Science journalism has snowballed around a fascinating simulation from June that examines the dimensions of David Deutsch’s closed timelike curves (CTCs) and seems to suggest that the "grandfather paradox"—the question of what happens when you go back and time and prevent your own existence, which would presumably prevent you from going back in time, which would in turn prevent you from changing the past in the first place, and so on—can be resolved on a quantum level. Well, probably:
"Instead of a human being going back to kill the ancestor, imagine a fundamental particle goes back in time to reverse the particle-generating machine that created it. Due to the reversal, the machine emits a particle-the particle-back into the CTC. Deutsch insisted that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to reverse the machine with the same probability value, imbuing itself at creation with a probability of one half of going back to flip the switch.
"If the particle were a person, that person would be born with a one-half probability of killing their grandfather, giving their grandfather a one-half probability of escaping demise; a good enough probability to close the loop and escape the paradox."It’s all little more than a thought experiment at the moment because we have no way of knowing for sure whether Deutschian CTCs even exist outside of simulations, whether they’d behave like the simulations do if they did, whether objects larger than photons can travel back in time, whether those objects would behave like photons if they did, and so on. All we have is a basic understanding of how a photon might behave if it were to travel through a CTC, assuming our understanding of quantum mechanics and probability is complete (and inasmuch as we don’t have a grand unifying theory yet, we already know it isn't).
But the real elephant in the room, as far as I’m concerned, is time itself. Until we have a better sense of what it is (and right now we’re in no position to know), we’re only assuming that we can speak of traveling “backward” or “forward” through it in any more than a metaphorical sense. It’s useful to say we’re moving “backward” and “forward” through time because we conceptualize ideas better through a spatial lens, but the fact that a description is useful doesn’t mean that it’s accurate.
So yes, the CTC simulation provides one way that the grandfather paradox can hypothetically be resolved—and it’s a worthwhile and provocative study, for that reason. But time travel is just as much of a hot mess as it was six months ago, and the nature of time remains one of the most interesting (and least understood) questions that theoretical physics asks. And science journalists who think we’ve already definitively answered that question, and the questions regarding time travel that surround it, are making theoretical physics look a lot less exciting than it is.