Back to the Future: Does physics of Marty’s time travel add up?

By Joshua Sokol

Check the time circuits. It’s 21 October 2015. Today, Marty McFly arrives direct from 1985 to a future full of hoverboards, self-tying shoes, and rehydrated pizzas – plus two kids who look suspiciously like Michael J. Fox (the actor playing Marty) and a dad who looks suspiciously unlike Crispin Glover (who played the original George McFly but was not invited to reprise the role).

Back to the Future Part II might not be the best film in the series. It’s certainly not my favourite. But what it lacks in Oedipal issues and the Old West, it makes up in time travel shenanigans. Biff, the villain, brings his 1955 self a sports almanac, which creates a divergent – and weirdly dark – timeline that Marty and his classic mad scientist friend Doc Brown must then erase.

The kind of time travel where you can change the past always leads to paradoxes. How could anyone change the past if that change has already happened? But lest you think me yellow, I had to ask: could the physics of time travel with Doc’s hacked DeLorean ever really work?

If you’re gonna build a time machine into a car, why not do it with some style?
Let’s start with the DeLorean itself. Although Doc builds the time machine in 1985, the modded car is able to travel back to before its own creation at least twice – first to 1955 and then to 1885. But in 1992 – in our timeline – Stephen Hawking proposed that the laws of physics should forbid anything travelling backwards in time.

“It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” he wrote. So how does the DeLorean do it?

Physicist Kip Thorne thinks he has an answer: use a wormhole, a tunnel through space that connects two distant places. If one end of the tunnel is sped up to near the speed of light, it experiences time differently to the other end. By travelling through the tunnel, you could move back and forth to the future.

Where we’re going, we don’t need logic
Michio Kaku, a theoretical physicist at City College of New York, has a different idea. He says that according to the theory of general relativity, time flows like a river, and pumping 1.21 gigawatts of power into the DeLorean somehow allows you to jump streams. “If you go backwards in time you enter an alternate reality, a quantum alternate reality,” he says. “And Back to the Future, to my knowledge, is the only film that gets it right.”

That’s a point in favour of the alternative timelines sketched out on a blackboard in Part II. It also touches on my own theory about how to make Back to the Future work: every time Marty travels into the past, he’s making a new alternate universe. That way, Marty can make the world deviate from the one he comes from without creating a paradox. Right?

My density has brought me to you
I checked this theory with Caltech physicist Sean Carroll, who has criticised Back to the Future‘s version of time travel. He pointed out a flaw in my multiple universes scheme: Marty himself, when his limbs start to fade in the first film, seems to depend on events in the past of the universe he has travelled to.

That raises questions about how the multiple universes required would communicate with each other. And it doesn’t explain other paradoxes, like what is going on when people or things vanish from photographs after something changes. “When you’re in Back to the Future, and you’re looking at the photograph and there’s people disappearing in it, does the whole memory of that person ever existing disappear from your brain?” Carroll says. “And if not, why are their brains different from the picture?”

Carroll is pessimistic about finding a way to reconcile the films with reality. “Nothing that has ever been imagined by a scientist is sufficient to make sense of Back to the Future,” he says. “There is no single logical timeline that actually represents what is being shown to you.”

Maybe that explains why I still don’t have my hoverboard.

Image credits: Universal Pictures