[luk32]

Ok imagine you are seeing a screen and from behind you is coming a photon. How do you know whether there was a double slit behind you or not for a single photon?

Those are two situations that will lead to the same outcome and you don't know which happened.

Also reverse is possible. You have the same situation evolving into two different situations with double slit and interference.

[whatshisface]

You've just described the motivation for multiverse: Copenhagen has to stick in a concept of "measurement" that projects wavefunctions onto specific states. As a projection operator it's idempotent and many-to-one.

However, this mechanism is not necessarily a law of physics: we can explain all of the same results without it. So, deciding whether or not wavefunctions "actually" collapse is 100% philosophy.

[kgwgk]

While you no longer need wave functions to collapse, you need universes to branch out. The latter interpretation doesn't seem "better" than the former as far as being a reasonable description of the real world goes. And of course for all practical purposes they are equivalent.

[whatshisface]

The universes don't need an additional mechanism in order to branch out, wavefunctions will already do this if they're left alone (to be clear about what I'm saying, I mean that if you isolated some particles in a box they would start doing multiverse). The natural behavior of wavefunctions is to do multiverse, and if you want something else you have to introduce an additional mechanism that collapses them.

[kgwgk]

And what does it mean for a few-particles isolated system to start doing multiverse? Cannot you just describe it with a wave function without any branching out? The standard interpretation of quantum mechanics doesn't have any issues with the evolution of an isolated system, it doesn't require continous wave function collapse.

[whatshisface]

The isoated particles will, according to accepted physics, do everything that MV says we are doing. The discrete branches are a textbook illustration; it's really more of a continuous thing. There's still only one wavefunction, it just behaves in a way that can be compared to branching.

In a nutshell the idea of multiverse is that the entire universe evolves as an isolated system, without any wavefunction collapse.

[kgwgk]

I know, but you need to reconcile that with the universe we observe where looking at the system will find it in a definite state and not in a superposition. What is the multiverse response precisely? How is "branching magic" an improvement over "collapsing magic"?

[kgwgk]

Which existing mechanism gets you from that metaphysical multiverse to the physical universe?

[whatshisface]

The "multiverse" would be one big wavefunction (quite physical), and each "universe" would be a partition of the wavefunction in phase space. In this sense "multiverse" and "universe" would just be names for different parts of the wavefunction, like "The Rocky Mountains," and "Kona," would be names for features on a topo map.

[kgwgk]

Which basis is used to partition the wavefunction?

If you are going to tell me that it is according to the eigenvalues of the observable operator it's not that different from saying that there is a collapse on one of the eigenvalues of the measurement.

And the question remains for your "isolated particles in a box doing multiverse". How is the partition of the wavefunction done if there is no preferred basis?

Edit: Maybe in your interpretation the only "physical" thing is the universe described by its wave function and those infinite multiverses are just mathematical "projections" of that wavefunction. But then how can a mathematical operation without any physical substrate explain anything about the physical world?

[kgwgk]

You don't, but the photon does ;-)

Seriously, what do you mean by "will lead to the same outcome"?