A photo illustration created from inside Nakagin Capsule Tower.

To get around the “grandfather paradox,” some scientists theorize there could be multiple timelines. In these images of Nakagin Capsule Tower in Tokyo, Japan, time seems to pass at different rates.

But black holes are just the beginning. Physicists have also speculated about the implications of a much more exotic structure known as a wormhole . Wormholes, if they exist, could connect one location in space-time with another. An astronaut who enters a wormhole in the Andromeda Galaxy in the year 3000 might find herself emerging from the other end in our own galaxy, in the year 2000. But there’s a catch: While we have overwhelming evidence that black holes exist in nature—astronomers even photographed one last year—wormholes are far more speculative.

“You can imagine building a bridge from one region of space-time to another region of space-time,” explains Levin, “but it would require kinds of mass and energy that we don’t really know exist in reality, things like negative energy.” She says it’s “mathematically conceivable” that structures such as wormholes could exist, but they may not be part of physical reality.

There’s also the troubling question of what happens to our notions of cause and effect if backward time travel were possible. The most famous of these conundrums is the so-called “ grandfather paradox .” Suppose you travel back in time to when your grandfather was a young man. You kill him (perhaps by accident), which means your parent won’t be born, which means you won’t be born. Therefore, you won’t be able to travel through time and kill your grandfather.

Multiple timelines?

Over the years, physicists and philosophers have pondered various resolutions to the grandfather paradox. One possibility is that the paradox simply proves that no such journeys are possible; the laws of physics, somehow, must prevent backward time travel. This was the view of the late physicist Stephen Hawking , who called this rule the “ chronology protection conjecture .” (Mind you, he never specified the actual physics behind such a rule.)

But there are also other, more intriguing, solutions. Maybe backward time travel is possible, and yet time travelers can’t change the past, no matter how hard they try. Effingham, whose book Time Travel: Probability and Impossibility was published earlier this year, puts it this way: “You might shoot the wrong person, or you might change your mind. Or, you might shoot the person you think is your grandfather, but it turns out your grandmother had an affair with the milkman, and that’s who your grandfather was all along; you just didn’t know it.”

Which also means the much-discussed fantasy of killing Hitler before the outbreak of World War II is a non-starter. “It’s impossible because it didn’t happen,” says Fabio Costa, a theoretical physicist at the University of Queensland in Australia . “It’s not even a question. We know how history developed. There is no re-do.”

In fact, suggests Effingham, if you can’t change the past, then a time traveler probably can’t do anything . Your mere existence at a time in which you never existed would be a contradiction. “The universe doesn’t care whether the thing you’ve changed is that you’ve killed Hitler, or that you moved an atom from position A to position B,” Effingham says.

But all is not lost. The scenarios Effingham and Costa are imagining involve a single universe with a single “timeline.” But some physicists speculate that our universe is just one among many . If that’s the case, then perhaps time travelers who visit the past can do as they please, which would shed new light on the grandfather paradox.

( The Big Bang could have led to the creation of multiple universes, scientists say .)

“Maybe, for whatever reason, you decide to go back and commit this crime [of killing your grandfather], and so the world ‘branches off’ into two different realities,” says Levin. As a result, “even though you seem to be altering your past, you’re not really altering it; you’re creating a new history.” (This idea of multiple timelines lies at the heart of the Back to the Future movie trilogy. In contrast, in the movie 12 Monkeys , Bruce Willis’s character makes multiple journeys through time, but everything plays out along a single timeline.)

More work to be done

What everyone seems to agree on is that no one is building a time-travelling DeLorean or engineering a custom-built wormhole anytime soon. Instead, physicists are focusing on completing the work that Einstein began a century ago.

After more than 100 years, no one has figured out how to reconcile general relativity with the other great pillar of 20th century physics: quantum mechanics . Some physicists believe that a long-sought unified theory known as quantum gravity will yield new insight into the nature of time. At the very least, says Levin, it seems likely “that we need to go beyond just general relativity to understand time.”

Meanwhile, it’s no surprise that, like H.G. Wells, we continue to daydream about having the freedom to move through time just as we move through space. “Time is embedded in everything we do,” says Johnson. “It looms large in how we perceive the world. So being able to mess with time—I’m not surprised we’re obsessed with that, and fantasize about it.”

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Newton’s Views on Space, Time, and Motion

Isaac Newton founded classical mechanics on the view that space is distinct from body and that time passes uniformly without regard to whether anything happens in the world. For this reason he spoke of absolute space and absolute time , so as to distinguish these entities from the various ways by which we measure them (which he called relative spaces and relative times ). From antiquity into the eighteenth century, contrary views which denied that space and time are real entities maintained that the world is necessarily a material plenum. Concerning space, they held that the idea of empty space is a conceptual impossibility. Space is nothing but an abstraction we use to compare different arrangements of the bodies constituting the plenum. Concerning time, they insisted, there can be no lapse of time without change occurring somewhere. Time is merely a measure of cycles of change within the world.

Associated with these issues about the ontological status of space and time was the question of the nature of true motion. Newton defined the true motion of a body to be its motion through absolute space. Those who, before or shortly after Newton, rejected the reality of space, did not necessarily deny that there is a fact of the matter as to the state of true motion of any given body. They thought rather that the concept of true motion could be analyzed in terms of the specifics of the relative motions or the causes thereof. The difficulty (or, as Newton alleged, the impossibility) of so doing constituted for Newton a strong argument for the existence of absolute space.

In recent literature, Newton's theses regarding the ontology of space and time have come to be called substantivalism in contrast to relationism . It should be emphasized, though, that Newton did not regard space and time as genuine substances (as are, paradigmatically, bodies and minds), but rather as real entities with their own manner of existence as necessitated by God's existence (more specifically, his omnipresence and eternality).

1. Overview of the Scholium

2.1 the void, 2.2 aristotle's doctrines, 2.3 sixteenth-century innovations, 2.4 charleton and the seventeenth-century revival of atomism, 3. descartes' innovation.

4. Newton's Manuscript: De Gravitatione …

5.1 Arguments for Absolute Time

5.2 direct arguments for absolute space, 5.3 the arguments from properties, causes, and effects, 5.4 discriminating in practice between absolute and apparent motion, 6.1 what the major impediments are, 6.2 why they are indeed impediments, 7. newton's legacy, other internet resources, related entries.

Today, Newton is best known as a physicist whose greatest single contribution was the formulation of classical mechanics and gravitational theory as set out in his Philosophae Naturalis Principia Mathematica ( Mathematical Principles of Natural Philosophy ), first published in 1687, and now usually referred to simply as “Newton's Principia ”. Newton's views on space, time, and motion not only provided the kinematical basis for this monumental work and thus for the whole of classical physics up until the early twentieth century, but also played an integral role in Newton's general system of philosophy and theology (largely developed prior to the Principia ). Because Newton never drafted a treatise on, or even a digest of, this general system, his stature as one of the great philosophers of the seventeenth century, indeed, of all time, is no longer widely appreciated.

A “Scholium” at the beginning of the Principia , inserted between the “Definitions” and the “Laws of Motion”, lays out Newton's views on time, space, place, and motion. He begins by saying that, since in common life these quantities are conceived of in terms of their relations to sensible bodies, it is incumbent to distinguish between, on the one hand, the relative, apparent, common conception of them, and, on the other, the absolute, true, mathematical quantities themselves. To paraphrase:

Absolute, true, and mathematical time , from its own nature, passes equably without relation to anything external, and thus without reference to any change or way of measuring of time (e.g., the hour, day, month, or year).
Absolute, true, and mathematical space remains similar and immovable without relation to anything external. (The specific meaning of this will become clearer below from the way it contrasts with Descartes' concept of space.) Relative spaces are measures of absolute space defined with reference to some system of bodies or another, and thus a relative space may, and likely will, be in motion.
The place of a body is the space which it occupies, and may be absolute or relative according to whether the space is absolute or relative.
Absolute motion is the translation of a body from one absolute place to another; relative motion the translation from one relative place to another.

Newton devotes the bulk of the Scholium to arguing that the distinction between the true quantities and their relative measures is necessary and justified.

It is evident from these characterizations that, according to Newton:

space is something distinct from body and exists independently of the existence of bodies,
there is a fact of the matter whether a given body moves and what its true quantity of motion is, and
the true motion of a body does not consist of, or cannot be defined in terms of, its motion relative to other bodies.

The first of these theses was a point of major contention in 17th-century natural philosophy and one assailed by Newton's critics such as Leibniz, Huygens, and Berkeley. The second was not in general dispute. Descartes, Leibniz, and Berkeley all believed that, to put it in somewhat scholastic terms, the predicate ‘ x is in true motion’ is a complete predicate in the sense that it holds or fails to hold for any given body. (Huygens, at least in his post- Principia views, constitutes a special case.) Thus, for those who denied the first thesis, it was necessary to secure a definition, or an analysis, of what it means for a body be in true motion (and what determines the quantity of that motion), so as to be as adequate to the facts as Newton's characterization of true motion. The figures mentioned above all deemed that motion relative to other bodies is a necessary condition for true motion, although not, by itself, a sufficient condition.

Over the course of years, the consensus in the 17th and early 18th Centuries on thesis (2) was lost sight of, and it became common to characterize Newton's opponents as denying that there is a fact of the matter as to whether a body is in true motion and maintaining instead that all motion is merely relative motion . Thus, modern readers expect that Newton's Scholium on space, time, and motion should be read as arguing not only thesis (1) above, but also thesis (2), that all motion is not merely relative motion, but that some motions are true and absolute. Newton's arguments concerning motion, however, are designed to show, not that true motion is distinct from merely relative motion (which is granted by all), but rather that the only feasible analysis of true motion requires reference to absolute places, and thus the existence of absolute space.

In particular it has been assumed that Newton's so-called “rotating bucket experiment”, together with the later example of a pair of globes connected by a chord and revolving about their center of gravity, is supposed to argue, or provide evidence for, the existence of true, or absolute, motion. Not only is this false, but the two cases have distinct purposes in the framework of the Scholium. The rotating bucket experiment is the last of five arguments from the “properties, causes, and effects of motion” designed to show cumulatively that an adequate analysis of true motion must involve reference to absolute space. In contrast, the example of the revolving globes is intended to illustrate how it is that, despite the fact that absolute space is invisible to the senses, it is nonetheless possible to infer the quantity of absolute motion of individual bodies in various cases.

2. The Legacy from Antiquity

The most important question shaping 17th-century views on the nature of space, time and motion is whether or not a true void or vacuum is possible, i.e., a place devoid of body of any sort (including rarified substances such as air). Ancient atomism, dating back at least to the pre-Socratic philosopher Democritus (5th century, B. C.), held that not only is such possible, but in fact actually exists among the interstices of the smallest, indivisible parts of matter and extends without bound infinitely. Following Plato, Aristotle rejected the possibility of a void, claiming that, by definition, a void is nothing, and what is nothing cannot exist.

According to Aristotle, the universe is a material plenum, finite in extent, bounded by the outermost sphere of the fixed stars. Beyond that there is no void, i.e., empty places, since, as Aristotle defines ‘place’, the place of something is the outermost of “the innermost motionless boundary of what contains it.” Hence, since there are no boundaries outside the outermost celestial sphere, there are no places or space outside of it.

Time, according to Aristotle, is just the measure of motion, where by ‘motion’ he means change of any sort, including qualitative change. In order to define the uniformity of time, that is, the notion of equal intervals of time, Aristotle was guided by astronomical practice, which in antiquity provided the most practical and accurate measures of time. He identified uniform motion with the rate of motion of the fixed stars, a choice for which he found a dynamical justification in his celestial physics.

“Local” motion is but one species of motion, viz., change of place. Motion, in general, he defined as the actualization of potentiality, a notion commonly held in the 17th century to be so obscure as to be either useless or meaningless. However, as far as local motion is concerned, there is no difficulty as to what constitutes the true or absolute motion of a body in a finite geocentric universe. Indeed, elementary substances in the sub-lunar realm (earth, air, fire, and water) move of their own accord either up or down, i.e., toward the center or away from the center by their very nature. The celestial realm, beginning with the orbit of the moon, consists of an interlocking network of celestial spheres composed of a fifth element (aether), which by its nature is disposed to circular motion about the center of the of universe (i.e., the center of the earth). If the motion of this substance is taken to be the measure of time, the celestial spheres necessarily rotate uniformly. Since the net motion of an embedded sphere is the sum of its natural motion superimposed on the natural motions of the spheres in which it is embedded, and since the axes of rotation are in general set at slightly different angles in order to account for why the sun does not move on the celestial equator and the planets and the moon do not move strictly on the ecliptic (i.e., the path of the sun against the fixed stars), the motions of the moon, planets, and even the sun are not necessarily uniform. However, since the sphere of the fixed stars is embedded in no other celestial sphere in motion, the motion of the fixed stars is de facto the measure of all motion.

The motions spoken of so far are all natural motions of the substances in questions, motions induced by the body being the very substance that it is. In contrast, other motions, in which the cause of the motion is external rather that internal to the body, Aristotle subsumed under the concept of violent motion. Violent motion required for its continuation the constant application of an external cause.

Although Aristotle's views dominated medieval scholasticism, there occurred a renewed interest in atomism in the early 17th Century. Apart from general factors such as the Renaissance, Humanism, and the Reformation, specific innovations of the 16th Century made it attractive. Although Copernicus' introduction of a helio-static system was motivated by a strict adherence to Aristotle's dynamics of celestial spheres, it brought into question his terrestial physics. Galileo's telescopic observations of the surface of the moon and his discovery of moons orbiting about Jupiter brought into question the very distinction between the terrestial and the celestial. Moreover, the visibility of an abundance of new stars, apparently without end, suggested that the universe may in fact be without bound.

An important representative of the revival of atomism and its concomitant views concerning the void is Walter Charleton's Physiologia Epicuro-Gassendo-Charltoniana: Or a Fabrick of Science Natural, upon the Hypothesis of Atoms, “Founded by Epicurus, Repaired by Petrus Gassendus, Augmented by Walter Charleton” , which appeared in English in 1654, twelve years after Newton's birth. It is a text with which Newton became familiar as an undergraduate, and some of the core theses concerning time and space later put forth in the Principia and various unpublished manuscripts in Newton's hand can be found in Charleton. These include:

that time and space are real entities even though they fit neither of the traditional categories of substance or accident (i.e., property of a substance),
that time “flow[s] on eternally in the same calm and equal tenor,” while the motion of all bodies is subject to “acceleration, retardation, or suspension”,
that time is distinct from any measure of it, e.g., celestial motion or the solar day,
that space is “absolutely immoveable” and incorporeal,
that bodies, or “Corporeal Dimensions” are everywhere “Coexistent and Compatient” with the “Dimensions” of the parts of space they occupy,
that space distinct from body existed before God created the world and that God's omnipresence is his literal presence everywhere, and
that motion is the translation or migration of body from one place, as an immovable part of space, to another.

Charleton's arguments for his views concerning time have much the same tenor as those given by Newton in the Principia . In marked contrast, though, those for empty, immense, and immutable space are quite different. Charleton appeals to the explanation of such phenomena as rarefaction and condensation, the differences in “degrees of Gravity” of bodies, and the numerous ways in which bodies can interpenetrate at the micro-level in terms of solubility, absorption, calefaction, and diverse chemical reactions. However, Charleton does not introduce the terminology of “relative” time, “relative” spaces, or “relative” places, and nowhere raises concerns regarding true (absolute) motion versus merely relative motion. Oddly enough, although Charleton occasionlly mentions and criticizes Descartes with regard to other matters, no note of the fact is made that Descartes, a decade earlier, had proposed explanations, in detail or in outline, for just these sorts phenomena according to a system of nature in which the world is completely filled with matter and in which space distinct from body cannot exist. Descartes, it can be justly said, is the founder of the other main school of the “mechancal philosophy” of the 17th Century, which stood in direct opposition to atomism on the issue of the possibility of a vacuum and which adapted the Aristotelian doctrines on the nature of time, space, and motion to the new world view.

Although avowedly anti-Aristotelian in many regards, particularly on the view, shared with atomists, that all qualitative change on the macroscopic scale is reducible to the rearrangement and/or motion of matter on the microscopic scale, it was Descartes' ambition to carry out this program by retaining what is essentially Aristotle's notion of Prime Matter. The pure elements (earth, air, fire, and water) of Aristotle's physics could mutate into one another by alteration of the fundamental qualities definitive of them. These were the four haptic qualities of hot, cold, wet, and dry. Because of this, there had to be something distinguishable, at least in thought, from qualities that persist during elemental alteration. This quality-less substratum is what Aristotle referred to simply as matter, or as it is often called, Prime Matter, in order to avoid confusion with the macroscopically identifiable, quality-laden, homogenous portions of everyday objects. Unlike atomists, who attributed at least the quality of hardness (impenetrability) to the ultimate particles of matter, Descartes argued that matter, or synonymously, body [corpus] has no qualities whatsoever, but only quantity, i.e., extension. In other words, body and extension are literally one and the same [res extensa]. An immediate corollary is that there can be no vacuum, for that would require an extended region devoid of body --- a manifest contradiction. The task, then, was to show how all apparent qualities can be explained in terms of the infinite divisibility and rearrangement of extension with respect to itself. The task was grand indeed, for its goal was to develop a unified celestial and terrestrial physics that could account equally for the ductility of metals, magnetic attraction, the tides, the mechanism of gravity, the motion of the planets, the appearance and disappearance of comets, and the birth and death of stars (supernovae).

Descartes published his system of the world in 1644 as the Principles of Philosophy ( Principia Philosophae ). Part II of the Principles lays out the thesis of the identity of space (extension) and matter, develops a definition of motion in the “true, or philosophical sense”, and sets out the fundamental dynamical laws of his system. Motion, according to “the truth of the matter”, is defined to be “the translation of one part of matter, or one body, from the vicinity of those bodies, which are immediately contiguous to it and are viewed as if at rest, to the vicinity of others.” In consequence, Descartes points out, each body has a single motion proper to it (in contrast to the numerous relative motions that can be ascribed to it depending on which other bodies are selected in order to determine its place). It is this single proper motion that figures in his laws of motion. Of particular importance for Descartes' entire system, is that a body in circular motion has an endeavor [conatus] to recede from the center of rotation.

4. Newton's Manuscript De Gravitatione …

This fact, together with Descartes' contention that a body also participates in the motion of a body of which it is a part, makes it difficult to reconcile Descartes' system of the world with his definition of proper motion. Newton concluded that the doctrine is in fact self-refuting and that, where Descartes needed to, he had surreptitiously helped himself to a notion of space independent of body, particularly in order to assign the desired degree of centrifugal conatus to the planets and their satellites as they are swept about by celestial vortices of “subtle” matter.

The untitled and unfinished manuscript which begins “De Gravitatione et aequipondio fluidorum et solidorum …”, written perhaps a decade or more before the Principia , consists for the most part of an extensive and scathing critique of Descartes' doctrine of motion. The document, published for the first time in (Hall and Hall, 1962), is well worth the study for a glimpse at the development of Newton's thinking at a relatively young age. It manifestly embraces the doctrines of space and time later codified in the Principia . Notable, as well, is that each of the five arguments from the properties, causes and effects of motion advanced in the Scholium has a clearly identifiable antecedent in De Gravitatione . (See Rynasiewicz 1995 for details.) This makes it clear the extent to which the Scholium is concerned to argue specifically against the Cartesian system (as pointed out by Stein 1967), which Newton perceived to be the only other viable contender at the time.

5. Newton's Scholium on Time, Space, Place and Motion

The Scholium has a clearly discernible structure. Four paragraphs marked by Roman numerals I–IV follow the opening paragraph, giving Newton's characterizations of time, space, place and motion, respectively, as summarized in the third paragraph of Section 1 above. If we were to extend Newton's enumeration to the remaining paragraphs, then paragraphs V–XII constitute a sustained defense of the distinctions as characterized in I–IV. Paragraph XIII then states the general conclusion that the relative quantities are genuinely distinct from the respective absolute quantities and makes comments on the semantic issue of the meanings of these terms in the Bible. There follows one remaining, and quite extensive paragraph [XIV], which takes up the question how in practice one can ascertain the true motions of bodies and concludes: “But how we are to obtain the true motions from their causes, effects and apparent differences, and vice-versa, will be explained at length in the treatise that follows. For that is the end to which I composed it.”

In what follows, links have been inserted to the text of the Scholium according to the extended enumeration suggested above. Clicking on a link will open a new window in such a way that the reader can navigate back and forth between a given paragraph of the text and the commentary elucidating that paragraph.

Paragraph V appeals to the fact that astronomy distinguishes between absolute and relative time in its use of the so-called equation of time. This serves to correct for inequalities in the commonly adopted standard of time, the solar day, which most people mistakenly believe to be uniform. The solar day, defined as the period of time it takes the sun to return to zenith, varies by as much as 20 minutes over the course of a year. The standard of correction in the equation of time used in Ptolemaic astronomy was based upon the assumption that the sidereal day—the period of time it takes a fixed star to return to zenith—is constant, because the celestial sphere on which the fixed stars are located should not be assumed to speed up and slow down. With the demise of the Ptolemaic system and Aristotelian cosmology, this rationale was no longer compelling, and at least some astronomers, most notably Kepler, called into doubt whether the rate of rotation of the earth remained constant over the course of the year. (Kepler considered that its rotation would be faster when closer to the sun due to an excitatory effect of the sun.) Thus, the issue of the correct measure of time occupied considerable attention in 17th Century astronomy, especially because the ability to measure the rate of rotation of the earth is equivalent to the problem of determining longitude, which, for sea-faring nations, was critical for navigation (and hence military and economic dominance). Huygens' pendulum clock provided the first terrestrial candidate for a decently accurate measure of uniform time. Newton mentions this, as well as the eclipses of the moons of Jupiter, an alternative method based on Kepler's period law.

The invocation of the need for an equation of time in astronomy is not just an appeal to a well entrenched scientific practice. In the course of his discussion, Newton explains why he thinks the need is justified. Although he will argue in Book III of the Principia that the diurnal rotation of the earth is uniform, this is a contingent fact. It could have been otherwise. Indeed, it could have been that there are no uniform motions to serve as accurate measures of time. The reason is that all motion is subject to being accelerated or retarded (by the application of external forces). In contrast, absolute time (which is nothing other than duration or the perseverance of the existence of things) remains the same, whether the motions be be swift, slow, or null.

Paragraph VI defends the thesis of the immobility of (absolute) space, which against the backdrop of Descartes, clearly means that the parts of space, just as the parts of time, do not change their relation with respect to one another. Newton argues that the parts of space are their own places, and for a place to be moved out of itself is absurd. A more expansive antecedent of this argument occurs in De Gravitatione , applied specifically to time: if yesterday and tomorrow were to interchange their temporal relations with respect to the remainder of time, then yesterday would become today, and today yesterday. Thus, Newton held an interestingly holistic identity criterion for the parts of space and time.

Newton devotes five full paragraphs to justifying his characterization of the distinction between absolute and relative motion. The first three present arguments from properties of absolute motion and rest, the next presents an argument from their causes, and the final an argument from their effects. The force of these has confused modern commentators for a combination of reasons which, historically, are difficult to untangle. Since only those not already prejudiced by those commentaries, directly or indirectly, will find what follows unusual, it is best to defer an autopsy of those reasons until Section 6, after an exposition of the arguments.

Suffice it to say for the moment that it is a common misunderstanding that in these arguments Newton intends to develop empirical criteria for distinguishing cases of absolute motion from merely apparent motion and thereby to disprove the thesis that all motion is merely relative motion. To the contrary, the arguments take as their point of departure the assumption, common to Cartesian and Aristotelian philosophy, that each body has a unique state of true motion (or rest). Throughout the arguments, the terms ‘true motion’ and ‘absolute motion’ are treated synonymously. At issue is whether true motion (and rest) can be reduced to some special instance of relative motion (or rest) with respect to other bodies. In announcing at the outset of these arguments that “absolute and relative rest and motion are distinguished by by their properties, causes, and effects”, Newton indicates his intent to show that they cannot, at least if true motion and rest are to have those features we generally associate, or ought to associate, with them.

Argument 1 from Properties [ Paragraph VIII ] Property : Bodies that are truly at rest are at rest with respect to one another. Conclusion : True rest cannot be defined simply in terms of position relative to other bodies in the local vicinity.

Reasoning : Suppose there were a body somewhere in the universe absolutely at rest, say far away, in the region of the fixed stars, or even farther. (Whether or not that body might ever be observed doesn't enter into what follows.) Clearly it is impossible to know just from considering the positions of bodies in our region relative to one another whether any of these latter bodies maintains a fixed position with respect to that hypothetical distant body. To amplify, let B be one of the local bodies, C the relative configuration over time of the set of local bodies, and A the far distant body at absolute rest. The specification of C alone fails to establish the position of B relative to A over time. In particular, C fails to establish whether B is relatively at rest with respect A, which, by the property stated above, is a necessary condition for B to be absolutely at rest. Hence, specification of the local configuration C underdetermines whether or not B is at absolute rest. Thus the conclusion: it is impossible to define what it is for a body such as B to be at absolute rest [i.e., to give necessary and sufficient conditions for when it is that B is at rest] simply in terms of how B fits into the local configuration C.

Argument 2 from Properties [ Paragraph IX ] Property : If a part of a body maintains a fixed position with respect to the body as a whole, then it participates in the motion of the whole body. Conclusion : True and absolute motion cannot be defined as a translation from the vicinity of (the immediately surrounding) bodies, viewing the latter as if they were at rest.

Reasoning : Newton first introduces two considerations that can be taken either to support, or to illustrate, or to amplify upon the import of the stated property. The first is that if a part of a rotating body is at rest relative to the body as a whole, it endeavors to recede from the axis of rotation. The second is that the impetus of a body to move forward arises from the combination of the impetus of its parts.

From the property it follows that if those bodies surrounding a given body move (either rotationally or progressively forward as a fixed configuration) while the surrounded body is at rest relative to the surrounding ones, then the surrounded body partakes in the (true) motion of the group of surrounding bodies. Hence, if the surrounding bodies move truly, then so does the surrounded body. But according to the (Cartesian) definition of motion—which identifies the true motion of a body with its transference from the vicinity of immediately surrounding bodies, regarding the surrounding bodies to be as though they are at rest—it would have to be said (wrongly) that the surrounded body is truly at rest. Hence that definition is untenable.

Argument 3 from Properties [ Paragraph X ] Property : Anything put in a moving place moves along with that place, and hence a body participates in the motion of its place when it moves [relatively] away from that place. Conclusion : The complete and absolute motion of a body cannot be defined except by means of stationary places.

Reasoning : From the property, the [relative] motion of a body out of a given place is only part of the motion of the body if the place in question is itself in motion. The complete and true motion of the body consists of its motion relative to the moving place added vectorially to whatever motion the place may have. Should the place be moving relative to a place which is in turn moving, then the motion of that place must be added, and so on. Barring infinite regress, the sum must terminate with a motion relative to a stationary place.

Addended Argument : After deriving this conclusion, Newton amplifies upon the consequences. The only places that are stationary are all of those that that stay in fixed positions with respect to one another from infinity to infinity, and since these always remain stationary, they make up what Newton calls immobile absolute space.

The Argument from Causes [ Paragraph XI ] Causes : the forces impressed upon bodies. The major premise is that application of a [non-zero net] force on a body is both a necessary and sufficient condition for either generating or altering its true motion. More specifically: (A) Impressed force is a necessary condition for generating or altering true motion (but not, as remains to be shown, merely relative motion). (B) Application of a [non-zero net] force is a sufficient condition for the generation or alteration of true motion (but not, as will be shown subsequently, merely relative motion). Conclusion : The true motion of an individual body cannot be defined as any particular sub-instance of its motion relative to other bodies.

Reasoning : Newton seeks to establish that application of a positive net force to a body is neither a necessary not a sufficient condition for the generation of motion relative to other bodies. The two lines of reasoning are given separately, call them ‘Prong A’ and ‘Prong B’, respectively.

Prong A : To be established is that, although an impressed force is necessary for the generation or alteration of true motion in a body, it is not necessary for the generation of motion relative to other bodies. The reasoning is quite simple: pick a given body and merely apply the same [accelerative] force to all other bodies in question. These other bodies will then remain in the same relative configuration with respect to one another, but a relative motion with respect to the original body [to which no force has been applied] will either be generated or altered.

Prong B : To be established is that, although an impressed force is sufficient for the generation or alteration of true motion in a body, it is not sufficient for the generation of motion relative to other bodies. Again, the line of reasoning is quite straightforward. Consider an arbitrarily given body amongst a system of bodies and simply apply the same [accelerative] force to all bodies in question. Then, despite the fact that a force has been impressed upon the originally given body, there is neither generation nor alteration of relative motion with respect to the remaining bodies.

The Argument from Effects [ Paragraph XII ] Effects : the forces of receding from the axis of rotational motion [centrifugal endeavor]. The major premise is that the centrifugal endeavor of bodies [or parts of bodies] to recede from the axis of rotation is directly proportional to the quantity of the true circular motion. Conclusion : True rotational motion cannot be defined as relative rotation with respect to the surrounding bodies.

Reasoning : The line of reasoning is in fact parallel to the preceding argument from causes, although this may not be completely perspicuous due to the fact that the correlates of the two prongs above are here stages of a single on-going experimental situation, the so-called “rotating bucket” experiment, which, Newton intimates, he actually performed. In order to set up this experiment, one suspends a bucket using a long cord and by turning the bucket repeatedly, winds up the cord until it is strongly twisted, then fills the bucket with water. During the course of the experiment, the degree to which the water tries to climb up the sides of the bucket is used as a measure of its centrifugal endeavor to recede from the center. Newton uses the experiment to establish that centrifugal endeavor is neither a necessary condition nor a sufficient condition for the existence of relative circular motion [of the water] with respect to its surroundings [the bucket].

Stage 1 : When the bucket is first released, it rotates rapidly with respect to the rest frame of the experimenter while the water remains at rest with respect to the experimenter. In other words, there is rapid relative motion of the water with respect to the bucket. However, the surface of the water remains flat, indicating that it has no tendency to recede from the axis of relative rotation. Thus, the existence of centrifugal endeavor in the parts of a body is not a necessary condition for the body to be rotating relative to its surroundings. That is, such relative rotation with respect to immediately adjacent bodies need not produce any centrifugal endeavor in the parts of the body to recede from the axis of relative rotation.

In the further course of the experiment, as the bucket continues to rotate, the water gradually begins to rotate with it, and as it does so, begins to climb up the sides of the bucket. Eventually, according to Newton, the water acquires the same rotation of the bucket relative to the lab frame, at which point we have the following situation.

Stage 2 : The water and the bucket are at relative rest, yet the water has achieved its highest ascent up the sides of the bucket, indicating a maximum centrifugal endeavor to recede from the axis of common rotation. Hence, the existence of centrifugal endeavor is not a sufficient condition for the presence of relative circular motion between a body and its surroundings, i.e., if a body, or rather its parts, have a centrifugal endeavor to recede from a central axis, it does not follow that there is a relative circular motion of the body with respect to its immediate surroundings.

Astrophysical Application . After deriving the conclusion, Newton uses the premises of the first two arguments from properties, together with the premise of the argument from effects, to critique the vortex theory of planetary motion. According to that theory, each of the planets (and most notably the earth) is relatively at rest with respect to the “subtle” matter of the celestial vortex of our own sun. Hence, according to Descartes' own definition of true motion (as well as his explicit insistence), they have no true motion. However, it is manifest that they do not maintain fixed positions with respect to one another. So, according to the property invoked in the first argument, they cannot [all] be truly at rest. Moreover, from the property invoked in the second argument, they partake in the circular motion of the solar vortex [assuming that motion to be true motion, as Descartes implicitly assumed]. Finally, because they would accordingly participate in the true circular motion of this hypothetical vortex, they should have an endeavor to recede from the axis of its rotation.

This completes the sequence of arguments from the properties, causes, and effects of motion. The next paragraph [ XIII ] states the cumulative conclusions of the arguments marshalled beginning with the arguments for absolute time in paragraph V: “Hence relative quantities are not the quantities themselves, whose names they bear, but are only sensible measures of them (either accurate or inaccurate), which are commonly used in place of the quantities they measure.” Having made his case, Newton comments on the ordinary language meaning of the terms for these quantities in order to address contemporary issues of dogma and heresy.

Galileo's condemnation by the Catholic Church for asserting that the earth is in motion was still recent history at the time Newton composed the Principia . Descartes, who lived in reach of Papal authority and feared similar fate, had found a clever way of espousing Copernicanism without falling prey to accusation of heresy. According to his definition of motion “properly speaking”, he contends, the earth is truly at rest.

In Newton's system of the world as set out in Book III of the Principia , the earth patently moves absolutely. In anticipation, Newton indicates how to reconcile this with scripture by observing that, if usage determines the meanings of words, then in ordinary discourse (including the Bible) the terms ‘time’, ‘space’, ‘place’, and ‘motion‘ are properly understood to signify the relative quantities; only in specialized and mathematical contexts do they denote the absolute quantities. (Keep in mind Newton's title, The Mathematical Principles of Natural Philosophy .) He proceeds to chastise Descartes on two counts, first for doing violence to the scriptures by taking them to refer to the absolute quantities, and second, for confusing the true quantities with their relative measures.

Having argued his case that true motion consists in motion with respect to absolute space, and thus having dealt to his satisfaction with the metaphysics of motion, Newton turns in the final paragraph of the Scholium to epistemological strategies available on his account. On an Aristotelian or Cartesian account, one can directly observe the allegedly absolute motion of a body if both it and its immediate surroundings are visible. In contrast, because the parts of absolute space are not directly accessible to the senses, it is very difficult, Newton confesses, to ascertain the true motion of individual bodies and to discriminate them in practice from the apparent motions. “Nevertheless,” he remarks in a rare moment of wit, “the situation is not entirely desperate.” Evidence is available in part from apparent motions, which are the differences of true motions, and in part from the forces, which are the causes and effects of true motions.

Newton illustrates with an example. Imagine a pair of globes, connected by a cord, revolving about their common center of gravity. The endeavor of the globes to recede from the axis of motion is revealed by the tension in the cord, from which the quantity of circular motion can be estimated. Furthermore, whether the direction of their revolution is clockwise or counterclockwise can be detected by applying forces to opposite faces of the globes to see whether the tension in the cord increases or decreases. All this can be done in empty space where no other bodies are present to serve as points of reference.

Suppose now that, in addition to the globes, there is second system of bodies maintaining fixed positions with respect to one another (for example, the fixed stars). If the two systems are in a state of relative rotation, one cannot gauge from just the relative rotation, which, if either, is at rest. However, from the tension in the cord connecting globes, one can establish whether the relative rotation is due entirely to the absolute rotation of the system of globes. Supposing so, the second system of bodies can then be exploited to provide an alternative technique for determining whether the globes revolve in a clockwise or counterclockwise direction—one simply consults the direction of rotation relative to the stationary system.

At this point Newton cuts off the Scholium, explaining that the whole point of having written the treatise to follow is to show how to infer the true motions from their causes, effects, and apparent differences, and conversely the causes and effects from either the true or the apparent motions.

6. Common Impediments to Understanding the Scholium

As remarked in Section 5.3 above, the purpose of the arguments from properties, causes, and effects has been widely misunderstood in both the historical and philosophical literature, and as a consequence, so too the relation of these to the example of the revolving globes in the final paragraph. Some diagnosis as to why may help those readers already steeped in tradition to overcome certain prejudices they bring to the Scholium and may also serve to further illuminate the framework in which Newton and his contemporaries struggle with the problem of motion.

(1) Newton's stated intention in the Scholium is to maintain that absolute space, time, and motion are genuinely distinct from their relative counterparts. For the case of space, this clearly amounts to arguing the existence of an entity distinct from body in which bodies are located—something denied by relationists. Similarly, for the case of time, this involves arguing the existence of an entity distinct from the succession of particular events in which the events are located—again, something denied by relationists. It may seem then as a matter of course that, for the case of motion, Newton should argue for existence of something denied by relationists, presumably, absolute motion.

(2) It would amount to a virtual petitio principii were Newton to rest a case for absolute motion on the existence of absolute space. Hence, one would expect him to appeal to various physical phenomena that might provide independent warrant. Now it is well known that Newton's laws satisfy the principle of Galilean relativity, according to which there can be no experimental test to determine whether a system is at rest or in a state of uniform rectilinear motion. However, Newton's laws do support a distinction between inertial and non-inertial motion in that they predict, in non-inertial frames, the appearance of so-called “fictitious forces,” for instance, centrifugal forces in rotating frames, resulting in a tendency for bodies to recede from the axis of rotation. Since this is exactly the effect involved in the rotating bucket experiment, it is tempting to interpret Newton as marshaling it as a case in which this phenomenon suggests independent warrant for the existence of absolute motion.

(3) Moreover, since the same effect is operative in the example of the revolving globes, it is hard to see why that example does not serve the very same purpose. In fact, in his famous critique of Newton in the Science of Mechanics , Ernst Mach, in quoting from the Principia , cut out all of the intervening text to make it appear as though the two are but variant examples in the development of a single argument.

(4) Finally, the choice of language in Motte's 1729 translation, which is the basis for the most widely available twentieth century English translation by Cajori, tends to reinforce the presumption that the arguments from properties, causes, and effects seek to identify phenomena that empirically distinguish absolute from (merely) apparent motion. In the Cajori version, the conclusions of the first three arguments, the arguments from the properties of motion and rest, read:

… it follows that absolute rest cannot be determined from the position of bodies in our regions. [ Paragraph VIII ]
…the true and absolute motion of a body cannot be determined by the translation of it from those which only seem to rest; [ Paragraph IX ]
Wherefore, entire and absolute motions can be no otherwise determined than by immovable places; [ Paragraph X ]

Thus, it is tempting to assume that both the argument from causes and the argument from effects are likewise concerned to identify an empirical signature of absolute motion by which it can be distinguished from (merely) apparent motion. (Reading the arguments in this fashion, only the argument from effects, which deals with the centrifugal effects of circular motion, appears to help Newton's cause—a commonly registered complaint.)

It will be more illuminating to respond to these in reverse order.

(Ad 4) It is an artifact of Motte's translation that the Latin verb definiri (passive infinitive) is rendered occasionally as ‘be determined’ rather than as ‘be defined’. According to seventeenth-century English usage, either choice is acceptable. In appropriate contexts, the two function as synonyms, as in the Euclidean axiom, “Two points determine a line.” Motte's practice conforms with this. The conclusion of the argument from effects, ‘definiri’ is translated as ‘be defined’:

And therefore this endeavor does not depend upon any translation of the water in respect of the ambient bodies, nor can true circular motion be defined by such translation. [ Paragraph XII ]

If one now goes back and substitutes ‘be defined’ for ‘be determined’ into the conclusions from the arguments from properties quoted above, they take on, to the modern ear, a different meaning. They make claims as to what constitutes an adequate definition of the concepts of true, or absolute, motion and rest.

(Ad 3) We have already seen how paragraph XIII signals the conclusion, not just of the arguments from properties, causes, and effects, but the direct arguments for absolute time and absolute space as well, which, altogether, Newton takes establish the ontological distinction between the absolute and the relative quantities. That the next paragraph, in which the globes are introduced, concerns a different, epistemological issue would be apparent were it not for another artifact of the Motte translation, this time involving the Latin verb ‘distinguere’. Newton uses the word again and again, almost thematically, in characterizing and arguing for the ontological distinction between the absolute and the relative quantities; and Motte renders it in English as ‘to distinguish’. Unfortunately, the English verb appears in the Motte translation one more time at the start of the final paragraph:

It is indeed a matter of great difficulty to discover, and effectually to distinguish, the true motions of particular bodies from the apparent;

But in the Latin, the word ‘distinguere’ is nowhere to be found. Rather, the sentence reads:

Motus quidem veros corporum singulorum cognoscere, & ab apparentibus actu discriminare , difficillimum est;

Thus, to the Latin reader, it is clear that Newton is moving on to a different consideration.

(Ad 2) What has been said in connection with (4) suffices against the false expectations developed in (2). However, there may remain some sense that, even on a proper reading, Newton tried to bluff his way past the principle of Galilean relativity. Newton indeed acknowledges the principle, though not by name, in Corollary V to the laws of motion:

The motions of bodies in a given [relative] space are the same among themselves whether that space is at rest or moves uniformly in a straight line without uniform motion.

And there is no reason to think that he did not appreciate the limitation it poses for experimentally differentiating between absolute rest and uniform motion in a straight line. A particular instance of Corollary V is the solar system as a whole. Assuming the absence of external forces, it follows (from Corollary IV to the laws) that the center of gravity of the solar system is either at rest or moves uniformly in a straight line. But which? Because of Corollary V, when Newton wishes to attribute a definite state of motion to the center of mass of the solar system in Book III, he must introduce the hypothesis that “The center of the system of the world is at rest.” Should this not be some source of embarrassment?

Apparently not. Immediately following the hypothesis, he writes:

This is conceded by everyone, although some contend it is the earth, others the sun, that is at rest in the center. Let us see what follows from this.

According to Newton, the attribution of a state of absolute rest to one or the other of these bodies is universally taken for granted. What does confound all conventional wisdom in what follows is that neither the earth nor the sun is at rest, but rather the center of gravity of the solar system.

(Ad 1) Although arguing that absolute space and absolute time are distinct from any relative spaces and relative times involves, in each case, arguing for the existence of an additional entity, it does not follow that, in arguing that absolute motion is distinct from relative motion, Newton is obliged to argue yet another existence claim. Unfortunately, the term ‘absolute motion’ is prone to be read in two distinct ways. On one reading, it means, as a matter of stipulative definition, ‘change of absolute place’. In this sense of ‘absolute motion’, the existence of absolute motion (or more precisely, the possibility of the existence of absolute motion) follows immediately from the existence of absolute space and absolute time. As indicated before, nothing further needs to be said. On the other reading, ‘absolute motion’ is synonymous with ‘true motion’. And as we have just seen, Newton finds no reason to doubt that his audience does not grant that a body is either truly at rest or truly in motion. The venerable tradition that takes motion and rest to be contraries has yet to be questioned. So it is not incumbent on Newton make a case for the reality of absolute motion in the sense of true motion. What is incumbent is for him to argue that true motion just is change of absolute place. And that is the purpose of the arguments from properties, causes, and effects.

Newton's views on space, time, and motion dominated physics from the 17th Century until the advent of the theory of relativity in the 20th Century. Nonetheless, these views have been subjected to frequent criticism, beginning with contemporaries, such as Leibniz and Berkeley, and continuing on to the close of the 19th Century, most notably with Ernst Mach, whose writings influenced Einstein. In the early twentieth century, Newton tended to be cast as a metaphysical dogmatist by the early philosophical interpreters of relativity, in particular Hans Reichenbach. Unfortunately, that stigma has tended to linger.

More recent scholarship reveals a more sober picture of why Newton felt fully justified in positing absolute space, absolute time, and absolute motion. Moreover, the novel feature of special relativity, the rejection of absolute simultaneity—something that never occurred to any of Newton's earlier critics—necessitated only that absolute space and absolute time be replaced with an absolute space-time (Minkowski spacetime). And although Einstein's development of general relativity was in large part motivated by a desire to implement a general principle of relativity, to wit that all motion is relative motion, that it succeeds in doing so was questioned shortly after the theory was introduced. As for the question of the absoluteness of space-time in general relativity, it no longer has the character of something which acts without being acted upon, as Einstein himself pointed out. The space-time metric tensor not only encodes for spatiotemporal structure, but also represents the gravitational potentials, and thus gravitational energy. By Einstein's famous equation for the equivalence of energy and mass, it follows that the gravitational field possesses mass. Only, since gravitational energy can not be localized in terms of an energy density tensor, but is possessed by the field holistically, neither can this mass be localized. Thus, philosophical controversy as to whether space-time can exist without matter becomes tendentious according whether one counts the gravitation field as something material or not.

Thus, the question whether the revolution in our views about space and time in the last century vindicates Newton's critics as more philosophically astute becomes a misplaced one. The distinction between what counts as matter in contrast to empty space presupposed in the earlier debates has been eclipsed by possibilities undreamt of before the introduction of modern field theory and relativity. [ 1 ]

Primary Sources

Charleton, Walter, 1654, Physiologia Epicuro-Gassendo-Charltoniana: or a Fabrick of Science Natural Upon the Hypothesis of Atoms , London: Tho. Newcomb. Reprinted with indices and introduction by Robert Hugh Kargon, New York and London: Johnson Reprint Corporation, 1966.
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Andrew Motte's 1729 translation of the Principia
Voltaire on Descartes and Newton

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Einstein's Theory of Special Relativity

Special relativity: It's like normal relativity, but special.

Special relativity equation (E=mc^2) on a chalkboard.

What was physics like before relativity?

How did einstein come up with special relativity, what does e = mc^2 mean, time dilation, special relativity and quantum mechanics, additional resources.

Albert Einstein 's 1905 theory of special relativity is one of the most important papers ever published in the field of physics. Special relativity is an explanation of how speed affects mass, time and space. The theory includes a way for the speed of light to define the relationship between energy and matter — small amounts of mass (m) can be interchangeable with enormous amounts of energy (E), as defined by the classic equation E = mc^2.

Special relativity applies to "special" cases — it's mostly used when discussing huge energies, ultra-fast speeds and astronomical distances, all without the complications of gravity . Einstein officially added gravity to his theories in 1915, with the publication of his paper on general relativity .

As an object approaches the speed of light, the object's mass becomes infinite and so does the energy required to move it. That means it is impossible for any matter to go faster than light travels. This cosmic speed limit inspires new realms of physics and science fiction, as people consider travel across vast distances.

Before Einstein, astronomers (for the most part) understood the universe in terms of three laws of motion presented by Isaac Newton in 1686. These three laws are:

Objects in motion or at rest remain in the same state unless an external force imposes change. This is also known as the concept of inertia .
The force acting on an object is equal to the mass of the object multiplied by its acceleration. In other words, you can calculate how much force it takes to move objects with various masses at different speeds.
For every action, there is an equal and opposite reaction .

Newton's laws proved valid in nearly every application in physics, according to Encyclopedia Britannica . They formed the basis for our understanding of mechanics and gravity.

But some things couldn't be explained by Newton's work: For example, light.

To shoehorn the odd behavior of light into Newton's framework for physics scientists in the 1800s supposed that light must be transmitted through some medium, which they called the "luminiferous ether." That hypothetical ether had to be rigid enough to transfer light waves like a guitar string vibrates with sound, but also completely undetectable in the movements of planets and stars.

That was a tall order. Researchers set about trying to detect that mysterious ether, hoping to understand it better. In 1887, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang , physicist Albert A. Michelson and chemist Edward Morley calculated how Earth's motion through the ether affected how the speed of light is measured, and unexpectedly found that the speed of light is the same no matter what Earth's motion is.

If the speed of light didn't change despite the Earth's movement through the ether, they concluded, there must be no such thing as ether to begin with: Light in space moved through a vacuum.

That meant it couldn't be explained by classical mechanics. Physics needed a new paradigm.

According to Einstein, in his 1949 book " Autobiographical Notes " (Open Court, 1999, Centennial Edition), the budding physicist began questioning the behavior of light when he was just 16 years old. In a thought experiment as a teenager, he wrote, he imagined chasing a beam of light.

Classical physics would imply that as the imaginary Einstein sped up to catch the light, the light wave would eventually come to a relative speed of zero — the man and the light would be moving at speed together, and he could see light as a frozen electromagnetic field. But, Einstein wrote, this contradicted work by another scientist, James Clerk Maxwell, whose equations required that electromagnetic waves always move at the same speed in a vacuum: 186,282 miles per second (300,000 kilometers per second).

Philosopher of physics John D. Norton challenged Einstein's story in his book " Einstein for Everyone " (Nullarbor Press, 2007), in part because as a 16-year-old, Einstein wouldn't yet have encountered Maxwell's equations. But because it appeared in Einstein's own memoir, the anecdote is still widely accepted.

If a person could, theoretically, catch up to a beam of light and see it frozen relative to their own motion, would physics as a whole have to change depending on a person's speed, and their vantage point? Instead, Einstein recounted, he sought a unified theory that would make the rules of physics the same for everyone, everywhere, all the time.

This, wrote the physicist, led to his eventual musings on the theory of special relativity, which he broke down into another thought experiment: A person is standing next to a train track comparing observations of a lightning storm with a person inside the train. And because this is physics, of course, the train is moving nearly the speed of light.

Einstein imagined the train at a point on the track equally between two trees. If a bolt of lightning hit both trees at the same time, the person beside the track would see simultaneous strikes. But because they are moving toward one lightning bolt and away from the other, the person on the train would see the bolt ahead of the train first, and the bolt behind the train later.

Einstein concluded that simultaneity is not absolute, or in other words, that simultaneous events as seen by one observer could occur at different times from the perspective of another. It's not lightspeed that changes, he realized, but time itself that is relative. Time moves differently for objects in motion than for objects at rest. Meanwhile, the speed of light, as observed by anyone anywhere in the universe, moving or not moving, is always the same.

One of the most famous and well-known equations in all of human history, E = mc^2, translates to "energy is equal to mass times the speed of light squared." In other words, wrote PBS Nova , energy (E) and mass (m) are interchangeable. They are, in fact, just different forms of the same thing.

But they're not easily exchanged. Because the speed of light is already an enormous number, and the equation demands that it be multiplied by itself (or squared) to become even larger, a small amount of mass contains a huge amount of energy. For example, PBS Nova explained, "If you could turn every one of the atoms in a paper clip into pure energy — leaving no mass whatsoever — the paper clip would yield [the equivalent energy of] 18 kilotons of TNT. That's roughly the size of the bomb that destroyed Hiroshima in 1945."

One of the many implications of Einstein's special relativity work is that time moves relative to the observer. An object in motion experiences time dilation, meaning that when an object is moving very fast it experiences time more slowly than when it is at rest.

For example, when astronaut Scott Kelly spent nearly a year aboard the International Space Station starting in 2015, he was moving much faster than his twin brother, astronaut Mark Kelly, who spent the year on the planet's surface. Due to time dilation, Mark Kelly aged just a little faster than Scott — "five milliseconds," according to the earth-bound twin. Since Scott wasn't moving near lightspeed, the actual difference in aging due to time dilation was negligible. In fact, considering how much stress and radiation the airborne twin experienced aboard the ISS, some would argue Scott Kelly increased his rate of aging.

But at speeds approaching the speed of light, the effects of time dilation could be much more apparent. Imagine a 15-year-old leaves her high school traveling at 99.5% of the speed of light for five years (from the teenage astronaut's perspective). When the 15-year-old got back to Earth, she would have aged those 5 years she spent traveling. Her classmates, however, would be 65 years old — 50 years would have passed on the much slower-moving planet.

We don't currently have the technology to travel anywhere near that speed. But with the precision of modern technology, time dilation does actually affect human engineering.

GPS devices work by calculating a position based on communication with at least three satellites in distant Earth orbits. Those satellites have to keep track of incredibly precise time in order to pinpoint a location on the planet, so they work based on atomic clocks. But because those atomic clocks are on board satellites that are constantly whizzing through space at 8,700 mph (14,000 km/h), special relativity means that they tick an extra 7 microseconds, or 7 millionths of a second, each day, according to American Physical Society publication Physics Central . In order to maintain pace with Earth clocks, atomic clocks on GPS satellites need to subtract 7 microseconds each day.

With additional effects from general relativity (Einstein's follow-up to special relativity that incorporates gravity), clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. That effect adds microseconds to each day on a GPS atomic clock, so in the end engineers subtract 7 microseconds and add 45 more back on. GPS clocks don't tick over to the next day until they have run a total of 38 microseconds longer than comparable clocks on Earth.

Special relativity and quantum mechanics are two of the most widely accepted models of how our universe works. But special relativity mostly pertains to extremely large distances, speeds and objects, uniting them in a "smooth" model of the universe. Events in special (and general) relativity are continuous and deterministic, wrote Corey Powell for The Guardian , which means that every action results in a direct, specific and local consequence. That's different from quantum mechanics, Powell continued: quantum physics are "chunky," with events occurring in jumps or "quantum leaps" that have probabilistic outcomes, not definite ones.

Researchers uniting special relativity and quantum mechanics — the smooth and the chunky, the very large and the very small — have come up with fields like relativistic quantum mechanics and, more recently, quantum field theory to better understand subatomic particles and their interactions.

Researchers striving to connect quantum mechanics and general relativity, on the other hand, consider it to be one of the great unsolved problems in physics. For decades, many viewed string theory to be the most promising area of research into a unified theory of all physics. Now, a host of additional theories exist. For example, one group proposes space-time loops to link the tiny, chunky quantum world with the wide relativistic universe.

Check out this time dilation calculator from Omni Calculator .
Explore Einstein's thought experiments in this video from PBS Nova .
Go back to the source and read Einstein's explainer in this translated edition of his book, Relativity: The Special and General Theory (Dover, 2001).

This article was originally written by Elizabeth Howell and has since been updated.

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter.

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You Lost Your Passport. Now What?

Summer travel season is upon us. Do you know where your passport is?

An illustration of a traveler, with a rolling suitcase and holding a passport in one hand, walking in front of the open pages of a passport.

By Seth Kugel

While just about every other document is accessible online, a lost passport is one of the last analog emergencies that can derail an international trip.

Here’s a guide to replacing a lost passport according to how fast you need it, how much money you have to spare and where you live. (The process for last-minute renewals of expired passports, by the way, is fairly similar.)

If you’ve got at least three weeks

You’ll be OK. On the State Department website, at travel.state.gov/passports , first report your passport lost and then follow the process for a replacement. On the website, you’ll find a list of 7,500 passport acceptance facilities — including post offices, public libraries and clerk of court offices — where you can make an appointment or, in most cases, come during scheduled walk-in hours. They’ll verify your documents and send them to the State Department, and you’ll get your passport in the mail.

Before the day of your appointment, check what you need to bring , a list that will include at least proof of U.S. citizenship, payment (acceptable forms vary by facility), the right forms (for lost passports, that’s the DS-11 and DS-46) and at most facilities, a properly taken photo .

The “routine” processing time to replace a passport takes six to eight weeks and costs $165; choose the “expedited” service, for an additional $60, to receive your passport in two to three weeks.

You might even get yours “faster than advertised,” said Matt Pierce, a managing director of passport services for the State Department, noting that the pandemic-era backlog was cleared up as of December.

Avoid delays by carefully following instructions, and consider spending an additional $19.53 for faster shipping.

When time is tighter

If your trip is less than three weeks away, you’ll need to take the extra step of going through one of 26 passport processing offices across the United States. If you’re doing it without an outside expediter, you must make an appointment online or over the phone, up to 14 days in advance of your trip.

There are no more walk-ins, an option before the pandemic, but the State Department has increased capacity to eliminate the need for them, Mr. Pierce said, and the offices give special priority to documented “ life-or-death emergencies ” of immediate family members. You’ll need to bring all of your documents, and proof you are traveling soon — like a plane or cruise reservation.

Things can get dicey if you need an appointment in the next day or two. In such cases you can call for an appointment, or contact your local member of Congress.

Laurie Lee, the chief executive of Chicago-based Swift Passport and Visa Services , an expediter, has seen cases where clients on the verge of missing a wedding or another once-in-a-lifetime event spend hours refreshing the site to find a last-minute appointment, and even end up booking flights across the country if they can find an opening. That, of course, costs both time and money.

At the passport office, the process will be similar, except that you’ll likely be able to pick up your passport later in the day. In most cases, said Mr. Pierce, you won’t have to return the next day, but it can happen. (You can also have your passport mailed to you, if you have time.)

Using an expediter

If you have at least three business days and are willing to spend hundreds of dollars for help, consider Swift or one of the other 200-plus agencies registered with the State Department as couriers, or expediters.

Expediters are especially helpful for people who live far from the nearest passport office and cannot or do not want to drive or fly there. But they can also benefit anyone having trouble getting a timely appointment, because they have reserved slots that allow them to bring a client application directly to certain passport offices. Swift, for example, can bring 10 applications a day to the Boston office, and five to the Chicago one. Depending on urgency, Swift’s assistance costs between $155 and $599.

To work with an expediter, you will still need to go to your local passport acceptance facility, like a post office, where workers verify and seal your documents in an envelope, which you then send (or hand-deliver) to the expediter. They’ll take care of the rest, and get your new passport back to you.

What can go wrong

If you’re traveling to a passport agency, leave plenty of time to get there. Screwing up something on the form can also lead to delays that cost you your trip.

“Common errors include signing in the wrong place, not putting the date on their application, having an incorrectly sized passport photo or not being framed correctly in the photo,” said Steve Diehl, chief corporate development officer of CIBTvisas , a large expediter.

Once you get it

When your passport arrives, make digital and paper copies. Change your number on trusted traveler programs like Global Entry, and remember that if your old passport had a visa in it for the country you’re headed to, you’ll also need to rush a replacement of that.

If you do find your old passport later, keep it as a souvenir: It is no longer valid.

If you lose it while abroad

First of all, try not to. Unless you are traveling in a country that strictly requires you to have your passport on your person at all times, stow it in a hotel safe and carry around a photocopy, plus your drivers license.

But if you do lose the passport or have it stolen, report it online to protect yourself from identity theft and then contact the nearest U.S. embassy or consulate for an emergency appointment. You’ll need that paper or a digital copy of your passport, plus similar documents to what you would need for a replacement in the United States. You may receive either an actual replacement passport or an emergency version, usually good for one year.

In a real emergency, you can try a last ditch maneuver: Ask the airline to contact U.S. Customs and Border Patrol to seek permission for you to enter the country without a passport.

Preventative measures

Several expediters advised that many of their clients misplaced their passports during recent moves, so when you pack up your home, be sure to remember where your passport is packed.

Oh, and before you give up on that lost passport, check your home copier or scanner. “I can’t tell you how often people find it in their copy machine,” said Ms. Lee.

Seth Kugel is the columnist for “ Tripped Up ,” an advice column that helps readers navigate the often confusing world of travel. More about Seth Kugel

Open Up Your World

Considering a trip, or just some armchair traveling here are some ideas..

52 Places: Why do we travel? For food, culture, adventure, natural beauty? Our 2024 list has all those elements, and more .

The Alaska Highway: On an epic road trip, a family plots a course from Alaska to the Lower 48, passing through some of Canada’s most spectacular scenery .

Minorca: Spend 36 hours on this slow-paced Spanish island , which offers a quieter and wilder retreat than its more touristy neighbors.

Japan: A new high-speed train stop unlocks Kaga, a destination for hot springs, nourishing food and traditional crafts , as an easy-to-reach getaway from Tokyo.

London: The Victoria and Albert Museum is a treasure trove of art and design. Here’s one besotted visitor’s plan for taking it all in .

Americans have between 4 and 6 hours of leisure time daily. We just have no idea how to use it.

The average American has way more "free" time than you might think.
But most of us spend the majority of our leisure time staring at screens.
Picking up a new hobby can be a good way to reset — but that's easier said than done.

It's another Tuesday night, and work is winding down. I send my last Slack messages for the evening, fire off a few emails, and shut my laptop with a false sense of finality as if I don't have to return to my desk in sixteen short hours.

I make the short trek from my at-home office to the couch, where I grab the remote and settle in for yet another night of "Real Housewives" viewing. Before I know it, four hours have passed. My eyes are heavy, and it's time for bed. So long, Tuesday.

That's how it goes on Wednesday and Thursday, too. Maybe I'll grab drinks with friends on Friday or switch out reality TV for the newest Netflix movie come Monday, but generally speaking, I spend most of my adult life eating, sleeping, working, and scrolling to the ambient sounds of the telly . It's a far cry from the schedule I kept as a teenager, sprinting from play rehearsal to swim practice and still finding time for homework and socializing in between.

It's true that Americans are overworked , overstressed, and generally awful at unplugging from work, two time-use researchers told Business Insider.

But that only tells half the story.

Americans, on average, have between four to six hours of leisure time every day, according to the American Time Use Survey , which measures the amount of time people spend doing various activities. In 2022, men spent an average of 5.6 hours engaging in leisure activities each day, while women clocked in 4.8 hours of free time, according to the study.

Five hours of free time a day? That can't be right! If I had 25 extra hours each workweek, I certainly would have mastered the piano or written a novel by now, right? Wrong. Instead, all I have to show for my free time is an ungodly knowledge of Bravo lore.

Researchers say I'm not alone in languishing away my leisure time. Several facets of American life, including our reverence for work , our failing social safety net , and the Puritanical ideals on which our country was founded, all play a role in Americans' seeming inability to unwind in meaningful ways, researchers said.

Two elements define leisure: choice and control, according to Brigid Schulte, author of "Overwhelmed: Work, Love, and Play when No One has the Time" and director of the Better Life Lab. People have to choose an activity freely and have control over the time they spend doing it.

That's why, for much of human history, leisure time was out of reach for the masses, restricted to those with the social standing and status to engage in it — namely, rich men.

"It used to be that having discretionary time and being able to engage in leisure activity was a mark you were of a high social class," said Liana Sayer, director of the University of Maryland's Time Use Lab. "If you could do what you wanted with your time, it meant other people were providing the necessities of life for you."

That changed with industrialization, Sayer said. But the idea that those who have more money also have more time is one that still holds true today. People who work steady, 9-to-5 jobs with predictable schedules are much more likely to find extra time in their day, Sayer said. Gig workers and hourly employees, on the other hand, are increasingly reliant on multiple jobs and unpredictable schedules.

Despite class differences, 95% of Americans over the age of 15 engaged in some kind of leisure activity on a typical day, according to the 2022 time-use survey. The leisure category encompasses pastimes like socializing, exercising, and reading for pleasure.

But the vast majority of Americans' leisure time is spent — you guessed it — in front of the television. Watching TV is the most popular leisure activity, accounting for an average of 2.8 hours a day — more than half of all Americans' leisure time.

Much has been made about America's co-dependant relationship with the tube. (Some influential time researchers have argued Americans' social skills started to decline when air conditioning and television became commonplace, allowing people to remain both comfortable and entertained without leaving their homes, Sayer said.) But whether the small screen rots our brains or helps us relax, the act of watching TV is often an inherently anti-social one, researchers said.

It's easy to flip on the TV after a busy day at work because it requires almost no planning and very little brain engagement. Partaking in an out-of-the-house activity or making plans with other people, meanwhile, takes organization and coordination.

Our reliance on TV is part of a larger trend that has seen Americans engage less and less with other people and pro-social institutions like church or volunteer groups, Sayers said, adding that this is a pre-pandemic pattern. While COVID-19 certainly shifted our approach to work and leisure, it can't be solely blamed for Americans' increasing loneliness.

Sexism and the safety net

There is also a gendered element at play when it comes to Americans' leisure time. Married mothers do about three times the amount of housework, and twice as much childcare as married fathers do, Sayer said. There's some evidence that men are starting to step up their contributions post-pandemic, according to Sayer. However, much of women's "free" time is still dedicated to household duties and parenting.

"Most women don't feel like they deserve leisure time," Schulte said. "They feel like they have to earn it."

That mindset is not unique to American women and dates as far back as biblical times, researchers said. Women have long been transforming their leisure time into productive activities, from starting sewing circles to socializing at the watering hole, Schulte said.

But Americans' toxic relationship with leisure isn't gender-specific. A valorization of "hard work" was built into the very founding of our country, Schulte said. As a result, Americans have an intrinsic desire to be busy; we take it as a point of pride to overwork ourselves and cultivate little societal respect for hobbies and recreation. And Americans have been getting busier and busier over the course of the last few decades, increasingly to the detriment of their civic and social life, Schulte said.

"A lot of that panic and anxiety can be tied back to the '70s and '80s and the dismantling of the social safety net," she added.

When it comes to prioritizing leisure time, America could take some cues from its European friends. Norwegians average more than six hours of leisure time each day. At the same time, the Belgians and Greeks pursue relaxation and hobbies for more than five-and-a-half hours each day on average, according to time use data from the Organization for Economic Co-operation and Development. The US ranked 21 in global leisure time.

But America today lacks much of the infrastructure that would allow people to fully embrace and prioritize their leisure time, researchers said. People can't take the time to learn a new skill or truly unwind without robust maternity leave, affordable childcare, a better work-life balance, and dependable healthcare in place, Schulte said.

European countries far outrank the US when it comes to social spending. In 2019, France spent nearly a third of its gross domestic product on services related to health, family, unemployment, housing, and other benefits, according to OECD data compiled by the World Economic Forum . Finland, Belgium, Italy, Germany, Spain, Japan, and the UK all dedicated 20% or more of their spending to social services, while the US ranked ninth with 18.7%.

"A lot needs to change big-picture with policymakers and business leaders," Schulte said of the US. "But people can't wait until then to pick up a new hobby."

Most adults struggle to remember what they even liked to do as a kid, which is one of the reasons TV has become the national default leisure activity. Schulte recommends starting small: Set a timer for 30 minutes each day and practice developing the muscle of first remembering what you like to do and then giving it a try.

Beginning ballet

The best way to recover and "refresh your soul," as the Greeks described it, is to completely detach from work and take a proper break, according to Ciara Kelly, a lecturer in work psychology at The University of Sheffield.

Hobbies are particularly good for that, Kelly said, citing a 2019 study she led that found people who engaged in hobbies enjoyed improved confidence and saw benefits at their jobs.

The study's findings resonated with me. I had been an activity-driven adolescent, someone who found purpose and community in my hobbies and passions. I missed having an identity outside my work and media consumption.

So, I did what any rational 26-year-old journalist would: I signed up for a beginner's ballet class.

It was terrifying. I hadn't worn ballet slippers since I was four years old. I had no idea what the French words flying out of my teacher's mouth meant. My balance was terrible, and my flexibility left much to be desired.

In those first few weeks, I came dangerously close to quitting, nearly falling prey to the achievement-oriented culture that runs rampant in America.

"We're focused on doing and being the best — even in yoga classes — people have written about trying to outdo others as if we're in constant competition," Schulte told me. "But leisure requires none of that."

So, I kept going back. Even though I wasn't the best one in my class. Even though I sometimes (often) looked silly. And for 50 minutes every Monday night, I feel like a kid again.

If you enjoyed this story, be sure to follow Business Insider on Microsoft Start.

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Destination of the Year 2024: Costa Rica

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Picture yourself on a white-sand beach, surrounded by palm trees. In the distance, surfers ride the breaking waves. Behind you rises a dense rainforest, where white-faced capuchin monkeys jump from branch to branch. Welcome to Costa Rica .

I grew up in Costa Rica, and while I’ve lived all over the U.S., I'm always drawn back home. In the northwest, a dry tropical forest stretches down the Nicoya Peninsula. The laid-back beach towns that dot this part of the Pacific coast include Nosara, a mecca for surfing and yoga, and my favorite, the lesser-known San Juanillo, which still has the charm of a small fishing village. Farther south, Santa Teresa has an intriguing mix of cultures and superb international cuisine.

Venture inland and you’ll reach the Guanacaste pampas, with its sabaneros — Costa Rican cowboys — and rich pre-Columbian and colonial history. I love to walk through the town of Nicoya, one of the first Spanish settlements in the country and the home of the Chorotega people.

On the Caribbean side of the country, you’ll find the freshwater canals and serene rivers that meander through the rainforest of Barra del Colorado National Wildlife Refuge and Tortuguero National Park. In Limón, the vibrant Afro-Caribbean culture includes the sounds of calypso and an aromatic cuisine built around strong spices and coconut milk.

If hiking is your passion, try the Camino de Costa Rica, a 174-mile trail that cuts across the country: starting in Barra del Pacuare on the Atlantic coast and eventually winding into the Dota Mountains — the place where I grew up.

Off the Osa Peninsula, in southern Costa Rica, is the Golfo Dulce, a sanctuary for Pacific humpback whales. The region is defined by mangrove swamps and Corcovado National Park, which protects one of the most diverse ecosystems in the world — and where it is possible to see a jaguar strolling along a white-sand beach.

I could go on and on. There is no single recipe for enjoying Costa Rica. Any road you take will likely bring the same result: a sense of wonder and a desire to return. We Costa Ricans are relatively few — the population barely exceeds 5 million. But we’re proud of our shared idea that things will always turn out fine and that life is, above all, beautiful.

— Ronny Rojas

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If 2024 is your year for retirement, then one, congratulations, and two, we've got a destination for you to consider moving to in your golden years.

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15 Underrated Places to Travel This Summer, From Kosovo to Quebec

By Jamie Spain

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Planning the perfect summer trip can take weeks of research, preparation, and organization to ensure it goes off without a hitch. But before you get to the planning and booking phase, you'll need to decide where to go. There's a seemingly endless amount of great options out there, and while there's nothing wrong with heading to the same familiar spot over and over again, sometimes you want to get a little off the beaten path and go somewhere you haven't been before. Whether you're in the mood for a nature escape with jaw-dropping vistas, hiking paths, and secluded lodges, or are looking for a big city excursion surrounded by trendy restaurants, unique neighborhoods, and great shopping, we've got ideas for you.

From islands to national parks , here are 15 underrated summer travel ideas, perfect for avoiding the crowds while you explore someplace new.

Domestic destinations:

International destinations:

Domestic destinations

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Finger Lakes, New York

A fan-favorite of New York City residents, the New York Finger Lakes are unquestionably idyllic. The perfect destination for a summer weekend , or a weeklong trip with the family, this region is home to some of the most beautiful waterfronts. Dotted with adorable small towns , the 11 long, slender lakes that make up this area are exactly what East Coast summer dreams are made of. Spend your trip navigating nearby state parks; hiking, biking, and exploring; and enjoying everything that lake life has to offer, from jet-skiing to swimming.

Where to stay: Check out the Gould Hotel , a modern boutique property in Seneca Falls; book a night at The Lake House on Canandaigua for an airy, design-forward stay; or head east to the Inns of Aurora .

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Voyageurs National Park, Minnesota

Completely immerse yourself in nature at Minnesota's Voyageurs National Park. This gorgeous destination is ideal for getting in some of the best stargazing you could ever imagine. Just like dozens of other national parks , this one has an International Dark Sky Park certificate, designating it as a location that is particularly good for viewing the night sky and stars. Unlike the others, however, Voyageurs is one of the least-visited national parks in the country, meaning you'll be able to truly connect with nature and enjoy seclusion. It's ideal for boat rides and swimming, as it's primarily made up of waterfronts and islands—almost 40 percent of the park is water.

Where to stay: Nearby Cantilever Hotel is great for those interested in exploring the park, but not wanting to stay too far away from civilization. Those who are a little more daring will enjoy the Kettle Falls Hotel , a property that is only accessible by boat or seaplane. If you're really looking to be one with nature, you can rent a canoe and camp at a tent site.

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Isle Royale National Park, Michigan

If you're looking for a summer vacation where you can explore an area relatively untouched by civilization, look no further than Isle Royale National Park . A favorite of my family, this park is uniquely difficult to get to (the main island is best reached by ferry), so the only other travelers you'll find here are extreme adventurers and in-the-know locals. It's best to visit during the summer months since Lake Superior is quite cold and foggy throughout the rest of the year. My family heads to this park in the summer months in hopes of spending a few days backpacking , canoeing, and hiking through the near-untouched environment—admiring the nature and wildlife along the way.

By CNT Editors

By Todd Plummer

By Caitlin Morton

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By Lale Arikoglu

Where to stay: While the actual park is best experienced through camping, those who aren't interested in totally roughing it can stay at the Rock Harbor Lodge , centrally located on the main island. Or, book this nearby rustic Airbnb cabin with panoramic views of Lake Superior for the days leading up to your national park visit.

15 Underrated Summer Travel Ideas for 2024

Catalina Island, California

Just a hop, skip, and a jump from Los Angeles , Santa Catalina Island is quite literally heaven on Earth. Hopeful travelers can either take the one-hour Catalina Express ferry over to the island (it departs daily from San Pedro, Long Beach, and Dana Point) or charter a private yacht to get them there in style. Whether you're looking to hike the Trans Catalina Trail or simply indulge in some tasty seafood while wandering through the quaint town of Avalon, Catalina Island is a great break from the hustle and bustle of LA. Head a bit farther inland to explore El Rancho Escondido , the acclaimed Arabian horse ranch that's also home to Rusack Vineyards.

Where to stay: For day-trippers or those who plan to extend their vacation to include a stay in Los Angeles, there are a variety of hotels and Airbnbs to choose from, including The Beverly Hills Hotel, Dorchester Collection , The Malibu Beach Inn , and the luxe Chateau Marmont Estate on Airbnb. If you're hoping to stay on-island, the plush Bellanca Hotel is the way to go.

San Juan Islands, Washington

A 2019 Readers' Choice Award winner, named one of the best islands for beautiful scenery, this collection of more than 400 islands off the coast of Washington is ideal for a Pacific Northwest weekend getaway. The three most popular (and largest) islands to explore are Orcas , San Juan, and Lopez—all easily accessible by ferry. Travelers can enjoy a laidback vacation with comfortable temperatures and a relaxed atmosphere: hop on a bike and explore the area; spend time sailing, hiking, and fishing; or take a walk to admire all of the islands' natural beauty, from the mountains to the lakes.

Where to stay: Glampers, campers, and regular hotel-goers will all adore the Lakedale which has accommodations for every type of traveler.

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Crystal River, Florida

Home to one of the best beaches in Florida , Crystal River is unlike most other cities you'll find in the Sunshine State for one major reason: It's one of the only places in the world where people can swim alongside manatees . If you've ever wanted to truly get up close to these gentle giants, this is the city to visit. Head to the Crystal River National Wildlife Refuge to see your fill of the endangered animal, and spend some time taking in the natural beauty of the nearby springs and parks.

Where to stay: The Plantation Resort on Crystal River is home to a variety of different activities including a 27-hole championship golf course and a family-friendly resort with 196 accommodations.

International destinations

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There's a reason Kosovo is on our list of the Best Places to Go in 2024, and it has everything to do with the jaw-dropping scenery that can be found in every corner of this country. Outdoorsy folks who want to spend their time hiking and biking through untainted valleys, mountains, and countryside will appreciate the chance to explore the newest European country. Travelers who who are looking for a taste of the young nation's history can head to the national museum; afterward, explore the streets of Prizren taking in the architecture as well as the historic mosques and churches.

Where to stay: Book a night at the Hotel Gracanica in the quiet suburb of Prishtina, or pamper yourself at the luxe Ujevara e Drinit Resort near Peja. For something a little more traditional, enjoy tasty Kosovo dishes as well as classic comforts at the rural Ariu Guesthouse . (Check out our guide on where to eat, play, and stay in Kosovo for even more recommendations.)

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Budapest, Hungary

Skip Western Europe and instead head to the beautiful capital of Hungary . Luxuriate in the mineral-rich thermal baths you'll find around the city , or spend your days exploring the historic Castle District, walking along the Danube, and meandering through the city streets. You can even take a faux hot air balloon trip to enjoy the best view in the city. If you plan your trip for August, it may coincide with the Sziget Festival , one of the largest music and cultural festivals in all of Europe.

Where to stay: Book a night at 2024 Gold List winner, The Four Seasons Gresham Palace Budapest , or spend a few nights at the adults-only Hotel Clark Budapest . (Check out our guide on where to eat, play, and stay in Budapest for more recommendations.)

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Copenhagen, Denmark

There's no better time to visit Copenhagen than during the summer months when the weather is warm, the days are exceptionally long, and the waterfront views are breathtaking. Aside from being ranked as one of the happiest countries in the world , sustainably-minded travelers will appreciate how eco-focused the place is, and enjoy a trip to CopenHill, the multi-use waste-to-energy plant with a ski slope, climbing wall, and cafe. It's also worth checking out the Tivoli Gardens, Rosenborg Castle, and the picturesque Nyhavn harbor .

Where to stay: Check out our favorite hotels in Copenhagen , including the grand and historic Hotel d'Angleterre , the Nimb Hotel which was originally built as a castle in 1909, and the modern Villa Copenhagen .

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Buenos Aires, Argentina

From awe-inspiring museums to culture-packed tours to dance halls perfect for learning to tango , there's no shortage of amazing things to do and see in Buenos Aires . Whether you're spending the day simply walking the streets and taking in the sights; exploring the colorful La Boca; watching a soccer match; learning to tango; or going to the Colón Theatre to see the opera, this will be a trip to remember.

Where to stay: Some of our favorite hotels in Buenos Aires include the luxe Palacio Duhau ; the Jardín Escondido , once home to Francis Ford Coppola; and the Faena Hotel Buenos Aires located right by the water in Puerto Madero.

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Sardinia, Italy

There's so much more to Sardinia than just a beautiful coastline and wonderful beaches. While the long, warm days are perfect to spend by the crystal blue waters of the Mediterranean Sea, travelers who want a little more substance to their vacation can head inland to experience the island's charming towns, rustic cuisine, and ancient ruins.

Where to stay: Rest your head at the Cascioni Eco Retreat , set on a 90-acre nature reserve with gorgeous views of the surrounding land. The colorful Su Gologone Experience Hotel is another great option, as is the luxurious, high-end Hotel Cali de Volpe .

Another location on our list of the Best Places to Go in 2024, Mauritius is about 1,200 miles east of Madagascar . From its gorgeous coastlines with pristine beaches to the luxury accommodations and unparalleled nature, this island destination is nothing short of paradise. Head here during the Northern Hemisphere's summer months to avoid wet season. Your main activities on Mauritius will be water-based, from diving and snorkeling to sitting on the beach and admiring the coastline. If you need a break from relaxing, take a trip to the Bel Ombre Nature Reserve, the Botanic Gardens, or the capital city of Port Louis.

Where to stay: Check out our favorite hotels in Mauritius , including the four-time Readers' Choice Award winner and two-time Gold List winner, One&Only Le Saint Géran . You can also book a stay at the legendary resort, Lux Belle Mare , or the kite-surfers' paradise of Riu Palace Mauritius .

Kyoto, Japan

Instead of heading to the mega-popular Tokyo or the bright and lively Osaka, consider touching down in Kyoto , one of the best cities to experience traditional Japanese culture. Spend the day exploring the temples and shrines that are dotted throughout the city; take a walk through Gion, the “Geisha District;” or while away a few hours in the Kyoto National Museum. The best times to visit Japan are at the very end or beginning of summer when the season overlaps with spring and fall, and while it'll certainly be a bit warmer during the peaks of summer, it's no less beautiful.

Where to stay: From the Ace Hotel to the Shinmonzen , there is no shortage of great hotels in Central Kyoto. If you're interested in something a little different, tour guide Sara Aiko from Traveler 's Ask a Local series recommends taking a trip to Moksa , a gorgeous property in the north of Kyoto surrounded by temples and nature.

Nothing says romance like staying in an overwater bungalow in Bora Bora. The winter in French Polynesia corresponds with the summer in the Northern Hemisphere, meaning June, July, and August experience less humid and more comfortable temperatures. This time period also falls during Tahiti 's dry season, so you don't have to worry about being rained out. You could easily spend your days simply lounging in your bungalow and slipping into the crystal waters whenever you feel, but Bora Bora is also ideal for snorkeling, scuba diving, and taking a boat or a 4x4 island tour.

Where to stay: Bora Bora is particularly known for its luxury overwater bungalows. Conrad Bora Bora Nui is a more private option with exceptional snorkeling and coral, The St. Regis Bora Bora Resort is perfect for honeymooners and families alike, and the Four Seasons Resort Bora Bora is home to over 100 bungalows, fine dining, and an exceptional spa.

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Quebec, Canada

When you think of summer travel, your first thought is probably an island getaway or a far-flung destination that you've only read about in books. But, don't overlook all of the beauty that our northern neighbor has to offer. Quebec City is home to all the same type of charm you'll find in Europe but without the exceptionally long flight to get there. Spend some time in Old Quebec City —home to some of the oldest streets in North America—taking in the history and culture that make this city worthy of its title as a UNESCO World Heritage Site . Or, head to Montreal , where you can enjoy a variety of museums, the open-air market, and great culinary options. Don't forget to explore the nature and French architecture found in the greater province.

Where to stay: For a wonderful stay in Quebec City, Hotel 71 is a six-time Readers' Choice Award winner while Auberge Saint Antoine Quebec City and Le Germain Hotel Quebec are both five-time winners. For a wellness-focused retreat , head to Monastère des Augustines . In Montreal , check out the Auberge du Vieux-Port on the St. Lawrence River, or Hotel William Gray in Old Montreal.

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Where Does the Concept of Time Travel Come From?
The Concept of Time Travel From Ancient Myths to Modern Science
What is Time and How to Time Travel
5 Ways to Travel Through Time
Time travel theories date back to the 9th Century BCE
How To Explain Time Travel

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Time travel Concept l
Is Time Travel Really Possible
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Where Does the Concept of Time Travel Come From?
One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the ...
Is Time Travel Possible?
In Summary: Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.
Time Travel
Time Travel. First published Thu Nov 14, 2013; substantive revision Fri Mar 22, 2024. There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is ...
Time travel
The first page of The Time Machine published by Heinemann. Time travel is the hypothetical activity of traveling into the past or future.Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction. In fiction, time travel is typically achieved through the use of a hypothetical device known as a time machine.The idea of a time machine was popularized by H ...
Is time travel possible? An astrophysicist explains
Time travel is the concept of moving between different points in time, just like you move between different places. ... But for now, we'll have to enjoy the idea of time travel in our favorite ...
Is Time Travel Possible?
Time traveling to the near future is easy: you're doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein's special theory of ...
Time Travel Probably Isn't Possible—Why Do We Wish It Were?
Time travel exerts an irresistible pull on our scientific and storytelling imagination. Since H.G. Wells imagined that time was a fourth dimension —and Einstein confirmed it—the idea of time ...
Is time travel even possible? An astrophysicist explains the science
Time travel is the concept of moving between different points in time, just like you move between different places. ... But for now, we'll have to enjoy the idea of time travel in our favorite ...
Is time travel even possible? An astrophysicist explains the science
Time isn't the same everywhere. Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes, or hypothetical tunnels in space that could create shortcuts for journeys across the universe.If someone could build a wormhole and then figure out a way to move one end at close to the speed of light - like the hypothetical spaceship ...
Time travel
Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science ...
Time travel theories date back to the 9th Century BCE
Of all time travel's paradoxes, here's the strangest of them all: hop on a TARDIS back to 1894 and the concept didn't even exist. "Time travel is a new idea," explains New York-based author James ...
The Great Debate: Could We Ever Travel through Time?
Billings: So I love the idea of time travel! And in fact I do it all the time—like most everyone else I'm traveling into the future at one second per second. I'm less of a fan, though, of ...
Exploring the Reality of Time Travel: Science Fact vs ...
Time travel, a longstanding fascination in science fiction, remains a complex and unresolved concept in science. The second law of thermodynamics suggests time can only move forward, while Einstein's theory of relativity shows time's relativity to speed. Theoretical ideas like wormholes offer potential methods, but practical challenges and ...
Can we time travel? A theoretical physicist provides some answers
Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the ...
Time Travel and Modern Physics
Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. ... such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things ...
What Einstein and Bill Gates Teach Us About Time Travel
The theoretical underpinnings of time travel date back to 1905, when Albert Einstein wrote down his special theory of relativity that showed space and time are intimately linked, and to 1916, when ...
Time travel: five ways that we could do it
Time travel using light. Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling ...
Time Travel
Time Travel. Time travel is commonly defined with David Lewis' definition: An object time travels if and only if the difference between its departure and arrival times as measured in the surrounding world does not equal the duration of the journey undergone by the object. ... Different scientific ontologies result in different ideas of what ...
How would time travel affect life as we know it?
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Where Does the Concept of Time Travel Come From?

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Time after time

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Is Time Travel Possible?

How do we know that time travel is possible?

Can we use time travel in everyday life?

In Summary:

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Can we time travel? A theoretical physicist provides some answers

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Time is a river

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What Einstein and Bill Gates Teach Us About Time Travel

Time travel: five ways that we could do it

Cathal O’Connell

1. Time travel via speed

Computer solves a major time travel problem

2. Time travel via gravity

3. Time travel via suspended animation

4. Time travel via wormholes

5. Time travel using light

How would time travel affect life as we know it?

Key Takeaways

Possibilities and Paradoxes of Time Travel

Frequently Asked Questions

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Time travel for travelers? It’s tricky.

Dueling theories

Multiple timelines?

More work to be done