It would appear that longstanding and tricky paradox of time travel, that one who travels backward in time could conceivably influence it to the point where the time travel couldn’t have happened is dead. All hail the longstanding and tricky paradox.
This, the “grandfather paradox,” is so named because of the thought experiment used to illustrate it. A time traveler could theoretically go back in the past and murder his own grandfather, which would render the time traveler non extant, and thus would lead to the paradox of the time traveler never having been able to go back in time in the first place.
The same holds true in a less dramatic fashion for subatomic particles like quarks and photons, which hold a much higher likelihood of ever traveling into the past, based on the strange, spooky behavior they’ve demonstrated for the people who’ve studied and been alarmed by them thus far. If a particle could physically travel to the past, it could bump up against itself and alter its position so that it may not have the opportunity to have gotten to the point where it traveled into the past.
This little conundrum made time travel fundamentally dangerous, if not impossible. Until a guy named Seth Lloyd — who Wired Magazine says you’d like to have a beer with, fyi — applied a principle of quantum mechanics called postselection to the problem and rubbed the paradox out like it was his own grandfather.
Where to begin? Postselection is the ability to select out paradoxical situations to prevent them from ever happening. Based on Lloyd’s experiments with photons, using postselection, which you could reasonably liken to programming the past — in this case, selecting the option that it will be impossible for you to kill your own grandfather. As Wired’s Laura Saunders explains, by making that selection, the universe would expectedly make attempts to kill your grandfather exaggeratedly difficult to an almost cartoonish degree. The laws of probability would skew toward preventing the event from taking place, so anything that could go wrong in your attempt to kill your grandfather most likely would. The event with the highest probability would be you giving up out of sheer frustration your attempt to kill your grandfather.
Perhaps most startling about this application of postselection to the grandfather paradox of time travel is that it came out of experiments with photons. At some point during experiments where he and his buddies messed with photons, pushing them into paradoxical situations that I can’t begin to wrap my mind around, Seth Lloyd thought something like, “Hey, I think we just solved that old grandfather paradox,” and went and wrote a couple papers about it.