My regular reader (Mrs Trellis of North Wales) will know that I have a fascination with the various different interpretations of quantum physics. My own preferred take on the matter is, of course, the many-worlds interpretation, along with the game-theoretic derivation of the Born rule as outlined by David Deutsch and others. In the many-worlds interpretation, essentially, every time there are two possible courses of action at the subatomic level, the universe splits into two, and both actions happen, one in each universe.
Or not — because one of the unfortunate things about quantum physics is that while it makes perfect sense on its own terms, as soon as one starts to translate it into words the words become lies, so every word of that description of many-worlds is wrong. But it gives a flavour of the true meaning, more or less…
However, two weeks ago a new interpretation was published in Physical Review — Many Interacting Worlds.
According to this interpretation, there are a large finite number (but tending to infinity as we approximate quantum physics) of totally separate classical universes. Each of these is a single universe, with a single history that goes in a straight line, and a totally deterministic set of physical laws. However, when those universes get too close together, a force acts on them to bounce them apart in opposite directions.
(By “close together” here we mean close in configuration space, so another way of looking at this rule would be to say that it stops any two universes being too similar, by making them randomly different from each other).
This gets rid of the need of the Schrödinger wave function, which has always been the major barrier to people understanding the quantum world. To see the difference, let’s look at the Schrödinger’s cat experiment, and what would happen in the different interpretations.
In the Copenhagen interpretation, the cat is both alive and dead until the moment you look. At that point, the universe goes “oh no! Someone’s looked! I’d better decide what to be” and just chooses at random, and the cat is either alive or dead.
In the Many-Worlds interpretation, as soon as the cat is locked in the box the universe starts splitting a huge number of times, into universes with cats that are alive and universes with cats that are dead. When you look in the box, you discover which type of universe you’re in.
In this new Many-Interacting-Worlds interpretation, on the other hand, there are already loads of universes, all very alike, where you put cats in boxes. In some of them, the cat is alive, and in others, the cat is dead, but there’s been no branching, the cat in your world was *always* going to be alive or dead, but you just didn’t know which one until you looked in the box.
(Fans of SF and comics will recognise this as close to the multiverse of, for example, DC Comics, where universes don’t split off from ours at different points in history, but just exist elsewhere and have parallel histories).
Now, to my mind the new interpretation is, while interesting, not one that will take over from the Many-Worlds interpretation. It’s very clever (insofar as I follow the mathematics, anyway — it’s been many,many years since I looked into this stuff properly, and I could no more than get the gist of what they were saying), but it’s not parsimonious — it implies the existence of vastly more “stuff” than there needs to be to get the same results, and just doesn’t have the same beauty as (my understanding of) Many Worlds.
But it does have one advantage that may make it appeal to others — it means you’re in a single, real, world with a single, real, history. You’re never going to split into two, and your observations don’t magically change reality. The knowledge that there are many, many copies of you in separate universes will be, for some people, much more acceptable than the knowledge that the different future possibilities will all be happening to “the same you”, even though for all practical purposes the results seem the same.
So if you’re attached to the idea that you’re living in a single reality with one past and one future, and that it works by proper deterministic laws of physics with no spooky magic stuff going on, then Many-Interacting-Worlds gives you that option.
Personally, I don’t think that that makes sense — I think it makes much more sense to have one gigantic multiverse running by the Schrödinger rule than to have a huge number of separate universes. There’s an old rule of thumb I use that in cosmology the only three numbers that make sense are 0, 1, and infinity. It makes sense to have none of something, or just one single unique thing with a single explanation, but say there are fifty-two universes and you have to ask “why fifty-two? What’s special about that number? Why not fifty-one or fifty-three?”. This new interpretation talks about a large finite number N of universes — no matter how big N is, one has to ask “why not N+1 or N-1?”
But I know there are some of you who disagree with me, so I thought I’d pass this on. If nothing else it’s got Luboš Motl annoyed, which is always fun.
Aha! Wondered when you’d get onto this!
For me it’s a natural next question: how different does universe A have to be from universe B before there’s any point speaking of them as though they are different? If you can’t tell, then you can’t say. There could be a difference that has no perceptible effect on causality. There might not even be distinct individual universes, so much as “bands” of universes that can’t really be told apart. If somewhere outside the particle horizon a lone electron is actually a lone positron, perhaps you can’t travel to the universe where that’s so, because there’s not really any reason to think you’re not already in it — after all, you’re never going to see that particle: inflation removed its ability to affect you.
But a force…you could measure that, theoretically.