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Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

A common theme in science-fiction media , time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point far in the future . But how much of the idea is based in reality? Is it possible to travel through time? 

Is time travel possible?

According to NASA, time travel is possible , just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light , which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation , according to Live Science, is how one’s perception of time is different to another's, depending on their motion or where they are. Hence, time being relative. 

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Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories. 

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time .

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY. 

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging. 

“The tricky point of that is if you can find a physical, realistic, way to do it,” she said. 

Alonso-Serrano said wormholes and warp drives are tools that are used to create this curvature. The matter needed to achieve curving space-time via a wormhole is exotic matter , which hasn’t been done successfully. Researchers don’t even know if this type of matter exists, she said.

“It's something that we work on because it's theoretically possible, and because it's a very nice way to test our theory, to look for possible paradoxes,” Alonso-Serrano added.

“I could not say that nothing is possible, but I cannot ignore the possibility,” she said. 

She also mentioned the anecdote of  Stephen Hawking’s Champagne party for time travelers . Hawking had a GPS-specific location for the party. He didn’t send out invites until the party had already happened, so only people who could travel to the past would be able to attend. No one showed up, and Hawking referred to this event as "experimental evidence" that time travel wasn't possible.

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USA TODAY is exploring the questions you and others ask every day. From "How to watch the Marvel movies in order" to "Why is Pluto not a planet?" to "What to do if your dog eats weed?" – we're striving to find answers to the most common questions you ask every day. Head to our Just Curious section to see what else we can answer for you. 

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!

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).

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.

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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Time Travel Isn't Possible ... or Is It?

Special relativity teaches us that the three dimensions of space and the solitary dimension of time are woven together like a fabric. It's impossible to think of them as separate entities, only a singular unified entity — space-time. We can't think of motion through space without being mindful of motion through time, and vice versa. Left-right, up-down, back-forth and past-future are all on equal footing.

And yet, time does seem a little different. We have complete freedom of movement within space, but we cannot avoid our future. Time seems to have an "arrow," whereas the spatial dimensions are ambidextrous. Given the unity between time and space, it leads to the obvious question: Is time travel , of any sort, possible? Under any circumstances? At all? [ How Time Travel Works in Science Fiction (Infographic) ]

Into the future: Sure

Oddly enough, the answer is yes! We cannot avoid moving into our futures, but we can control the rate that we move through time. This is a consequence of another lesson from relativity: Not all clocks are the same.

The speed at which you move through space determines the speed at which you move through time. In the succinct phrase: Moving clocks run slow.

IF you could build a big enough rocket (don't ask me how, that's an engineering problem) to provide a constant acceleration of 1g (9.8 meters per second per second; the same acceleration as provided by the Earth's gravity at its surface), you could reach the center of the Milky Way galaxy — a healthy 20,000 light-years away — in just a couple decades of your personal time.

You could stop for a few hours, have a picnic near Sagittarius A* (the black hole at the center of the galaxy), and then hop back in to your rocket and come back to Earth.

By the time you return you'll be eligible for retirement benefits, if the institution providing those benefits is even around, because while you only traveled for a few decades according to the clock on your ship, about 40,000 years would've passed on the Earth.

Closing the loop

Time is relative, but it still flows in the same direction for everyone. To ask if we can go into reverse is the domain of general relativity (GR) — this is the mathematical language we use to not only understand gravity, but the full connection between space-time and motion.

In GR, we ask a slightly more technical question: Is there any arrangement of matter and energy (the stuff that warps space-time) to permit the existence of closed time-like curves, or CTCs? I know this is jargon but it's a fun phrase to toss around at parties. "Curve" here means a path, "time-like" means you never go faster than the speed of light, and "closed" means it returns to its starting point — in other words, its own past.

So, Oracle of Einstein, are CTCs permitted? Yes! Well….

The possibilities are finite

There are about half a dozen known configurations of space-time that allow CTCs, or time travel into the past. For example, Kurt Gödel (of Gödel's Incompleteness Theorem fame) discovered that if the expansion of the universe was accelerating (which it is) and the universe is also rotating, CTCs would be allowed and we could travel into our past on a whim.

As far as I can tell, Gödel used this solution to point out to Albert Einstein that perhaps GR wasn't all it was cracked up to be — I mean, come on, shouldn't any self-respecting theory of the natural world avoid such an obviously absurd solution?

But Gödel's point was moot — all observations indicate that the universe is not rotating, so that particular solution does not apply to our universe, and time travel into the past is verboten.

Ah! But what if we were to construct an infinitely long massive cylinder and set it spinning on its axis near the speed of light. It would drag on space-time around it, and certain paths around that spinning cylinder would end up in their own past. Good thing there are no infinitely long massive cylinders in the universe, or we might have to worry.

Wait, I've got one: If you make a wormhole (a shortcut between two distant locations in space-time) and send one end racing off near the speed of light and bring it back, the normal time-dilation effects would put one end in the "future" of the other, so you could waltz right through the wormhole throat and end up in your past. What's that? Wormholes require "negative mass" to exist, and negative mass does not exist in the universe? Well, hmm.

Into the past: Nope

It's the same story every time (pardon the too-hard-to-resist pun). For every scenario we concoct in general relativity to allow CTCs and time travel into our own past, nature finds a way to confound our plans and rule out the scenario.

What's going on? General relativity allows — in principle — time travel into the past, but it appears to be ruled out in every case. It seems like something funny is afoot, that there ought to be some fundamental rule to disallow time travel. But there isn't one. We can't point to any particle interaction at the subatomic level that clearly prevents the formation of CTCs.

mach The 7 Biggest Unanswered Questions in Physics

The inevitable progression of time from the past to the future resembles another indomitable law of nature: entropy. That's the iron law of thermodynamics that states that closed systems go from ordered to disordered. (This law explains why an egg will never just happen to unscramble itself if you leave it alone long enough). Is time linked to entropy? Maybe, but that's the subject of another article….

This story was originally published on Space.com.

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Would you really age more slowly on a spaceship at close to light speed?

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Every week, the readers of our space newsletter, The Airlock , send in their questions for space reporter Neel V. Patel to answer. This week: time dilation during space travel. 

I heard that time dilation affects high-speed space travel and I am wondering the magnitude of that affect. If we were to launch a round-trip flight to a nearby exoplanet—let's say 10 or 50 light-years away––how would that affect time for humans on the spaceship versus humans on Earth? When the space travelers came back, will they be much younger or older relative to people who stayed on Earth? —Serge

Time dilation is a concept that pops up in lots of sci-fi, including Orson Scott Card’s Ender’s Game , where one character ages only eight years in space while 50 years pass on Earth. This is precisely the scenario outlined in the famous thought experiment the Twin Paradox : an astronaut with an identical twin at mission control makes a journey into space on a high-speed rocket and returns home to find that the twin has aged faster.

Time dilation goes back to Einstein’s theory of special relativity, which teaches us that motion through space actually creates alterations in the flow of time. The faster you move through the three dimensions that define physical space, the more slowly you’re moving through the fourth dimension, time––at least relative to another object. Time is measured differently for the twin who moved through space and the twin who stayed on Earth. The clock in motion will tick more slowly than the clocks we’re watching on Earth. If you’re able to travel near the speed of light, the effects are much more pronounced. 

Unlike the Twin Paradox, time dilation isn’t a thought experiment or a hypothetical concept––it’s real. The 1971 Hafele-Keating experiments proved as much, when two atomic clocks were flown on planes traveling in opposite directions. The relative motion actually had a measurable impact and created a time difference between the two clocks. This has also been confirmed in other physics experiments (e.g., fast-moving muon particles take longer to decay ). 

So in your question, an astronaut returning from a space journey at “relativistic speeds” (where the effects of relativity start to manifest—generally at least one-tenth the speed of light ) would, upon return, be younger than same-age friends and family who stayed on Earth. Exactly how much younger depends on exactly how fast the spacecraft had been moving and accelerating, so it’s not something we can readily answer. But if you’re trying to reach an exoplanet 10 to 50 light-years away and still make it home before you yourself die of old age, you’d have to be moving at close to light speed. 

There’s another wrinkle here worth mentioning: time dilation as a result of gravitational effects. You might have seen Christopher Nolan’s movie Interstellar , where the close proximity of a black hole causes time on another planet to slow down tremendously (one hour on that planet is seven Earth years).

This form of time dilation is also real, and it’s because in Einstein’s theory of general relativity, gravity can bend spacetime, and therefore time itself. The closer the clock is to the source of gravitation, the slower time passes; the farther away the clock is from gravity, the faster time will pass. (We can save the details of that explanation for a future Airlock.)

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Is Time Travel Even Possible? An Astrophysicist Explains The Science Behind The Science Fiction

If traveling into the past is possible, one way to do it might be sending people through tunnels in space..

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.

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 .

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 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

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.

Adi Foord is an Assistant Professor of Astronomy and Astrophysics at the University of Maryland, Baltimore County. This article is republished from  The Conversation  under a  Creative Commons license . Read the  original article .

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Do we live in a rotating universe? If we did, we could travel back in time

Living in a rotating universe would be strange indeed.

We know that planets rotate, but what about the universe as a whole? No, the universe doesn't appear to rotate; if it did, time travel into the past might be possible.

Although people throughout antiquity had argued that the heavens rotate around the world, in 1949, mathematician Kurt Gödel was the first to provide a modern formulation of a rotating universe. He used the language of Albert Einstein 's theory of general relativity to do so, as a way of honoring his friend and neighbor at Princeton, Einstein himself.

But this process of academic "honoring" went in a different direction than you might suspect, because Gödel used the example of a rotating universe to show that general relativity was incomplete.

Related : Was Einstein wrong? The case against space-time theory

Gödel's model of a rotating universe was rather artificial. Besides the rotation, his universe contained only one ingredient: a negative cosmological constant that resisted the centrifugal force of that rotation to keep the universe static.

But the artificial nature of that universe didn't bother Gödel. Instead, his main point was that general relativity allowed for the possibility of a rotating universe at all. And Gödel used his rotating universe to show that general relativity allowed for time travel into the past, which should be forbidden. 

Taking the universe out for a spin

Living in a rotating universe would be strange indeed. For one, all observers would consider themselves the center of rotation. This means that if you parked yourself somewhere and ensured that you were absolutely still, you would see the universe wheeling around you. But if you picked up and moved anywhere else, even to a distant galaxy , you would always still see the universe rotating around your new position.

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This is incredibly hard to visualize, but it's not much different from the idea that in an expanding universe , all observers see themselves as the center of expansion.

The farther you go from any one observer, the greater the rate of rotation. And this isn't merely a rotation of stuff but a rotation of space-time itself. This means that light, which is always forced to follow the curvature of space-time, makes for some strange journeys. A beam of light sent out from an observer will curve away as it gets swept up in the rotation of space-time. At some distant point, the rotation will be too much, and the light will turn around and return to the observer.

This means there's a limit to how far you can see in a rotating universe, and beyond that, all you'll observe is duplicate images of your own past self.

This strange behavior doesn't apply only to light. If you were to get in a rocket and blast off through a rotating universe, you, too, would get caught up in the rotation. And because of that rotation, your movement would double back on itself. When you returned to your starting point, however, you would find yourself arriving before you had left.

In a manner of speaking, a rotating universe would be capable of rotating your future into your own past, allowing you to travel back in time.

Sitting still

This was Gödel's major objection to general relativity. That theory, being our ultimate understanding of space and time , should not allow for backward time travel, because time travel into the past violates our notions of causality and introduces all sorts of nasty time-travel paradoxes. The fact that relativity did not automatically make time travel impossible signaled to Gödel that Einstein's theory was incomplete.

Thankfully, we see no signs that we live in a rotating universe. If the cosmos were rotating, then light coming from opposite directions of the sky would be redshifted in one direction and have an equivalent amount of blueshifting in the other. Astronomers have applied this test to surveys of distant galaxies and even to the cosmic microwave background , which is the light left over from when the cosmos was only 380,000 years old. The conclusion of these tests is that if the universe is rotating, it's doing so at a rate of less than 10^-17 degrees per century. 

— Is time travel possible?

— Why time-traveling tachyons probably don't exist

— What is the grandfather paradox?  

But Gödel's objection still stands. Since 1949, physicists have concocted other ways for general relativity to allow for backward time travel, wormholes , faster-than-light-speed "warp drive" (known as Alcubierre drive), and special paths around infinitely long cylinders. But all those contrivances rely on some sort of exotic physics that breaks our understanding of how the universe works, like matter with negative mass.

But Gödel's rotating universe is simply a matter of observational test, not a fundamental break with known physics. We could have found ourselves in a rotating universe just as easily as we find ourselves in an expanding one. There's nothing in our knowledge of physics that prevents this kind of universe from existing, so there's nothing in our knowledge of physics that prevents backward time travel.

Perhaps Gödel is right, and we have more to learn about the universe.

Learn more by listening to the "Ask A Spaceman" podcast, available on  iTunes  and askaspaceman.com . Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter .

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Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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Admin said: A rotating universe would be capable of rotating your future into your own past, allowing you to travel back in time. Do we live in a rotating universe? If we did, we could travel back in time : Read more

• mokeshame I think the universe is rotating but some early assumptions have been made in this article. Like the one that we should red and blueshift every lightbeam. Instead red and blueshift are direct evidence that the universe is rotating and because of that appears to be moving away from us. And it goes faster because the universe on a grand scale will ever rotate faster. This is the endgoal of al conservation of energy. Conserve it in rotations as a way of catapulting the universe to all time high rotation speeds. So in the end the speed isnt conservated any more and goes into a strait line. The universe does this to work in on itself. We see actually one particle that can manage itself. Builds his own structure and forms, with no limits. Just like Lego. Spacetime is the exactly the same as the forming of rotations, so it can not be distorted by it i think. A while ago i saw a matmathhecian that exactly formulated it the way i see it, her name was M. Duchin. I think to understand the base and interactions of the universe we must see it as an geniometrical system. Reply
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Is time travel possible? An astrophysicist explains

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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A beginner's guide to time travel

Learn exactly how Einstein's theory of relativity works, and discover how there's nothing in science that says time travel is impossible.

Everyone can travel in time . You do it whether you want to or not, at a steady rate of one second per second. You may think there's no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein 's theory of relativity , we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you'd see it travel a light-year of distance in just over a year of time. 

That's obvious enough, but now comes the weird part. For astronauts onboard that spaceship, the journey would take a mere seven weeks. It's a consequence of relativity called time dilation , and in effect, it means the astronauts have jumped about 10 months into the future. 

Traveling at high speed isn't the only way to produce time dilation. Einstein showed that gravitational fields produce a similar effect — even the relatively weak field here on the surface of Earth . We don't notice it, because we spend all our lives here, but more than 12,400 miles (20,000 kilometers) higher up gravity is measurably weaker— and time passes more quickly, by about 45 microseconds per day. That's more significant than you might think, because it's the altitude at which GPS satellites orbit Earth, and their clocks need to be precisely synchronized with ground-based ones for the system to work properly. 

The satellites have to compensate for time dilation effects due both to their higher altitude and their faster speed. So whenever you use the GPS feature on your smartphone or your car's satnav, there's a tiny element of time travel involved. You and the satellites are traveling into the future at very slightly different rates.

But for more dramatic effects, we need to look at much stronger gravitational fields, such as those around black holes , which can distort space-time so much that it folds back on itself. The result is a so-called wormhole, a concept that's familiar from sci-fi movies, but actually originates in Einstein's theory of relativity. In effect, a wormhole is a shortcut from one point in space-time to another. You enter one black hole, and emerge from another one somewhere else. Unfortunately, it's not as practical a means of transport as Hollywood makes it look. That's because the black hole's gravity would tear you to pieces as you approached it, but it really is possible in theory. And because we're talking about space-time, not just space, the wormhole's exit could be at an earlier time than its entrance; that means you would end up in the past rather than the future.

Trajectories in space-time that loop back into the past are given the technical name "closed timelike curves." If you search through serious academic journals, you'll find plenty of references to them — far more than you'll find to "time travel." But in effect, that's exactly what closed timelike curves are all about — time travel

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There's another way to produce a closed timelike curve that doesn't involve anything quite so exotic as a black hole or wormhole: You just need a simple rotating cylinder made of super-dense material. This so-called Tipler cylinder is the closest that real-world physics can get to an actual, genuine time machine. But it will likely never be built in the real world, so like a wormhole, it's more of an academic curiosity than a viable engineering design.

Yet as far-fetched as these things are in practical terms, there's no fundamental scientific reason — that we currently know of — that says they are impossible. That's a thought-provoking situation, because as the physicist Michio Kaku is fond of saying, "Everything not forbidden is compulsory" (borrowed from T.H. White's novel, "The Once And Future King"). He doesn't mean time travel has to happen everywhere all the time, but Kaku is suggesting that the universe is so vast it ought to happen somewhere at least occasionally. Maybe some super-advanced civilization in another galaxy knows how to build a working time machine, or perhaps closed timelike curves can even occur naturally under certain rare conditions.

This raises problems of a different kind — not in science or engineering, but in basic logic. If time travel is allowed by the laws of physics, then it's possible to envision a whole range of paradoxical scenarios . Some of these appear so illogical that it's difficult to imagine that they could ever occur. But if they can't, what's stopping them? 

Thoughts like these prompted Stephen Hawking , who was always skeptical about the idea of time travel into the past, to come up with his "chronology protection conjecture" — the notion that some as-yet-unknown law of physics prevents closed timelike curves from happening. But that conjecture is only an educated guess, and until it is supported by hard evidence, we can come to only one conclusion: Time travel is possible.

A party for time travelers 

Hawking was skeptical about the feasibility of time travel into the past, not because he had disproved it, but because he was bothered by the logical paradoxes it created. In his chronology protection conjecture, he surmised that physicists would eventually discover a flaw in the theory of closed timelike curves that made them impossible. 

In 2009, he came up with an amusing way to test this conjecture. Hawking held a champagne party (shown in his Discovery Channel program), but he only advertised it after it had happened. His reasoning was that, if time machines eventually become practical, someone in the future might read about the party and travel back to attend it. But no one did — Hawking sat through the whole evening on his own. This doesn't prove time travel is impossible, but it does suggest that it never becomes a commonplace occurrence here on Earth.

The arrow of time 

One of the distinctive things about time is that it has a direction — from past to future. A cup of hot coffee left at room temperature always cools down; it never heats up. Your cellphone loses battery charge when you use it; it never gains charge. These are examples of entropy , essentially a measure of the amount of "useless" as opposed to "useful" energy. The entropy of a closed system always increases, and it's the key factor determining the arrow of time.

It turns out that entropy is the only thing that makes a distinction between past and future. In other branches of physics, like relativity or quantum theory, time doesn't have a preferred direction. No one knows where time's arrow comes from. It may be that it only applies to large, complex systems, in which case subatomic particles may not experience the arrow of time.

Time travel paradox 

If it's possible to travel back into the past — even theoretically — it raises a number of brain-twisting paradoxes — such as the grandfather paradox — that even scientists and philosophers find extremely perplexing.

Killing Hitler

A time traveler might decide to go back and kill him in his infancy. If they succeeded, future history books wouldn't even mention Hitler — so what motivation would the time traveler have for going back in time and killing him?

Killing your grandfather

Instead of killing a young Hitler, you might, by accident, kill one of your own ancestors when they were very young. But then you would never be born, so you couldn't travel back in time to kill them, so you would be born after all, and so on … 

A closed loop

Suppose the plans for a time machine suddenly appear from thin air on your desk. You spend a few days building it, then use it to send the plans back to your earlier self. But where did those plans originate? Nowhere — they are just looping round and round in time.

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Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortean Times, How It Works, All About Space, BBC Science Focus, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.

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Gödel proved that, mathematically speaking, time travel is physically possible

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Time travel

• Thread starter ericc
• Start date Feb 27, 2024
• Space Travel & SpaceX
• Feb 27, 2024

I have a problem with Steven Hawking’s suggestion that if time travel were possible there would be loads of tourist from the future here already. There could not be any from the past as a time travel has not been invented yet. As far as the future is concerned there has not been any yet to come back from. How can you touch the future unless you are there. At that stage it becomes the present. We can only be there together. Or can someone explain why my thinking is wrong  

Classical Motion

• Feb 28, 2024

It's like star travel. It's impossible. Time can not be changed.....with velocity or anything else. Time and length never change. ALL star light has an absolute velocity. Because time and length do not change. If the light of 13 BLY ago has the same velocity, then time and length were the same at that time too. If one could find light with a different velocity, then one might have some evidence of these science fiction theories. And since no one knows the dynamic of mass/light interaction, there is a good chance that we have measurement error. Especially with high rates of change motion. Such as light or spinning particles. The goal in the study of motion is to remove all probability, all randomness, all chaos....... from it. These are man made veils, and after you remove them, the only thing that remains is understanding.  

ericc said: I have a problem with Steven Hawking’s suggestion that if time travel were possible there would be loads of tourist from the future here already. There could not be any from the past as a time travel has not been invented yet. As far as the future is concerned there has not been any yet to come back from. How can you touch the future unless you are there. At that stage it becomes the present. We can only be there together. Or can someone explain why my thinking is wrong Click to expand...

We can look into the past but not go there. We can go into the future but not look at it ahead of time. Time travel into the past is impossible because: - It would require two universes, one for the home base and one where the traveler went to. It would violate conservation of mass. - Heat flow would need to be reversed. Heat won't flow from cold to hot. Entropy law violation. - You could change history to where you were never born. It would violate causality.  

billslugg said: We can look into the past but not go there. We can go into the future but not look at it ahead of time. Click to expand...

billslugg said: Time travel into the past is impossible because: - It would require two universes, one for the home base and one where the traveler went to. It would violate conservation of mass. Click to expand...

billslugg said: - Heat flow would need to be reversed. Heat won't flow from cold to hot. Entropy law violation. - You could change history to where you were never born. It would violate causality. Click to expand...

• Apr 7, 2024

i've always been fascinated by the thought of travelling through time and as theoretical science says that time travel is possible but our limited knowledge of physics at this time dictates faster than light speed is not possible and if it was then the greatest minds say that we can travel into the future but can't travel back in time and my problem with this is if travelling into the future could happen then it doesn't take a genius to come to the conclusion that our future has already been set and has already been fated to happen, has happened will happen and if that is wrong and our futures have yet to be written then it wouldn't be possible to travel into our future "correct" this has bugged me for many many years now and i have came to only one conclusion that fate exists and free thought doesn't mean a thing because no matter what choice or decision made by us in life won't be able to change the future which is fated to happen has happened will happen but i realized that the only thing we can change is our destiny  

• Apr 8, 2024

Do we live in the future, or is there a future of this same place in a parallel universe  

Anumi said: Do we live in the future, or is there a future of this same place in a parallel universe Click to expand...

Anyone can assert the existence of an infinite number of universes since our universe is defined as all we can see. We can look into the past all we want, but we cannot go into the past and change events such that today's status is changed. Such a tool does not exist since, if it did, it would be all over late night TV commercials, which it isn't. We can go into the future at whatever rate we choose, it's just a matter of acceleration ability. We cannot look into the future. See above reasoning.  

billslugg said: Anyone can assert the existence of an infinite number of universes since our universe is defined as all we can see. We can look into the past all we want, but we cannot go into the past and change events such that today's status is changed. Such a tool does not exist since, if it did, it would be all over late night TV commercials, which it isn't. We can go into the future at whatever rate we choose, it's just a matter of acceleration ability. We cannot look into the future. See above reasoning. Click to expand...

We live in the present. Because our senses can not sense the past or the future, only the present. All life forms, except man, only concept the present. Only man can imagine, guess and reason the past and the future. Or attempts too. We rationalize time. To make sense of life. Not knowing that looking up, senses the past. And we can't make sense of that either. In the future when we realize light, we will see that time and length are omnipresent. And all motion can ONLY occur.......in the manner that is does. One universe. There are no other possibilities. Chaos, randomness and probability are strictly forbidden. And in all that area and all the time of it, only in one little spec(here)........there is choice. We can't change action, but we can choose which action. And we think we are gods.  

Classical Motion said: We live in the present. Because our senses can not sense the past or the future, only the present. All life forms, except man, only concept the present. Only man can imagine, guess and reason the past and the future. Or attempts too. We rationalize time. To make sense of life. Not knowing that looking up, senses the past. And we can't make sense of that either. In the future when we realize light, we will see that time and length are omnipresent. And all motion can ONLY occur.......in the manner that is does. One universe. There are no other possibilities. Chaos, randomness and probability are strictly forbidden. And in all that area and all the time of it, only in one little spec(here)........there is choice. We can't change action, but we can choose which action. And we think Click to expand...

What fine answers, or try's for answers. And I'm just realizing why a time traveler can travel into the past light cone speeding up in the future of past histories and change history as soon as they arrive anywhere by their sheer presence anywhere. To use Einstein's phrase concerning "spooky action at a distance" concerning any distance whatsoever, the universe splits! There are then two SPACETIME universes (observable past (t=+1) and unobservable future (t=-1) (the REALTIME (t=0))), two observers then (observable past (t=+1) and unobservable future (t=-1) (the REALTIME (t=0))), and two travelers then (observable past (t=+1) and unobservable future (t=-1) (the REALTIME (t=0))). The observable traveler (t=+1) is observed to speed from the past (t=+1) to rendezvous with the observer (t-0). The unobservable traveler (t=-1) (t=0), though, is speeding from the future (t=-1) unobserved to the rendezvous with the observed observer (t=+1) and unobservable observer (t=0). Upon arrival all those travelers arriving )t=0) from the future (t=-1) change histories. Spooky actions (mergers -->|<-- from and of pasts and futures, from and of past light cone and future light cone, effected in REALTIME)) NOT at a distance! I already understood and had described much of the above before, except neglecting to realize that REALTIME (t=0) travelers in coming out of the future histories (t=-1) future light cone will always CHANGE histories!  

• Apr 12, 2024

Classical Motion said: We live in the present. Because our senses can not sense the past or the future, only the present. All life forms, except man, only concept the present. Only man can imagine, guess and reason the past and the future. Or attempts too. We rationalize time. To make sense of life. Not knowing that looking up, senses the past. And we can't make sense of that either. In the future when we realize light, we will see that time and length are omnipresent. And all motion can ONLY occur.......in the manner that is does. One universe. There are no other possibilities. Chaos, randomness and probability are strictly forbidden. And in all that area and all the time of it, only in one little spec(here)........there is choice. We can't change action, but we can choose which action. And we think we are gods. Click to expand...

Atlan0001 said: What fine answers, or try's for answers. And I'm just realizing why a time traveler can travel into the past light cone speeding up in the future of past histories and change history as soon as they arrive anywhere by their sheer presence anywhere. To use Einstein's phrase concerning "spooky action at a distance" concerning any distance whatsoever, the universe splits! There are then two SPACETIME universes (observable past (t=+1) and unobservable future (t=-1) (the REALTIME (t=0))), two observers then (observable past (t=+1) and unobservable future (t=-1) (the REALTIME (t=0))), and two travelers then (observable past (t=+1) and unobservable future (t=-1) (the REALTIME (t=0))). The observable traveler (t=+1) is observed to speed from the past (t=+1) to rendezvous with the observer (t-0). The unobservable traveler (t=-1) (t=0), though, is speeding from the future (t=-1) unobserved to the rendezvous with the observed observer (t=+1) and unobservable observer (t=0). Upon arrival all those travelers arriving )t=0) from the future (t=-1) change histories. Spooky actions (mergers -->|<-- from and of pasts and futures, from and of past light cone and future light cone, effected in REALTIME)) NOT at a distance! I already understood and had described much of the above before, except neglecting to realize that REALTIME (t=0) travelers in coming out of the future histories (t=-1) future light cone will always CHANGE histories! Click to expand...

• Apr 14, 2024

another thing that bothers me about travelling into the future is that if u do travel into our future then u can't travel back intoour past so u couldn't come back  

The expanding universe does not expand into something since it has no edge. There is nothing "outside" the universe to expand into. The universe just gets larger by itself.  

I see things differently. Entropy is an ordered process. It has no chaos or randomness. You will find no disorder in this cosmos. All is moving in the only way it can move. Life has real time choice. And results are not known until choice is made. This is your mystical concepts. But take choice away, and all is predictable. Like the rest of the cosmos. Life can not be included in the discussion of the cosmos and the motion of it.. Life is super cosmos. Super motion. Choice motion. It's not used in this cosmos. Only locally here. Don't use a rouge tumor(earth) to describe the whole body organism(cosmos).  

• Apr 15, 2024

billslugg said: The expanding universe does not expand into something since it has no edge. There is nothing "outside" the universe to expand into. The universe just gets larger by itself. Click to expand...

• Jul 23, 2024

E.B.E1 said: another thing that bothers me about travelling into the future is that if u do travel into our future then u can't travel back in toour past so u couldn't come back Click to expand...

Gibsense said: This is an assertion dependent on a flat universe of infinite extent. If we substitute 'boundary' for 'edge' then there is a justification for asking what is 'outside the boundary'. Clearly, I suggest that if the universe is spherical a boundary exists. Click to expand...

Gibsense said: It may still be the case that the sphere is all that there is however nowadays discussion of the existence (or not) of branes is quite acceptable if unproven; the fashion changes. Click to expand...

• Jul 25, 2024

Helio said: The only "boundary" or "edge" is in our limited observability beyond this region. The use of "flat" is not in the sense of something like a pancake. It means that two parallel rays will stay parallel regardless of time and distance, ignoring the bumps along the way and assuming perfect flatness. An "open" or "closed" universe alters these rays from remaining parallel. Click to expand...

Gibsense said: Flat means parallel as per the surface of a cylinder for example but not excluding flat as in a pancake. A closed universe has a boundary (except for any ongoing expansion). I keep making that mistake of not saying 'and Euclidian' - or just Euclidian. Click to expand...

Gibsense said: I expect the only reality of anything outside the universe is spatial dimensions (any number, minimum 4) without space as we know it. And, they (dimension concepts) would only exist because 'something' exists. I would not expect an equivalence to our space which contains virtual particles. It is just conceptual allowing for 'something else' other than our universe to be real. To say 'Nothing' is too much Click to expand...

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

Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County

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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

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.

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.

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.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

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