They are referring to many worlds interpretations of quantum mechanics that argues that the quantum wave function is as real as anything. As real as say electromagnetic fields. If this is true then the mathematics say that the universe branches or splits at each quantum event.
To the best of my knowledge there's no way for branched universes to ever interact with each other again, so why would a layman even care that they exist, let alone be profoundly changed by that fact?
I don’t know the science of this possibility, but you should check out the short story Anxiety is the Dizziness of Freedom by Ted Chiang. He writes directly about a future where we have a tool to communicate between branches universes.
That's science fiction, though. The Everettian Many Worlds Interpretation is just that: An interpretation of a set of equations. And pretty much by definition the worlds cannot interact in any way. Even if there were some obscure way to differentiate between MWI and, say, the Copenhagen interpretation, I wouldn't expect it to become feasible anytime soon. But as far as I know there cannot be any experiments differentiating between the two (or other interpretations), because they're simply different interpretations of the same equations. They predict the same experimental outcomes.
Sorry I didn’t clarify. It is fiction, but I think it describes a plausible reaction of the general public (the layman of the comment I replied to) to this hypothesis if it was discovered to be true. Don’t want to spoil the good part of the story beyond that!
I think their mere existence is profoundly alienating. For example anytime you make a choice to be kind, there's another universe where you make the choice to be cruel in the same circumstance. Likewise the opposite. The result is that nothing you do really matters in the multiverse, everything happens anyway regardless of your choices. You're just choosing to experience one of countless paths. Arguably it could even be said to be selfish, you're consigning another version of yourself to the cruel path.
I think it's not dissimilar to absolute determinism. Many people, even irrelegious, have a deep aversion to the idea that their choices may be deterministic. You might say why does it matter if they cannot be predicted, and of course the path is one that your brain decides to take, not something that is being forced on it. Still lots of people like to imagine some sort of weird quantum consciousness woo that lets brains make free thoughtful choices independent of physical laws.
One caveat here is that even if every outcome is taken in some branch of the world, they don’t have equal likelihood. So while there might exist a world where a person makes that cruel decision, it may only be a small fraction. I imagine we could ask, how do we feel about our expected behavior averaged across all worlds?
The result is that nothing you do really matters in the multiverse, everything happens anyway regardless of your choices.
That's ridiculous. It's like saying there's no point in playing a videogame because other people are playing the same game differently. If it doesn't affect you why do you allow it to make you care or not care? Personally I think it comes from this sort of arrogant thinking where people are aware they are an insignificant cosmic bug but also are somehow above caring about things at the level of a cosmic bug. It's an ass-backward way of thinking, this "I don't matter so I don't care". A better way of thinking is "I don't matter, so I can care about things that also don't matter, because they're no more insignificant than I am." If a dog turned to us and said "I can never write a symphony, therefore gnawing on bones is pointless" we would call him a foolish dog for refusing to engage with reality at his own level, for thinking that things so far beyond him should make any difference to him and how he lives his life.
To quote James Watson, "I don't think we're here for anything, we're just products of evolution. You can say 'Gee, your life must be pretty bleak if you don't think there's a purpose' but I'm anticipating a good lunch."
Do branches ever “unsplit”? I mean, if U1 splits into U2 and U3, U2 into U21 and U22, U3 into U31 and U32, is it possible that (say) U21 and U31 might be identical? In which case, couldn’t you say they are the same, and a case of U2 and U3 merging/converging?
But if you are going to say that - why describe it as a “split”, as opposed to two separate universes which happen to identical in contents up to some point, non-identical thereafter?
Also, what you say about dollars is true because they have certain properties - effectively they have two different criteria of identity, one economic, the other physical. What is the criterion of identity for universes/branches?
>But if you are going to say that - why describe it as a “split”, as opposed to two separate universes which happen to identical in contents up to some point, non-identical thereafter?
But they are (presumed under this model to be) splitting. You posited a case where two later happen to be identical, but that doesn't mean they merge. You'd just have two of them.
>Also, what you say about dollars is true because they have certain properties - effectively they have two different criteria of identity, one economic, the other physical. What is the criterion of identity for universes/branches?
I'd guess that there is none, like electrons. But though those are even more indistinguishable than dollars, you can still have two of them.
> But they are (presumed under this model to be) splitting.
Okay, but what's the actual model here? Let's talk about two different versions of many-worlds:
Version 1: Universes splitting
Version 2: No "splitting" per se ever actually occurs, universes are identical up to time t, different thereafter
Is there any difference between 1 and 2 from a physics perspective–any difference in the maths, or in conceivable results of observation or experiment? Or are they just different ways of wording it? A metaphysical/philosophical difference, but not an empirical/mathematical one?
Also, if the theory is symmetric with respect to time, you'd expect merger as well as splitting, since the former is just the time reversal of the latter – isn't QM supposed to have T-symmetry? Does MWI have T-symmetry?
Right, I’m not sure whether this refers to finding another earth-like planet with JWST (interesting and plausible though not related to the many-worlds interpretation of quantum), or seeing another version of the Earth with JWST that followed a different path in possible many-worlds quantum outcomes (seemingly impossible!).
Here’s a very very good explanation of this:
https://m.youtube.com/watch?v=nOgalPdfHxM