I love a good time travel story. There’s something about the possibility of jumping ahead to far flung futures or traveling to the past, experiencing a famous event live or changing a regretted mistake for the better. (That never goes wrong…)
I’ve been thinking about time travel a lot recently, after reviewing two trade paperbacks that hinge on the concept. Scout Comics’ Forever Forward follows a group of friends stuck jumping forward in time, hoping to find a future society that has the technology to send them back again. And of course, Marvel’s X-Men: Days of Future Past: Doomsday is a sequel/prequel to one of the best time travel stories ever written in the comics medium.
But unfortunately, I don’t actually believe time travel can or ever will exist. Well, actually, we’re all traveling in time right now – just always in one direction, and normally at a constant rate. What I mean, of course, is that I don’t think jumping far into the future or traveling to the past can or will ever happen. And as a physicist, I have my reasons.
Remember when I said we “normally” travel through time at a constant rate? Einstein’s theory of special relativity explains how an object – including a human – accelerated to velocities close to the speed of light will actually experience time more slowly than those who stay put.
For example, while a person accelerated to 99% the speed of light ages only one day, everyone remaining on Earth would age a whole week. In this way, jumping forward in time – similar to what happens in Forever Forward – could actually theoretically happen. This idea is generally referred to as the Twin Paradox. This was a narrative component of Christopher Nolan’s film Interstellar.
The problem is, it would take an overwhelming amount of energy to accelerate a human (in a spaceship) to the required speed – an amount of energy that we don’t now and likely never will have access to. So, this kind of jumping forward in time will probably never happen. And even then, this only works going forward in time.
Other stories, like the most recent Flash movie, use Einstein’s special relativity to argue that moving faster than the speed of light would allow a person to travel backward in time. This is just science fiction, though. Special relativity clearly states that nothing can travel faster than the speed of light. Sorry.
But, Einstein’s other theory of relativity – general relativity – explains that both space and time can be curved by large masses or amounts of energy. One could imagine a spacetime curved to such an extreme extent, that time loops back around on itself.
In fact, the famous mathematician Kurt Gödel actually found a solution to Einstein’s so-called field equations that allows for looped timelines. In such a Gödel-universe, there exists for every point in spacetime a timeline that connects to any other point in spacetime, no matter if that point is in the future or the past. So, from any point in time, you could travel to any other point in time.
The problem is that Gödel’s solution doesn’t have anything to do with the properties of the universe we live in. It describes a rotating universe, whereas our universe is an expanding one. And even in such a Gödel-universe, the amount of energy necessary for traveling backward in time would be staggering.
Back in our universe, to curve spacetime to the extremes needed to even imagine unusual jumps in time, you’d need either extreme amounts of energy (do you see a pattern here?), or an extremely massive object. Black holes are such extremely massive objects that curve spacetime to the limits of what physics can understand.
But that’s kind of the problem. Physics can’t yet describe what goes on inside a black hole. I know Matthew McConaughey’s character in Interstellar travels into one in order to send a message to the past, and then returns back to our solar system. But in reality, traveling into a black hole would kill you long before you got all the way into it.
So, although many sci-fi stories base their time travel narrative on the principles of physics – especially Einstein’s dual theories of relativity – the current state of science pretty much rules all of that out. But, it’s still fun to imagine the possibilities of time travel in books, films, or comics. In this four-part series, I’m going to look at the main ways time travel is depicted in science fiction. Then we’ll look at if – assuming it were physically possible – such time travel could logically exist.
AIPT Science is co-presented by AIPT and the New York City Skeptics.
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