[loup-vaillant]

I thought this experiment was explained just as well by decoherence? Under the many world interpretation, there's no instant communication, or hidden variable, we just happen to learn in which world we are by performing the measurement.

The laws of physics are still deterministic as far as I can tell.

[furgooswft13]

This just pushes back the point of non-determinism. Instead of the measurement, or wave function collapse, or decoherence or whatever you want to call it being not deterministic, the "world" you end up in is.

If even that point is to be considered deterministic then there has to be some previous state of something to lead to it, and we're back to non-local hidden variables.

This is one of the many reasons I dislike the many-worlds theory, and it's seeming popularity of late. It does not actually solve non-determinism, it just shuffles the deck a little, while also tacking on an ever expanding number of alternate universes, all the while remaining as non-falsifiable as any other QM interpretation.

If I had to choose between two non-local hidden variable theories, I'd choose the one without the constant branching alternate worlds.

Non-local hidden variables are almost by definition unknowable to us, so I'm not sure what we gain by pretending all this meta-physical philosophizing is actual science. It did make for a good TNG episode though.

[loup-vaillant]

Many Worlds is popular for a reason: it's one of the simplest interpretation that fits the facts. The scientist in you might dislike that it came about later than the Copenhagen interpretation without any experiment to distinguish them. The Bayesian in me just applies Occam's razor.

Just realize that a collapse interpretation is the same as a many worlds, with the additional assumption that the blob of amplitude you don't live in is somehow flattened to zero. The "it's just maths" interpretation isn't much better: the maths tell us that we have two worlds worth of amplitudes, yet for some reason the one we don't observe isn't real. This is as ridiculous as believing that stuff you send outside the observable universe ceases to exist once it crosses the boundary.

As for how far it pushes non-determinism… I'd say quite far. Under the MWI, the universe is entirely deterministic. What is not is just our subjective experience. What was originally a Physics problem is now an *anthropic" problem, similar to what you would get if you were to copy & paste humans, or do mind uploading. The more interesting mystery in my opinion is more about why the Born statistics are the way they are, instead whatever else we could imagine them to be.