You don’t need a DeLorean to see that this year is going to bring a lot of Back to the Future references and time travel talk. It’s fitting, then, that physicists have actually succeeded in constructing a quantum time machine. Well, sort of. (Isn’t that always the way with quantum physics?) It turns out, one can create subatomic systems under one set of conditions, and they will also behave as if they were under a different set of conditions. The first intuition is probably to imagine doing something slowly as a way to understand how it will work quickly, like a dancer or a martial artist practicing moves at quarter speed to get the motions right. But that’s not what I mean; it’s more like using slow and fast to simulate sweet and salty.
In this case, they are using two photons to simulate what it would be like for a single photon to interact with itself in the past. Since we don’t know if it’s actually possible for a photon to travel through time that way, it’s a pretty neat trick to do experiments with time travel anyway. And the good news is that their results match up with what we’d expect if the universe prevents grandfather paradoxes. In other words, if time travel is possible, it’s less like Back to the Future and more like 12 Monkeys or Primer. Probably more significant is that may provide the means to experiment at the intersection of quantum mechanics and relativity (without going into a black hole).
We often imagine time travel as a means to go back and fix things. But if you can’t change the past, what event would you go back to participate in exactly how it already happened?