Time travel is a staple of many of our favorite movies. But what about all the science happening as you blast to the past?  How much of what we see on the silver screen is actually possible and scientifically accurate?

We decided to find out. Physics and astronomy professor Shane Larson sat down with us, and we picked his brain for the answers to our time travel questions from movies.

First and foremost – is it actually possible to time travel?

Larson says people always ask him that.

“The answer is yes,” he said. “Because you and I got to right now from yesterday. We traveled to the future.”

Yeah, yeah, we know. But time travel is a lot more complex, and according to Larson there are good reasons for why we ponder it in our spare thoughts as well as in movies.

“It’s what we do in our heads all the time. We revisit our past and say, ‘Wow, was that really the best I could’ve done?’” Larson said. “And thinking about time travel allows you a way, particularly in the context of movies, to explore that. That’s why this discussion appeals to us. It appeals to us ... because we all like to melt our brains.”

So, without further ado, here is what we learned about time travel on the silver screen.

Expect some movie spoilers for Interstellar, Harry Potter and the Prisoner of Azkaban, Contact and Avengers: Endgame!

Interstellar (2014)

We begin with what is considered to be an example of what “good movie science” is all about. Produced by Nobel Prize-winning physicist Kip Thorne, who also served as a scientific consultant for the film, Interstellar gets a lot right. Professor Larson was a postdoc in Thorne’s research group and would go to screenings of Interstellar to do Q&As.

“By and large... everything in that movie is great and really accurate, and not very speculative in terms of all the time and the gravity," Larson said. "All the way up to the point where [Cooper’s] spaceship falls into the black hole. After [Cooper falls into the black hole], [the science is] way speculative.”

Wait, so it’s accurate that Cooper would age a few hours and his daughter would age decades when he goes to that planet with super tall waves near the black hole?

Yup! This is all due to time dilation and Einstein’s law of general relativity, which Einstein spent a decade working on, from 1905 to 1915. Massive objects, such as stars and black holes and (even to a very minor extent) humans bend the fabric of the universe, a.k.a. spacetime.

“Because space and time are part of spacetime, when massive objects like black holes bend space very strongly, they must also bend time very strongly,” Larson said.

This bending of time itself causes time to slow down relative to observers who are not in the presence of that strong gravity. Relative to Cooper, who is near the immense gravity of the black hole, his daughter Murph will experience time passing at a faster rate. And that's exactly what happens in the movie – we see Murph’s transmission to Cooper when she’s an adult, aging decades in what is only a couple hours for Cooper.

So what’s going on with the black hole scene?

It’s important to know that a black hole forms around a point of infinite density, called a singularity, with gravity so strong that nothing, not even light, can escape. A human that gets closer and closer to the singularity would be spaghettified, the endearing technical term for being ripped apart by gravity into long strands of “spaghetti” due to the incredible difference in gravity between the gravitational pull in your feet versus your head. In the movie, Cooper is not spaghettified – but this is not science breaking down. According to Larson, spaghettification wouldn’t occur immediately if the black hole is sufficiently massive, like the one in Interstellar.  Eventually, Cooper would approach the singularity and be spaghettified, but not quite yet.  

There is a way, however, to avoid spaghettification and theoretically travel through time – just fall into a Reissner-Nordström black hole, which has an electrical charge attached to it. With these black holes, upon passing the event horizon – or the point beyond which light cannot escape the black hole’s gravity – you could maneuver your spaceship to avoid and “fly past the singularity” without being spaghettified. That makes the inside of a black hole a tunnel that comes out somewhere else.  

This end to the tunnel would be a white hole. But, in theory, this is possible. Additionally, spinning black holes (called Kerr black holes) are considered to be the most common black holes in nature. They could also allow for travelling through the “tunnel” within the black hole. Sound mind-numbing? We agree. Though there is not yet any proof of their existence in nature, these white holes could be a viable way to “escape” a black hole.

As far as what actually happened when Cooper fell into the black hole?

“What went on in Interstellar is this business about higher dimensionality," Larson said. "Which may or may not be about the way the world is.”

In other words, there may or may not be extra dimensions. If only Interstellar had gone the white hole route.

Harry Potter and the Prisoner of Azkaban (2004)

In the third installment of the Harry Potter series, it’s time travel that ultimately saves the day. Over the course of her third year at Hogwarts, Hermione Granger used a device called a time turner to go back in time and attend more classes (AND is in our DNA). At the end of the movie, she takes Harry back in time via the time turner in order to save Harry’s godfather, Sirius Black, from going to the wizard prison, Azkaban.

The same scene is shown twice, once through the eyes of past Harry, Ron and Hermione and once from Harry and Hermione’s point of view as they travel back in time and see themselves. There is one moment when Harry is being attacked by dementors, and would have suffered the “dementor’s kiss” (described in the book as a fate worse than death) had it not been for someone casting a patronus spell to send the dementors away. Harry believes it is his father, but when he travels back in time, he realizes that his father was never there. He has to cast the patronus in order to save himself.

But this can’t make sense, right? If Harry’s father was never there, how did Harry survive in order to go back in time and save himself? Who saved him the first time?

“These are endless loops,” Larson said. “What you think of as the past already has built into it the fact that you went back.”

What?

Okay, let’s back up. Remember when we said that space and time are ~connected~? While this is true, to our knowledge, there is an important distinction between space and time – direction. In space, for example, you can walk forward and backward. While we can go forward in time, it is not proven that we can actually go backward.

This is called the chronology protection conjecture – it hasn’t been proven, but it hasn’t been disproven. Larson said that in Harry Potter, the chronology is protected because their future selves always come back.

“Harry was saved by someone casting a really powerful stag patronus. Who was that? It was future Harry,” Larson said. “And he was always there and he always saved young Harry, because if he didn’t, young Harry wouldn’t have made it to the future to come back and save himself. It’s not a paradox; it’s a necessity.”

Contact (1997)

This movie, based on the Carl Sagan novel of the same name, tells the story of scientist Dr. Ellie Arroway who intercepts a message from an extraterrestrial civilization near Vega. At the film’s climax, Arroway enters a pod in a machine that sends her through a series of wormholes to visit the ET civilization. As she travels back to Earth, she becomes unconscious and wakes up as the pod falls into a safety net below the machine. To the scientists in the room, it looks as though Arroway’s pod has simply fallen through the machine, but Arroway thinks that 18 hours have passed. Although the others do not initially believe her, her recording device had taped 18 hours of static.

What’s a wormhole?

A wormhole is essentially a tunnel that connects two different places in spacetime. For Arroway, the wormhole connected her to the star Vega. If she didn’t travel through a wormhole, it would have taken her much longer – Vega is about 25 light years away. According to Scientific American, if Arroway traveled at the speed of the fastest human-made object – the Voyager spacecraft – it would have taken her approximately 490,000 years to reach Vega.

But with a wormhole, Arroway could get there in a fraction of the time. Imagine two points on a piece of paper. You could move from one to the other traveling linearly across the page, or you could fold the page and make a hole that connects the two points. Similarly, if you imagine a sphere, you could go from one point to the other across the sphere’s surface — or you could cut a tunnel through the middle, which would be a much shorter distance. These tunnels perform the function of wormholes.

Avengers: Endgame (2019)

The time travel in Endgame is “messy,” according to Larson.  Nonetheless, from a scientific perspective, there exists the possibility of the many worlds hypothesis, in which there are a seemingly infinite number of trajectories that our reality may take. But can we jump from one of these branches to another? In the movie, the Avengers are able to do so, and they use this fact within the physics of their movie universe to collect the Infinity Stones from previous years and places in a parallel universe.

Despite how messy the time traveling is in Endgame, however, Larson views it with strong optimism.

“We just like this exotic kind of weird paradoxical stuff," Larson said. "It’s just fun to think about. It’s a great thing to do on a Saturday afternoon around a barbeque grill.”

***

The truth is, we don’t have the technology for time travel yet and we are still trying to understand the physics of it. It’s crazy to think about, and maybe one day it’ll all make sense. But for now, we can continue to go to the movies and watch people travel through time on-screen, even if it’s wildly confusing.

“We’re in that phase of knowledge growth now when we’re just trying to understand,” Larson said. “And it’s a hard phase to be in, but it’s a fun phase to be in because you can talk about weird stuff. It melts your brain. Which is the thing we said we like.”