[kovrik]

I was always wondering about MWI and couldn't find the answer to one question. AFAIK Everett never mentioned Many Worlds in his paper. What he was talking about was a Universal Wave Function. In that sense, 'world' is just a 'quantum system', not 'Universe'. At what stage we started calling it Many Worlds?

When scientists say Many Worlds, do they actually mean Worlds as physical 'parallel universes' that pop up into existence?

Or they only mean that those are probable outcomes of our measurement (probable histories) that never actually happened (only one of them happened - the one we end up being it)?

[gpsx]

The words "many worlds" is misleading to a lot of people. As Carroll says, when a "measurement" occurs, nothing special happens. It is just normal quantum mechanics.

I think in general people accept that there can be a wave function for a cat with the states alive and dead. Cats are much more complicated than that. Presumably they have memory and inside that memory can be things like perhaps the result of a electron spin meansurement experiment. One state of the wavefunction of the cat might have the memory that the electron result was spin up. Another state of the cat might have the memory that the electron result was spin down.

This scenario above is a cat watching a electron spin measurement. Afterwards, the electron does not collapse into a single state upon being observed. Instead, it is still in two states but it is correlated with the memory of the cat (or entangled with the cat).

The belief, at least as I see it and I assume others believe this too, is that the cat has a conscienceness for each state of the wave function. So there is a "conscienceness" or "cat" that thinks the electron was measured as spin up. And then there is another that thinks that electron was measured as spin down. This is where the term many worlds comes from, the fact that there are two "consciences" (well, many consciences). I can see why people would think that part is weird. But how else should it work?

I guess it all comes down to what is the experience of a person (or cat) having a wave function and being in multiple state at the same time (just like all other objects in quantum mechanics). We are not external observers to the world, we are a part of it and we have a wave function too. Or, it would be more correct to say we are a part of the wave function of the system. There is not a separate wave function for each thing. There is just one wave function.

[millstone]

In MWI, all Worlds are physically real and may interfere with each other.

[sliken]

Sounds like that would be a testable hypothesis, right?

How exactly would they interfere?

[millstone]

All QM interpretations require interference. This is demonstrated by the two-slit experiment (for example). In this experiment, an electron passing through the two-slit shield generates an interference pattern. But if you detect which slit the electron passes through, you destroy the pattern. Why?

In textbook/Copenhagen interpretation, we say that when the electron passes through the slits the wavefunction has not yet been measured. But with the detector in place, you collapse the wavefunction to an eigenvector of position. By the uncertainty principle, the momentum uncertainty is now very high, so the electron shoots off in a random direction and cannot be expected to follow any pattern.

In MWI, we start with the interference pattern. But adding the detector does not "collapse" anything - instead it introduces lots of degrees of freedom. The particle passes through both the left and right slits, but the degrees of freedom ensure there is no fixed relative phase delta between these possibilities. The detector turns waves into "static noise", and so any interference pattern is lost. Both options are realized, but cannot detect the other possibility: distinct Worlds.

The key point is that MWI has no special role for measurement. The left-slit and right-slit worlds interfere, but with the detector in place the pattern is destroyed: the interference is uncoordinated and averages to zero, so undetectable. Whereas in textbook/Cophenhagen QM we say that the measurement results in a single outcome.

[n4r9]

It partly depends on how "world" is defined, but generally: no. No one has come up with a way to test it against other interpretations (or indeed any interpretation against any other, with the possible exception of dynamical collapse models).

I had a discussion with someone on HN about this a while ago and realised that "world" or "split" isn't necessarily synonymous with "superposition". Rather, I believe that a split occurs when the superposition entangles with the environment and causes sufficient decoherence that there is negligible probability of (measurable) interference.

In principle you could imagine a thought experiment where you had a super-powered quantum machine which finely could control the quantum state of a large, isolated room. In that situation you could imagine someone in the room conducting an electron spin measurement and looking at the outcome, before the machine enacts a reversal of the room's quantum wave-function, thus causing the two copies of the person to interfere. If we ever reach that level of technology, it will be fascinating to see how the interpretation debate progresses.