has time travel already happened

Time travel is theoretically possible, calculations show. But that doesn't mean you could change the past.

• Time travel is possible based on the laws of physics, according to researchers.
• But time-travelers wouldn't be able to alter the past in a measurable way, they say. 
• And the future would essentially stay the same, according to the reseachers. 

Imagine you could hop into a time machine, press a button, and journey back to 2019, before the novel coronavirus made the leap from animals to humans.  

What if you could find and isolate patient zero? Theoretically, the COVID-19 pandemic wouldn't happen, right? 

Not quite, because then future-you wouldn't have decided to time travel in the first place.

For decades, physicists have been studying and debating versions of this paradox: If we could travel back in time and change the past, what would happen to the future?

A 2020 study offered a potential answer: Nothing.

"Events readjust around anything that could cause a paradox, so the paradox does not happen," Germain Tobar, the study's author previously told IFLScience .

Tobar's work, published in the peer-reviewed journal Classical and Quantum Gravity in September 2020, suggests that according to the rules of theoretical physics, anything you tried to change in the past would be corrected by subsequent events.

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Put simply: It's theoretically possible to go back in time, but you couldn't change history.

The grandfather paradox

Physicists have considered time travel to be theoretically possible since Albert Einstein came up with his theory of relativity. Einstein's calculations suggest it's possible for an object in our universe to travel through space and time in a circular direction, eventually ending up at a point on its journey where it's been before – a path called a closed time-like curve.

Still, physicists continue to struggle with scenarios like the coronavirus example above, in which time-travelers alter events that already happened. The most famous example is known as the grandfather paradox: Say a time-traveler goes back to the past and kills a younger version of his or her grandfather. The grandfather then wouldn't have any children, erasing the time-traveler's parents and, of course, the time-traveler, too. But then who would kill Grandpa?

A take on this paradox appears in the movie "Back to the Future," when Marty McFly almost stops his parents from meeting in the past – potentially causing himself to disappear. 

To address the paradox, Tobar and his supervisor, Dr. Fabio Costa, used the "billiard-ball model," which imagines cause and effect as a series of colliding billiard balls, and a circular pool table as a closed time-like curve.

Imagine a bunch of billiard balls laid out across that circular table. If you push one ball from position X, it bangs around the table, hitting others in a particular pattern. 

The researchers calculated that even if you mess with the ball's pattern at some point in its journey, future interactions with other balls can correct its path, leading it to come back to the same position and speed that it would have had you not interfered.

"Regardless of the choice, the ball will fall into the same place," Dr Yasunori Nomura, a theoretical physicist at UC Berkeley, previously told Insider.

Tobar's model, in other words, says you could travel back in time, but you couldn't change how events unfolded significantly enough to alter the future, Nomura said. Applied to the grandfather paradox, then, this would mean that something would always get in the way of your attempt to kill your grandfather. Or at least by the time he did die, your grandmother would already be pregnant with your mother. 

Back to the coronavirus example. Let's say you were to travel back to 2019 and intervene in patient zero's life. According to Tobar's line of thinking, the pandemic would still happen somehow.

"You might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would," Tobar said, according to Australia's University of Queensland , where Tobar graduated from. 

Nomura said that although the model is too simple to represent the full range of cause and effect in our universe, it's a good starting point for future physicists.  

Watch: There are 2 types of time travel and physicists agree that one of them is possible

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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!

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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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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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 .

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.

The post Is time travel possible? An astrophysicist explains appeared first on Astronomy Magazine .

A Brief History of Scientists Hunting for Time Travelers

Time travel is possible—or at least a lot of serious physicists say so . It's probably not possible to pull it off in a souped-up Delorean, but there are wormholes, Tipler cylinders, and other Einstein-inspired theories for how it could work. Which raises the question: Why haven't we met any visitors from another time?

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It sounds like a silly question, but it's one that many scientists actually take very seriously. Meeting someone from the future would, of course, serve as definitive proof that we can indeed travel through time, and that would be a quite a simple way to solve a huge scientific riddle. So it's no surprise that a handful of enthusiasts and experts have staged experiments in order to attract the time travelers that could be hiding among us.

One of them is Stephen Hawking. The renowned physicist totally believes time travel is a scientific possibility , and even says he knows how to build a time machine . He also famously wondered, "If time travel is possible, where are the tourists from the future?" It's a good question. Here's how we've tried to answer it.

Why we think time travel is possible

Time travel's been one of man's wildest fantasies for centuries. But in the last century scientists came up with theories that suggested it was indeed plausible to take a leap into the future. Going back in time, unfortunately, is much more complicated .

According to Einstein's theory of general relativity, a wormhole could act like a bridge through space-time by connecting two distant points with a shortcut. Certain types of wormholes, it's theorized, could allow for time travel in either direction, but there are several major caveats of traveling back in time. Mainly, the simple fact that we'd need a method for creating wormholes, and once created, the wormhole would only allow us to travel as far back as the point in time when it was created.

Another option for time travel involves a phenomenon called time dilation , also based on Einstein's theories of relativity. It refers to the idea that time passes more slowly for a moving clock than it does for a stationary clock. The force of gravity also effects the difference in elapsed time. Thanks to the space program, we've actually been dealing with this effect for many years. This is why the clocks on the International Space Station tick just a little bit more slowly than clocks on Earth do.

The implications of this are huge. What if you took this to the extreme? If you jumped in a spaceship that flew super fast, time would pass more quickly for people on Earth. You could do a lap around the galaxy and return to Earth in the future. The question is, how far can we take it? And is it possible to go backwards through time, too?

Once again, we don't really know. And we won't really know exactly how that works until we try it, and at the moment, we don't really have the means to do that. One easy way to find out is simply to search for time travelers walking amongst us. No laboratory required! And that's exactly what several zany scientists have done.

The party approach

One of the earliest well-publicized attempts at finding time travelers was hardly scientific. (I'll get back to the real scientists in a second, I promise.) It happened in the early 80s, when computers and consumer electronics started merging science and science fiction in fascinating ways. It was also when the first space shuttle, Columbia , blasted off. These sorts of things got people thinking.

A group of artist types in Baltimore acted on these curiosities in 1982 with an event The New York Times described as "an epidemic of temporary lunacy." In March of that year, all nine planets were as close together as they'd been in almost 200 years, and a group that called themselves Krononauts gathered together to welcome "visitors from the futures." The Times reports, "The Krononauts drank, danced, and after midnight some of them took off their clothes." No time travelers actually showed up, but it sounds like everybody had fun.

Fast forward 30 years and a similar—although simultaneously entirely different—party took place in Cambridge, England. This time the ringleader was not a wild pack of twentysomethings but Stephen Hawking himself.

Hawking's party was wonderfully deliberate. There was champagne and snacks in a fancy room at Cambridge University, where a banner had been hung: "Welcome Time Travellers." Hawking had always suggested that time traveling tourists could be proof that time travel was possible, so he invited only them. Nobody showed up.

It's hard to tell how tongue-in-cheek Hawking's futurefest was supposed to be. On one hand, Hawking believes time travel is feasible, so he wasn't necessarily laughing at the idea. He might've even expected someone to come. But, in Hawking's words, "There's a twist." The sneaky scientist didn't tell anybody about the party until after it happened. Somebody in the future would have to find out about the event after the fact and hop in a time machine in order to hang out with the famous physicist.

Still, there's a good chance that Hawking was just trying to prove a point. As his friend and contemporary Kip Thorne explains in his book Black Holes and Time Warps , one of the most feasible methods for building a time machine would involve creating and manipulating wormholes . But this would only allow people in the future to travel back as far as the invention of the time machine itself. So if this is indeed how we might build a time machine and we haven't built it yet, it would be impossible for people from the future to travel back to Hawking's party.

The convention approach

A few years before Hawking's party, an ambitious MIT grad student tried a similar but nerdier approach. Instead of keeping the event a secret, Amal Dorai organized a whole convention about time travel and encouraged everybody to spread the world. Dorai wrote on his student website :

We need volunteers to publish the details of the convention in enduring forms, so that the time travelers of future millennia will be aware of the convention. This convention can never be forgotten! We need publicity in MAJOR outlets, not just Internet news. Think New York Times, Washington Post, books, that sort of thing. If you have any strings, please pull them.

Dreams do come true. The New York Times did publish a report . NPR's All Things Considered did a segment on the convention. Wired wrote a story . Heck, even Tina Fey made fun of it on Saturday Night Live .

The exposure makes pretty good sense. MIT is a famous university that tends to attract media attention anyway. The program was also filled with famous professors talking about time travel. And the premise itself, well, it makes for a pretty fun headline!

It still didn't work. "The convention was a mixed success," Dodai said in an update to the event website. "Unfortunately, we had no confirmed time travelers visit us, yet many time travelers could have attended incognito to avoid endless questions about the future." A similar sort of event happened around the same time in Perth, Australia, to the same disappointing end.

Now, one could argue that Dodai simply underestimated the power of the internet. It would've been hard to tell in 2005, when the convention went down, but the internet won the war against print. Dodai discouraged everyone from publicizing on the internet, because, in his words, "The World Wide Web is unlikely to remain in its present form permanently." It's still ticking in 2014.

There's probably a simpler explanation. It's the one the MIT physicists came up with, and the same one Hawking suggested: If we eventually discover a particular way to build a time machine that works, people would only be able to travel as far back as when the time machine was built.

The academic approach

You can probably already tell how this one's going to end. Earlier this year, a pair of physicists published the results of a pretty self-explanatory study, " Searching the Internet for evidence of time travelers ." Instead of staging some sort of event and counting on publicity to attract the people of the future, these scientists went on the hunt for evidence of where time travelers had been online. In a sense, they were searching for their digital footprints.

Robert Nemiroff and Teresa Wilson from Michigan Technological University cast their net wide . They searched Twitter. They searched Facebook. They searched Google, Google+, and even Bing. But they came up short. Between January 2006 and September 2013, they couldn't find a single mention of two terms from the future that people would not have known during that time. None.

It was a good effort. However, given the constraints of the experiment, they ended up searching for a very specific sort of time traveler. Why would that individual be doing putting words from the future on the internet? "A time traveller might have been trying to collect historical information that did not survive into the future, or might have searched for a prescient term because they erroneously thought that a given event had already occurred, or searched to see whether a given event was yet to occur," the paper explains.

That's a little bit silly, then, isn't it. It's no less silly than throwing a party without inviting anyone or hosting a convention celebrating technology that did not yet exist. No wonder these kinds of events ended up being fodder for sketch comedy shows.

The fact of the matter

All that said, there does seem to be a pretty concrete takeaway that's rooted in some widely accepted ideas from theoretical physics. We have not yet invented a time machine. If and when we do, we'll enter a new era of time travel. We will not, however be able to go any further back in time than when that machine was built.

That's all assuming that this hypothetical time machine can actually go back in time. Again, that's really difficult.

Time travel is real, though. In a way, the astronauts on board the International Space Station time travel every day, albeit by a few microseconds . The principles of relativity and the very nature of space-time make it possible. So if you really want to meet a time traveler, look no further than your friendly neighborhood astronaut.

Top image by Sam Woolley / Photos via Flickr / NASA

Curious Kids: is time travel possible for humans?

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Is time travel possible for humans? Jasmine, age 8, Canberra, ACT.

Hi Jasmine.

I wish! In books and movies, our favourite characters can use “time-turners” and treehouses to travel through time. Unfortunately, it isn’t that easy for people in real life. Let’s look at why.

First, there are two types of “time travel”: going back in time, and going forward in time.

Travelling to the past

As far as we know, travelling back in time is impossible. Even sending information back in time is difficult to imagine, because it can change things that have already happened, which should be impossible.

Say you broke your arm falling off the monkey bars. What if you could travel back in time and tell yourself to not go on the bars? If you were successful, you’d never fall and break your arm. But then you would have no reason to travel back in time. So what does this mean for your arm? Did it break, or not?

If thinking about this makes your head hurt, you’re not alone.

Time travelling is a confusing idea for most people. That’s because when we think of time, we think about it as going in a straight line, with one thing happening after another.

If we could travel back in time and change something that happened before, we would then change the order of that line. This would mean breaking a rule called “ causality ”.

Causality is the rule saying that a “cause” (your actions, for instance) happens before an “effect” (the result of your actions). In our monkey bar example, the cause is falling, and the effect is breaking your arm – which happens because you fell.

Causality is one of the unbreakable rules of the universe. Breaking it would have nasty consequences for the universe and all of us. Experts think that because the universe has this rule, travelling to the past must be impossible otherwise the rule would be broken all the time.

Travelling to the future

If going to the past is impossible, can we go forward in time to the future?

Well, technically we’re already travelling forward in time, because time is passing. Every second we travel one second into the future. But this happens to everyone, so it’s not really time travel, right?

Well, believe it or not, two people can feel time at different rates. Time passes differently for someone who is moving fast, compared to someone who is staying still. This is a very complicated idea called “time dilation”.

Someone flying from Sydney to Melbourne will feel time pass more quickly than someone waiting for them at the airport without moving, while the flight was in the air. So why don’t we notice this difference?

It’s because you have to be moving much, much faster than an aeroplane before you start to notice time dilation. Even if you flew all the way around the world, the time would only feel about a billionth of a second different to someone who stayed home.

Read more: Curious Kids: Does space go on forever?

The only way scientists even know about time dilation is because of amazingly accurate experiments that have measured it.

Unfortunately, this still can’t help us “time travel”. If you flew around the world for more than four million years, people on the ground would only have experienced one more second than you!

How fast can we go?

So if it’s all down to speed, the answer must be to go faster, right? If you could go fast enough for long enough, hundreds of “human” years could slip by on your journey, meaning you would feel like you were travelling into the future!

Unfortunately, a fast enough speed to do this would be close to the speed of light, which is the fastest speed anything can go. Light travels at about one billion kilometres every hour – that’s very, very fast.

The fastest human-made thing is NASA’s Parker Solar Probe, a spaceship sent to the Sun in August, 2018. But as fast as it is , it’s only 0.064% as fast as the speed of light. So light is more than 1,000 times faster!

All of this means that if humans want to visit the future, we’ve got a long, long way to go.

Looking back to the past

Ok, so we can’t time travel. But we can see into the past, every night.

Light has a fixed speed, as we just learned. It’s really, really fast, but things in the universe are so far apart that it still takes a long time for light to reach us from faraway stars and planets.

When light arrives from the Sun, the light we see actually left the Sun eight minutes and twenty seconds ago. This means we see the Sun as it was eight minutes and twenty seconds in the past . By the way, remember never to look straight at the Sun as it can damage your eyes.

The nearest galaxy to our Milky Way is the the Canis Major dwarf galaxy, which is 25,000 light years away. This means it takes the light 25,000 years to get here!

When we look at this galaxy through a telescope, we’re actually seeing it as it was more than 25,000 years ago. So although we can’t time-travel ourselves, we can look up to the sky and see the past every night.

Read more: Time travel is possible – but only if you have an object with infinite mass

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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.

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.

What did Albert Einstein invent?: Discoveries that changed the world

Just Curious for more? We've got you covered

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.

This article originally appeared on USA TODAY: Will time travel ever be possible? Science behind curving space-time.

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