The idea of parallel universes is popular in science fiction and, to a lesser extent, in science.
In its simplest form the idea is that every decision that is made spawns a completely different universe that is identical to ours except for the changes caused by that one decision.
One might think that given the sheer number of possible choices anyone, or anything can make (including the rate at which radioactive elements decay and the precise direction of the individual atoms that make up, well, everything) that the number of possible universes is infinite, and that (if the theory is true) literally anything could exist somewhere.
But that’s wrong.
Because the number of possible universes that could exist is not infinite.
Imagine that the universe was actually a chessboard. Moreover, that it was a chessboard with only 2 knights on it, one white and one black. Imagine further that this chessboard universe has been in existence for exactly 2 turns.
How many possible universes are there? The white knight could move up and left, or up and right, so that’s 2 possibilities. The black knight could move down and left, or down and right, so that’s another 2 possibilities.
So there are only 4 possible parallel chessboard universes (after 2 turns), not an infinite number.
And as far as I’m concerned the same rules apply to our universe as well.
Our universe has not been around for an infinite length of time. It has been around for 14 billion years, which is a lot. But it’s far from being infinite, and that’s important.
Likewise our universe does not have an infinite number of moving parts. It certainly has more than just 2 knights but the number of things that the universe contains is finite, even when you get down to subatomic particles. There may be so many of them that their number could never be counted. But even then their number is finite
And if you have a finite number of pieces, and a finite number of moves, and a finite amount of time to move in, then the number of possible combinations is also finite.
This may be hard to grasp because of the sheer volumes we are discussing. The number of possible configurations of atoms in our universe, after 14 billion years, is a number so huge that the only surface large enough to write it on is yo momma!
But it’s still a finite number.
One counter to this idea is the notion that there are still infinite possible configurations because you could always make minor variations to any action. For example, even though the black knight decided to move down and left it is possible to place him ever so slightly more to the left than in a previous configuration, without him actually leaving the square. So there are still an infinite number of possibilities.
But this is untrue for the following reason: you can’t continue to change things forever, you have to stop sometime.
On the chessboard universe you can’t keep moving the black knight to a different part of the same square forever. Sooner or later he will be in a different square, and the rules of chessboard universe prevent this.
Likewise, in our universe objects can only move in certain ways before you start breaking the laws of physics. The number of ways they can move is vast and diverse, but it’s still a finite number.
And a finite number of agents, with a finite number of possible moves, can only use the finite time available to them to move in a finite number of ways. So there is a finite number of parallel universes.
It is of course possible that there is no limit to the lifespan of the universe, and if the universe will exist for infinite time then, naturally, the number of possible universes that might exist at some point becomes infinite.
But that doesn’t apply right now. Right now our universe has had a finite amount of time in which to do stuff. So with finite agents and finite moves the number of possible combinations must be finite.
Secondly, even if the universe does have an infinite lifespan that still wouldn’t mean that “anything” is possible. Sooner or later the possible worlds would still end up butting against the laws of physics and certain combinations would become impossible. So even though there would be an infinite number of possible universes that is not the same as saying that “everything exists somewhere” or something like that. You’re still never going to get a universe with “magic” in it.
But what stops possible universes from breaking the laws of physics? Surely, it could happen that a parallel universe would be created where the laws of physics are slightly different from how they are now?
Well, actually, no. In fact many scientists believe that if any of the fundamental properties of the universe were even slightly different it’s unlikely that there would be much going on in the universe at all. So while tiny changes might be possible the more changes you introduce the less likely that possible world becomes. Pretty soon we hit impossibility again, which means that the number of possible universes with slightly different laws of physics is also finite, so the number of possible versions of those universes is also finite.
So the number of possible parallel universes is still a finite number.
The question is, of course, does any of this actually matter?
Well, I really don’t know. But I’ve always been puzzled by sci-fi narratives’ claims of infinite parallel universes so I thought I’d sit down and think about.
Soon cats intruded.
So if you know more about physics than I do (which isn’t hard) please post in the comments section and let me know what you think.
[Standard Disclaimer: this post was entirely my own opinion and was not paid for in any way, directly or otherwise, by anyone or anything that stands to gain in any way from the ideas expressed herein.]