Watch AMC’s Visionaries: James Cameron’s Story of Science Fiction, a six-part series about focusing on AI, dark futures, aliens, and time travel.

In September 2015, cosmonaut Gennady Padalka arrived back on Earth for the last time. He had just completed his sixth mission in space and broke the record for most cumulative time spent beyond Earth’s atmosphere—879 days. And because of these two-and-a-half years spent orbiting the planet at high speeds, Padalka also became a time traveler, experiencing Einstein’s theory of general Relativity in action.

“When Mr. Padalka came back from his adventures, he found the Earth to be 1/44th of a second to the future of where he expected it to be,” explains J. Richard Gott, Princeton physicist and author of the 2001 book Time Travel in Einstein’s Universe, “He literally traveled…into the future.”

While being a fraction of a second younger than if he had stayed on Earth isn’t mind-bending stuff, it nonetheless gave Padalka, the distinction as the “current time traveler record,” according to Gott.

Although not exactly a plutonium-charged DeLorean, time travel is anything but fiction. Real astrophysicists like Gott are pretty sure they know how to build a time machine, and intense speed—much, much faster than Padalka’s orbital jaunt—is the key ingredient.

A Time Travel Crash Course

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Until the 20th century, time was believed to be completely immutable and time travel a scientific impossibility. In the 1680s, Sir Isaac Newton’s thought time progressed at a consistent pace throughout the universe, regardless of outside forces or location. And for two centuries, the scientific world subscribed to Newton’s theory.

Until 26-year-old Albert Einstein came along.

In 1905, Einstein revealed his ideas on special relativity, using this framework for his theory of general relativity a decade later. Einstein’s universe-defining calculations introduced, well, lots of things, but also some concepts related to time. The most important being that time is elastic and dependent on speed, slowing down or speeding up depending on how fast an object—or person— is moving.

In 1971, four cesium beam atomic clocks flew around the world and were then compared to ground-based clocks. The resulting minuscule time difference proved that Einstein was onto something. There’s also another technology, tucked inside your smartphone, that also validates Einstein’s theory.

“WITHOUT EINSTEIN’S GENERAL THEORY OF RELATIVELY, OUR GPS SYSTEM WOULDN’T BE WORKING.”

“Without Einstein’s general theory of relatively, our GPS system wouldn’t be working,” says Ron Mallet, an astrophysicist and author of the book Time Traveler: A Scientist’s Personal Mission to Make Time Travel a Reality. “That’s also proof that Einstein’s [theories are] correct.”

But apart from this mutable version of time, Einstein also calculated the speed of light. At 300,000,000 meters (or 186,282 miles) per second, Einstein describes the figure as the “ultimate speed limit” and a universal constant no matter if one is sitting on a bench or traveling in a rocket ship.

The last bit of Einstein’s time-bending ideas suggest that gravity also slows time, meaning time runs faster where gravity is weaker like the vast emptiness among massive celestial bodies like the Sun, Jupiter, and Earth.

Fast forward a century later, and all of these theories—highly summarized, of course—now form the building blocks of astrophysics, and buried among all this expert-level math, Einstein also proved that time travel was possible.

The Subatomic Time Machine

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In fact, not only is time travel possible, it’s already happened—it just doesn’t look like your typical sci-fi film.

Returning to our time-traveling cosmonaut Padalka, his 1/44-second jump into the future is so minuscule because he was only traveling 17,000 miles per hour. That isn’t very fast, at least in comparison to the speed of light. But what would happen if we created something that could go much faster than geostationary orbit? We are not talking a commercial jetliner (550 to 600 miles an hour) or a 21st century rocket to the ISS (25,000 miles per hour), but something that could approach 186,282 miles per second?

“On a subatomic level, it’s been done,” says Mallett. “An example is…the Large Hadron Collider. It routinely sends subatomic particles into the future.”

The particle accelerator has the ability to propel protons at 99.999999 percent the speed of light, a speed at which their relative time is moving about 6,900 times slower compared to their stationary human observers.

“THE LARGE HADRON COLLIDER…ROUTINELY SENDS SUBATOMIC PARTICLES INTO THE FUTURE.”

So, yes, we’ve been sending atoms into the future and we’ve been doing it for the last decade, but humans are another matter.

Gott says given that we propel particles nearly the speed of light on a regular basis, conceptually, it’s rather simple for humans to time travel into the future. “If you want to visit Earth in the year 3000,” Gott says, “all you have to do is to get on a spaceship and go 99.995 percent the speed of light.”

Let’s say a human is put on such a ship and sent to a planet that’s a little less than 500 light years away (for example, Kepler 186f), meaning if they traveled at 99.995 percent of the speed of light, it would take them about 500 years to get there since they are going at nearly the speed of light.

Just a quick 550-light year jaunt to Kepler-186f.

NASA AMES

After a quick snack and a bathroom break, they would then turn around and head back to Earth, which would take another 500 years. So in total, it would take about a thousand years for them to arrive safely back home. And, on Earth, it would be the year 3018.

However, since they were moving so fast, the resulting time dilation wouldn’t seem like a thousand years for them since their internal clock has slowed. “[Their] clock will be ticking at 1/100th of the rate of the clocks on Earth. [They] are only going to age about 10 years,” says Gott. While a millennium would pass for us, for them it would be a decade.

“If we [on Earth] were watching through the window, they would be eating breakfast veeeerrry slooooowly,” says Gott, “But to [them], everything would be normal.”

But there is a massive gulf between what is theoretical and what is real. So how do we overcome the immense technological challenges of building a time machine?

The Not-So-Distant Future of Human Time Travel

The Parker Solar Probe will reach speeds of 430,000 mph—fast but nowhere near the speed of light.

Building a time-traveling spaceship may be the best place to start, but the engineering obstacles, at least for now, are enormous. For one, we are not even close to having a spaceship that can travel the speed of light. The fastest spacecraft ever created will soon be the Parker Solar Probe, which will launch this summer and travel only .00067 percent the speed of light.

There’s also the enormous amount of energy that would be needed to propel a ship to go that fast. Gott suggests that highly efficient antimatter fuel could be the key and other world agencies and scientists also think such a fuel could be a potentially invaluable piece to interstellar travel.

But ensuring the safety of the human cargo on such a futuristic mission would also be tricky. First of all, the ship would need to carry enough supplies, like food, water, and medicine, and be self-sufficient for the entire journey.

Then there’s the whole acceleration thing. To make sure our hypothetical traveler wouldn’t be obliterated by overwhelming g-forces, the ship would need to gradually and steadily accelerate. While steady 1g acceleration (like what we feel on Earth) for a long period of time would eventually get the ship to approach near speed of light, it would add to the length of the trip and minimize how far in the future one could go.

Using our 500-light-year planet example, Gott predicts that the steady acceleration of 1g up to near light speed would increase the aging of the time traveler to 24 years, “but you would still get to visit Earth in the year 3000,” says Gott.

To create a vehicle with these specifications would require a lot of time, resources, and money. But the same can be said for other massively ambitious experiments, like detecting gravitational waves and building the Large Hader Collider. A time machine could be the world’s next scientific megaproject.

The Trouble of Going in Reverse

But there is one big caveat to this theoretical portrait of real-world time travel—this machine doesn’t go in reverse. While Bill and Ted travel to the past to pick up Socrates with relative ease, in reality scientists and researchers need to find a way to circumvent the rules of physics in order to travel back in time.

Wormholes, black holes, cosmic strings, and circulating light beams have all been suggested as potential solutions for time-traveling to the past. The main challenge that astrophysicists are grappling with is figuring out is how to beat a light beam to a point in spacetime and back.

“THE TECHNOLOGY ISN’T FAR OFF…WE COULD DO IT IN THE NEXT TWENTY YEARS.”

Since the speed of light is the absolute maximum, physicists are concentrating on finding phenomena like wormholes, which could provide tunnel-like shortcuts that jump across curved spacetime and, in theory, beat a light beam to a particular point in spacetime.

While wormholes do work within the confines of Einstein’s theories of relativity, they have yet to be observed in space, and scientists have no concrete evidence that these galactic shortcuts would even work.

An artist rendering of a black hole ripping apart a star.

NASA

So while time traveling to the past may be the more exciting concept, scientists are much more likely to fling someone into the unknown future rather than the well-trodden past. But despite overwhelming odds—fiscal and scientific—Mallet believes the future of a time-traveling society is possible.

“What happened with going to the moon…we wanted to go there, Kennedy asked for it, and there was proper funding so we got there within a decade,” Mallet says. “The technology isn’t far off. If the government and taxpayers wanted to pay for it, we could do it in the next twenty years.”

For now, wannabe time travelers will still have to look to science fiction for its time travel fix, with some movies being much more accurate than others.

“A good movie… was the original Planet of the Apes,” says Mallett. “The astronauts thought they had landed on another planet that was ruled by apes, but what they found out…was that they had traveled so fast, that they had arrived into Earth’s future. That movie accurately depicts Einstein’s special theory of relativity.”

Oh…spoilers.