So there’s this great particle physics joke. It goes…hey wait! Come back! It’s really funny. It goes like this: ”The bartender says, ‘We don’t serve faster than light neutrinos here.’ A faster than light neutrino walks into a bar.”
OK, it doesn’t stand up to close scrutiny, and not just because neutrinos can’t walk. It’s because nothing can go faster than light, though if a neutrino or anything else could, it would (depending on how you define these things) travel through time, so it could indeed be in the bar before it enters it. But it can’t.
That joke got a lot of play back in 2012, when a preliminary announcement came out of Italy that a blast of neutrinos—electrically neutral subatomic particles—produced in a particle accelerator had been made to exceed the speed of light, which means they went faster than a cool 186,262 miles per second. The neutrinos were said to have broken that cosmic speed limit only by a teeny, tiny 62 billionths of a second. But a little bit faster than the speed of light is like a little bit dead. You either are or you aren’t—the rest is just a philosophical rounding error.
Happily for the entire edifice of advanced physics, the findings were disproven, the speed limit was enforced and the universe was set right again. Which brings us to About Time, a just-released Hollywood rom-com about a guy (Tim) who travels back in time to kiss a girl (Mary) he was too shy to kiss when they met before at a New Year’s party and then…something-something-something they fall in love. Or don’t. Or maybe both. The point is, don’t worry about spoilers here. (Full disclosure: I didn’t see the movie, but I’m the kind of person who really likes particle physics jokes, so did you think I would?)
The other point is, we all love time travel stories. The release of About Time was an occasion for a web post on the site Total Film called The 50 Best Time Travel Movies (with the absolutely delicious sub-head “A big ball of wibbly-wobbly timey-wimey stuff). That’s fifty, with a 5-0. Wikipedia lists the first of many, many time travel books as Memoirs of the Twentieth Century, published in 1733. And there have been hundreds since. (Full disclosure Number Two: I wrote one of them, a young adult novel published in 2011—still available from all fine vendors, in case you were wondering.)
Whenever popular books or movies with even a whiff of science to them come out, there’s always a wealth of how-much-did-the-director/author/screenwriter-get-right stories. Time.com weighed in with one when Gravity was released, and we’ve scolded About Time already for making mistakes about how things would or wouldn’t play out if you could travel in time.
Now let’s go one step deeper: What did About Time get right about that big if—about the possibility of actually traveling in time? That one’s easy: nothing. Completely, utterly, absolutely nothing. The corollary? What did it get wrong? Everything. That’s because—at least as far as every single thing we know about physics tells us—it can’t be done.
As Einstein first showed, exceeding the speed of light—and using that avenue to travel in time—is impossible because the faster you go, the more your mass increases. The greater your mass, in turn, the more energy it takes to accelerate you, which causes you to become more massive still and on and on until it would take infinite energy to make you go any faster, which is the central impossibility that keeps you on this side of the light-speed limit.
It’s true that the faster you go, the more time slows down for you but not for the people you left behind. That prevents movies like Planet of the Apes from running afoul of science—except for, you know, the talking apes. And Charlton Heston acting. But in that case we’re talking about time dilation, not time travel. The clock in a speeding spacecraft looks like it’s ticking away at the normal speed, as do the ones on Earth. That’s the relative part of relativity. This can technically take you into the future of the world that’s standing still, but you can accomplish the same just by sitting in a chair: A minute passes, you’re in the future!
Studies of a phenomenon known as quantum entanglement—in which two particles are forever linked, even if they’re separated by great distances—feel a little time travelish. That’s because the particles in effect communicate with each other instantaneously, so that the state of one affects the state of the other. Instantaneous ought to be even faster than light, but in this case it’s not. It is instead, as Einstein himself evocatively put it, “spooky action at a distance,” which explains both everything and nothing. That, however, is the nature of the quantum world, where common notions of before and after, now and later, cause and effect just don’t apply. Trying to understand time travel in a case like this is a little like trying to ask how many home runs are scored in a football game. None—they don’t exist there.
Some theorists have looked at a sort of a bank shot way of traveling through time, which involves first traveling through a cosmic wormhole—a portal in spacetime. That could work, sort of, on paper, since it sidesteps the light-speed barrier by taking a shortcut to the other side of the universe. The problem is, the passage through the wormhole would also rip you to shreds—which pretty much takes the com part out of the rom-com. (There might be an exception to this rule, as my colleague Michael D. Lemonick informs me. A wormhole trip would indeed kill you, “unless you propped it open with something called ‘exotic matter,’ which is not known to exist. But if it did, it could prop open a wormhole like nobody’s business.” I’d listen to Mike. He always knows what he’s talking about.)
So, About Time is impossible. And so is H.G. Wells’s The Time Machine and Christopher Reeve’s exquisite Somewhere in Time and Jack Finney’s brilliant Time and Again and on and on and on. And so what? It ain’t called fiction because it’s been peer-reviewed and fact-checked. It’s called fiction because it’s fun or moving or sad or sweet—and has the delicious freedom of being entirely made up. Science—for all its street cred—can’t touch that.