Time Travel, Free Will, P vs NP

So I’ve been thinking a lot about time travel lately, thanks to Harry Potter and the Methods of Rationality. If you haven’t heard about it already, it’s a fan fiction piece in which instead of the doofus Uncle Vernon, Harry is raised by Aunt Petunia and her scientist/Professor husband. And so armed with the scientific method, Harry navigates the wizarding world trying to understand the laws of magic through science. I love it; it introduced me to a plethora of scientific concepts and made me think about known ones much more deeply — time travel, positive bias, and of course, the scientific method itself. I will write about a few of these topics, starting of with the one which has been the subject of intense philosophical debates, a trope experimented by countless literary pieces — time travel.

An outline of this post:

Let’s start off with how the time turner was used in HP canon1. Quick revision: what’s a time turner? A device to travel back in time in the HP universe. Turn it N times to go back N hours. Alright. So in the Prizoner of Azkaban, Hermione is given a time turner to help with her academic workload2, i.e., attending classes in clashing time slots (oh, wouldn’t we all love a solution to that problem). Why wouldn’t such a device exist in a universe that has magic. But the time turner’s moment to shine is its role in saving Buckbeak, a magical creature, from being killed. You can watch that part from the movie or pick up the book, but the part that I got me into this rabbit-hole is this:

Let’s say it’s t=T, and you decide to go back in time to T-100. So now it’s t=T-100, and there’s U and UF — you from the future. UF watches U doing things up to t=T, at which point U decides to go back to T-100. UF sees U do that. Then what? U becomes UF’, and is watching a U. And UF is watching both of them?

So you see the time loop. Messy, ain’t it? How does the time loop get broken? How does life continue beyond t=T?

Well, the answer is simple in hindsight. UF sees U turning the time turner, but then for UF, U disappears from the face of the earth. Or from the face of this earth. U has gone to a parallel universe where everything else is the same, but t=T-100. UF just takes U’s place in this earth, and continues life beyond t=T. The time loop is never broken, and there are infinite such parallel universes. And now you know what I meant when I said the term universe was a little overloaded…

Alright, all good. It’s a little complicated, but makes sense so far, right? Now let’s go a little deeper into what’s actually happening when UF and U are co-existing.

Okay, so you’re UF, and you’re observing U (bumbling through life). Now of course the first thing that comes to most people’s minds when they think of why one would go back in the past is to correct some mistakes; more broadly speaking, make decisions differently.

Cool, let’s say you could. What would that result in? Let’s look at a concrete example. In your current world, at t=T, one of the regrets you have in life is never asking your crush out all that time ago, and now you have completely different lives in different parts of the world. Somehow you end up with a time turner in your hands. A wistful smile comes on your face as you go back to that time and ask your crush out. And they say yes! One thing leads to another, and this time at t=T, you are happily married and raising your kids with a backyard hockey rink. Yaay! But wait, but then didn’t it also happen that were at t=T without said crush? So there are two parallel universes now, one in which you are, and the other in which you aren’t. Huh. And this is what you might have seen in many sci-fi pieces over the years, my favourite being ‘Back to the Future’ — multiple parallel universes, aka multiverses.

But what if you couldn’t change the past? What if you could just go back in time and be a passive observer, not an active participator? What good would time travel be in such a scenario, you ask. Well, you could be a passive participator… Let me explain :P Let’s go back to the Prizoner of Azkaban. Harry goes back in time, and is curious to see who saved him from the dementor by casting the patronus from the other side of the lake. So he goes to the other side of the lake around the time the patronus is supposed to be cast (which he knows because he’s lived through this timeline from the other side of the lake once, almost dying in the process). He waits and waits to see his father appear and save him … until it hits him. Nobody is coming. He has to do it himself. He has to cast a corporeal patronus for the first time to save his own self! And he does! And … you guessed it … he did it the last time as well. When he (H) was living that timeline for the first time, being kissed by a dementor, his future self (HF) saved him. So he lived, found a time turner thanks to Hermione, went back in time to save himself … because it had already happened! The fact that he was alive and could use a time turner meant that he had already gone back in time to save himself so that he lived on to find a time turner and go back in time to save himself so that he lived on to find a time turner and … :P

Neat, huh? It gets better (or worse, depending on how you see it). So things that have happened have to happen. HF had to save H, because that had already happened (yes, yes, tenses are kinda meaningless at this point). In other words, HF does not have free will. Yes.


To be continued…


1: What do you expect to find when you google “HP canon”? :P
2: People snarkily remark, he he, that’s the only purpose teenagers would use time turners for, suuuure, how dumb… there are sO mAnY plOtHolEs, cOmE oN J.K. Rowling. Well, this is supposed to be a kids’ book, and b) anyone can criticize, only a handful have created something that has touched millions of people’s lives, if not billions. So calm down, have a nice intellectual conversation about the merits and shortcomings of something, but don’t criticize.

Written on April 30, 2020 +1950
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