hyc's Journal: Infinity and parallel universes

I watched a silly movie on the SciFi channel last night that got me thinking about time travel again. Before I get into that, I should lay out my interpretation of life, fate, and parallel universes.

Since the notion of parallel universes hasn't been completely laughed out of the realm of physics, here's one way to look at it. Think of a person's life, and every possible decision that's encountered. Plot it out and it's like a game tree, every decision point branches into alternate paths. When you talk about infinite parallel universes existing, it's basically saying that at every possible decision point, an incarnation of you actually went down each and every one of those paths. Your current consciousness led you down a particular path that brought you here, reading my journal entry. And there are an infinite number of copies of you all going down different paths of the tree. For every possible decision in your life, there is a parallel universe out there where you made that decision.

More inclusively, for every *particle* in the universe, there is an infinite number of parallel universes encompassing all the possibilities of that particles existence - collided with a photon here, missed the collision there, etc. etc.

"Infinity" is a nice, neat word, and sometimes it's easy to forget what it means. Here, we're talking about a near-infinite number (all the particles in the universe) each describing a decision tree with a near-infinite number of branch points (particles do decay after all). The number of parallel universes is stupendously huge, but is it truly infinite?

Is there such a thing as fate? Well, look at it this way - when your life begins, you have this huge tree of possibilities fanning out in front of you. But as soon as you make your first choice, huge sections of that tree are cut off from you. As you progress further in your life, further down the tree, some possibilities are much easier to reach, much more likely, and some are much farther away. Some are impossible to reach. So as you get further along in life, it looks like you're being pushed a certain way, it looks like you're being guided by "Fate." But all that means is that you're far along your decision tree. At any point in the past you could have chosen a different path, and then "Fate" would have looked completely different. And the theory of parallel universes says that there's a multitude of copies of you that *did* take those different routes in your life.

I started writing this entry thinking there was an infinity of possibilities, but now I think it's actually finite. Just a huge number, larger than anything we'll ever be able to count, which may be the same for practical purposes.

But just because there's a near-infinite number of possibilities in life doesn't mean that anything and everything is possible. Some fiction authors play with the notion of a "mirror universe" that's exactly the same as ours except the good guys here are evil there, and vice versa. It doesn't take much thought to realize that even with infinite parallel universes, a mirror universe is impossible. Just consider an evil person killing someone in universe and doing the exact opposite in the other universe - curing them. Now you have one person living in one universe, and dead in the other - the two universes are no longer perfect mirror images of each other.

The striking realization here is just because there are a huge, countless number of possibilities in life doesn't mean that *everything* is possible.

Coming back to the notion of time travel... If you think about the multiverse just being a huge mesh of branching decision trees, then you can define time travel as just a special case of "phase shifting" from parallel universe to another. At present, we don't have any way to interact with all the parallel universes around us. If we could phase shift to an adjoining plane, that would mean we've jumped off our current path of the decision tree and reappeared on a different branch. Time travel would just be a case of jumping off our current path to a point on the tree that's in the past.

But again, if you accept the notion that parallel universes exist and that every possibility is instantiated across these universes, that means that your arrival at that point in the past has inherently created its own branch of possibilities. This means it's impossible to change the past in such a way that it affects your present, because you've already traveled down the path you came, and anything you do in the past is just being done on a different thread. You can certainly "create a new future" but really all you're doing is pursuing a new path; the old one that you jumped off of is still doing its thing, just without you present.

So here's something else that's interesting - again, the idea is that for every conceivable possibility, there is a parallel universe out there where that possibility is fact. So if it's possible to create a time machine/phase shifter and jump around from point to point in this multiverse, then there are realities out there where it is being done. That means that at every point in human history where it has been technically feasible to create a time machine, it has been done.

But just because it was done doesn't mean that our life today will change, because we happen to live on a thread that has already happened exactly as it did. Anyone from the past/present/future who jumps to this exact spot in the multiverse will simply create a new branch, and part of our multiple incarnations will experience that, and part won't.

So, maybe the future is pre-ordained, inasmuch as we know that every possibility is realized, somewhere in the multiverse. And what we see as freedom of choice is merely one incarnation of ourselve's perception of our path along all of these branches of possibilities. What happened in our past is done and gone, unchangeable. If someone shows us a tool for traveling to the past, it can't erase that past, it can only create a new one.

I guess that solves the "go back and kill your grandfather" paradox, anyway...