A Brief History Of Possibly Obsolete Time Travel Paradoxes

As anyone knows from watching movies, the problem with time travel is that when you do it, you run a high risk not only of bumping into your grandfather (or something worse with your mother) but simply bumping into anyone, thus setting off a chain-reaction of events that changes the course of history that gave birth to, among other things, you.

But among a number of recent developments in theorizing time travel via quantum mechanics emerges a particular weirdness called postselection, which may make time travel paradoxes impossible.

Postselection is a rather freaky idea that seems to say that its possible to condition a set of probabilities in such a way as to exclude certain ones that are false. Like, a traditional computer solves an equation by running through every possible variable until it works. Postselection in quantum computing means that it's theoretically possible to run all of those equations at once, based on the outcome, which is postselected. The upshot is that it's theoretically possible to affect the history of a particle or person in the present.

With some help from another quantum oddity, teleportation, a person or particle should be able to go back in time without running into the many damning snags that come out in "classical" ideas of time travel. This works because the time travel is occurring in a quantum system and in a quantum system, probabilities rule all. So, anything that could be caused by time travel has at least a very tiny probability of happening, some things have bigger probabilities and, importantly, some have zero probability of happening—like, say, killing yourself in the past.

It's like this: A quantum state involves a vast superposition of possibilities of things in that state, all occurring at once, represented by that state's wavefunction. The impossible, the paradox, is excluded from that state. Think of a big rubber band vibrating. It's occupying a lot of different places, but it's a finite amount so there are plenty of different states that have nothing to do with the rubber band.

You are going back in time along that rubber band and you have free will, as does your environment, but wiithin that state, you only experience the possible things within that particular state. You might think that not being able to kill yourself in the past is negating free will, but you don't think that, right now, your jumping straight up to the moon or bringing the pet turtle you had as a kid back to life not being possible is negating free will, do you?

Hopefully, I'm explaining that right because, frankly, this is one of those quantum physics-meets-science fiction stories that I don't even believe that the study authors totally "get." But, whatever—let's have a looksee at some of those possibly obsolete paradoxes.

In the 1980 movie Somewhere in Time the protagonist (Christopher Reeve) meets an old woman who gives him a watch. Later on he becomes obsessed with a women in a 19th century painting wearing a gold watch. He's obsessed with her and figures out a way to go back in time, which he does and meets the woman and for-reals falls in love with her and gives her the watch. Unfortunately he gets booted back to the present and kills himself out of sadness. The woman, however, lives until the almost-present, keeping the watch with her. She meets the Reeve character at the beginning of the movie and gives the then-watchless protagonist the watch.

The problem is: where did the watch come from? It's eternally passing back and forth between the two of them but getting older and older and starting to fall apart, presumably. Unless the watch is getting "younger" when it goes back time and is thus never aging but, then again, how could that be without the Reeve character getting "younger" (and not existing in the past) too. Many-worlds?

Or how about in Back To the Future when Marty plays "Johnny B. Goode" and one of the band dudes calls his uncle, Chuck Berry, who hears the song and goes on to record it in the past so Marty can learn it in the future to go back in time to present it to Chuck Berry to record it so Marty can learn it in the future and. . .

You know this one. You go back in time, kill your grandfather, the death of whom disqualifies "you" from existing and coming back to kill him.

This is one is less obvious. Say, I hop in a time machine and travel back to yesterday. In yesterday, there are now two mes. There is also a me approaching the time at the "present" when I first got into the time machine, about to get into that time machine myself. That me hops in the machine that he'd been building and obviously planning on getting into at that "present" and does so and enters a world where there is not one other "me" but two. And so now/then there are three, and a long—endless, in fact—string of mes approaching the time machine at the present about to hop back to a yesterday.

And then, like, boom the universe is packed past the stars with me. Which of course means I'm not going to get into the time machine in the first place. Unless I'm trying to escape the overpopulated hell I created. (A world overpopulated by me is the least fun thing I could ever imagine.) In that case, maybe I'd try and go back to an earlier time to stop myself from building the time machine, but then again the same "me" that had the idea to go back to stop the earlier me at a slightly earlier time is going to get into the time machine right after me—and on and on—so that earlier time is going to be another overpopulated universe.

Kick this around: you travel back to a past that exists in the time since you were born. How are you not actually just traveling to your future?

The thing isn't that we don't know how this might work, it's that we can't know in an open system like a universe what effect a small change to the past might make for the general make-up of the "present" universe. The Wolfram Alpha big thinker is trying to say now that the entire universe in its bottomless complexity came from a pattern so basic a terrier could have nosed it together. Little things in large time spans can—not must—equal enormous things. But it's impossible to say.