Today, Oct. 21, 2015, is the date Marty McFly traveled forward in time to in Back to the Future Part II. Which is as good a reason as any to check in on mankind’s ages-long dream of time travel.

Are the Doc Browns of the world making any progress?

In a documentary released in June by the BBC called Dara O’Briain Meets Stephen Hawking, the astrophysicist put the kibosh on going back in time and helping your mom and dad get together while you invent rock ‘n roll. “If you jump into a black hole, you will meet an unpleasant fate,” Hawking told comedian O'Briain.

Not everyone agrees with him, though, and some scientists will go so far as to say it’s already a scientific possibility. But to understand where we still need to go in order to reach backwards time travel, we need to understand what we’ve already achieved.

Into the Future

Get ready for this: Mankind has already traveled into the future. In fact, there’s even a world record for the most time into the future a man has traveled, held by cosmonaut Sergei Avdeyev who traveled twenty milliseconds ahead of the rest of the earthbound while aboard the Mir space station.

His feat resulted from time dilation, a phenomenon where time experienced by a moving object goes slower than time experienced by a stationary object. Einstein himself put an equation to this effect, known as his theory of special relativity —basically, time is affected by speed. Colin Stuart, author of The Big Questions in Science, uses a train as an example. Suppose a man is bouncing a ball on a train as a woman on the platform watches him go by. For the man, the ball is moving at the normal speed for a bouncing ball. For the woman, the ball is moving at the normal speed plus the speed of the train. Therefore, both people are seeing the ball move at a different speed, relative to their position.

Scientists prove time is experienced differently in this situation by using clocks. A clock that has traveled in a jet around the world will have an earlier time at the end of the trip than a clock that stayed in the same location. Avdeyev is human proof of Einstein’s theory.

“The faster you move, the more time slows down for you,” said Dr. Ronald Mallett, a theoretical physicist at the University of Connecticut and author of Time Traveler, a memoir of his life studying time travel in an effort to go back and see his father again. “Your heart rate, your metabolism, they all slow down. You wouldn’t feel it though. If you’re traveling very fast, you don’t notice that you’re not aging at the same rate. But everyone else watching you would see it.”

Einstein’s theory of special relativity says time travel to the future is possible —and his theory of general relativity flies in Hawking’s face, saying travel to the past is also possible.

Back to the Past

Black holes and Einstein have something in common: gravity.

Einstein’s theory of general relativity says that gravity affects time. Again, using clocks, scientists can put a stationary one on the surface of the earth and shoot a second out into space where gravity is weaker. The clock on Earth runs slower because the stronger gravity is, the more time slows. The practical application proving the theory is GPS.

A signal is sent from a satellite in space to a GPS-enabled device on the ground. If you know what time the satellite sent the signal and what time the unit on-ground detected it, you can use the speed of light to calculate distance. But when the system was first set up, the GPS was giving incorrect locations. Scientists and engineers neglected to account for Einstein’s theory of general relativity. The clock in the unit on the ground was running slower than the clock in the satellite, leading to false GPS information.

A black hole is a collapsing star with a gravitational field that gets increasingly stronger as the star continues to collapse. All the light surrounding the star is sucked in by the gravitational pull. Combining that with general relativity, Einstein’s physics show that if you get close to a black hole, time will come to almost a full stop—and tends to twist into itself.

“My breakthrough was that if light can create gravity and gravity can affect time, then light can affect time,” Mallett said. “I was able to use Einstein’s theory and come up with a new approach that said that if you have a circulating beam of laser light, the loop of laser light could actually cause a twisting of space and time into a loop. What happens normally is that all of us move along a timeline of the past to the present to the future.

It turns out that in Einstein’s theory, if you have a gravitational field of a rotating black hole, there are ways of using gravity to twist that timeline into a loop. Imagine you’ve taken a piece of paper and written a line on it, and you take that paper and make a loop out of it. You’ve made time into a loop so you can go from the future back into the past.”

In other words, time becomes … a flat circle.

Photo via Dr. Ronald Mallett

Photo via Dr. Ronald Mallett

Dr. Seth Lloyd, a quantum mechanical engineering professor at MIT, found a similar aspect of black holes that enabled time travel to the past. He used quantum teleportation experiments to simulate escaping from a black hole.

“I know this sounds completely and utterly crazy, this whole thing,” he said. “But the fact of the matter is that Einstein’s theory of general relativity allows time travel to take place. There are these weird things called closed time-like curves. The way you’d experience it is you would see some funny region of space and you’d point your spaceship towards it. You’d go through this weird region and when you exited from it, you’d see another spaceship out there and it would be you about to enter the closed time-like curve in the future. So you’d actually end up in the past. It would be time travel. I don’t think Einstein liked this part of his theory, but it’s true.”

Of course, any theory is still just that — a theory. Scientists haven’t reached the Avdeyev level of proof yet because setting up real-time scenarios is pretty impractical.

“We don’t know our theory is correct. We set up a quantum teleportation experiment. We didn’t have a real closed time-like curve or a pair of rapidly rotating black holes, which we would have needed to do the real experiment. For some reason when we asked the workplace safety people at MIT, they objected to us setting up a pair of rapidly rotating black holes in the laboratory. The fools! Standing in the way of science,” he said, jokingly. “What were they thinking? But I think it could be done. Once we work out the kinks, then we might indeed be able to teleport people, cats, recalcitrant students, people like that back into time.”

Paradoxes and Problems

But here’s where it gets tricky. Going back in time introduces all sorts of paradoxes, everything from the well-known Grandfather Paradox (if you go back in time and kill your grandparents, it would negate your existence in the future, so how could you go back to begin with) to the Butterfly Effect (changing the tiniest thing in the past could have enormous consequences, or as Simpsons fans would say, “I wish, I wish I hadn’t killed that fish.”)

Mallett and Lloyd both have potential solutions for these issues. Mallett’s is based in quantum theory and says that for every decision we make an alternate universe is created in which we made the opposite choice.

“Imagine that today for lunch, you’re trying to make up your mind between having a fish sandwich and a cheeseburger,” he said. “As soon as you make a decision to have a fish sandwich, there’s a split of the universe and there’s a separate universe now in which you’ve also chosen the cheeseburger. Neither of you know about the other. What this predicts is that if you got into a time machine and went back to the past, the moment you arrive in the past, there would be a split in the universe. There would be a universe that you arrive in that would be different from the universe you came from.”

In this new universe, you could stop your grandparents from meeting each other with no consequence to you, because you wouldn’t be from that universe anyway. But, Mallett concedes, although it’s currently considered a viable theory, the universe could be much stranger than anyone expects, and one change in an alternate universe could completely obliterate the real universe.

Lloyd’s theory takes advantage of lab experiments he and his colleagues have completed but could also lead to complete destruction of, well, the world.

“[Using quantum teleportation,] we sent a photon backwards in time a few billionths of a second, and we did the thing that, of course, you would have it do,” he said. “We had it try to kill its former self and prevent itself from going back in time. There’s no Society for the Prevention of Cruelty to Photons yet. It was a photonic version of the Grandfather Paradox, where the time traveler goes back in time and instead of merely buying her grandfather a beer, which would be the sensible thing to do, she accidentally kills him. The one photon that tried to kill itself inevitably failed.”

Lloyd’s theory doesn’t allow for impossible things to happen, so the photon failed —showing that the universe won’t allow the Grandfather Paradox to happen. But that’s assuming we could get back in time in the first place. It’s much different with subatomic particles than it is with actual human life.

“You know those movies where it says ‘No animals were harmed in the making of this movie’? Our experiment wasn’t like that,” Lloyd said. “There were many failures before the photon actually managed to go backwards in time, and in the failures, the photon was utterly destroyed. So I would not really recommend being the first person to use a time device. That would be my advice to you. In case anyone offers to let you use their devices, take that tip from me.”

Time travel to the future isn’t immune to problems, either. When Avdeyev, for example, was up in Mir, the Soviet Union fell. He was in space for only a few hundred days and his country had dissolved by the time he got back. Imagine how different the world would be if someone traveled into the future by a thousand years instead of just twenty milliseconds.

And to add another level of trouble, anywhere near a time machine might just plain get crowded.

“Imagine if in one hundred years time, time travel tourism is rife and time travel agencies are setting up trips to the past,” Stuart said. “Where would you want to go? Probably the earliest time you could go, plus you’d get to meet the person who made the time machine. So the moment you turn your time machine on, you would be inundated with time travel tourists from the future popping out of the time machine you’ve just created, wanting to meet you, shaking your hand, have selfies with you, get your autograph. It only takes one person who wants to come back for the next million years, and suddenly you have a million people in your room. If you think about it too much, it really starts to mess with your brain.”