[sam-2727]

Interesting, although I struggle to see how this could be. The collapse (from our point of view, at least) of the wave function is pretty necessary in the math of QM. You can only observe the eigenstates of a wavefunction, which is what collapse is. To me, it seems that other theories are just disputing what the "collapse" fundamentally means. For example, Many Worlds theory (from my admittedly limited understanding) says that the wave function's eigenstates each become the new reality in different universes. Please correct me if I'm wrong here though.

[ravi-delia]

Decoherence is the normally proposed solution. Unlike collapse, it wouldn't be the only non-linear, non-unitary, discontinuous operator in all of quantum mechanics. It doesn't give Born's rule, but as long as we're allowed to derive Born's rule by saying "and the wavefunction obeys Born's rule" that's not a mark against it.

[lmm]

> The collapse (from our point of view, at least) of the wave function is pretty necessary in the math of QM.

The measurement results are mathematically necessary. The Copenhagen interpretation, that this means there is a physical collapse in the wavefunction, is not.

> To me, it seems that other theories are just disputing what the "collapse" fundamentally means.

If you substitute "collapse" for "measurement" this is pretty much true. What else could "different interpretations of a theory" mean?

[scotty79]

I think that QM is actually covering two very distinct things.

One is about what actually happens. This contains Shrodinger equation and similar things.

The other is about what results will we get if we poke the particles with macroscopic objects disturbing them beyond recognition. That's the all math where the word "measurement" is used.

Somehow we think the science of what happens to a frog when you poke it with a knife is a part of zoology. It's important, it might be even more important than zoology, but that's a different domain of science.

[ravi-delia]

That's all well and good in zoology, but since both the experiment and experimenter are quantum systems, QM has to explain any and all interactions between the two. Measurement can't be some extraneous thing to what's happening, since it too is happening. I think decoherence gets some points on that mark, I'd be shocked if anyone really thought collapse wasn't the product of some yet undiscovered (or at least unconsidered) aspect of quantum physics.

[jbay808]

> For example, Many Worlds theory (from my admittedly limited understanding) says that the wave function's eigenstates each become the new reality in different universes

That's perhaps the Narcissistic Many Worlds interpretation. Another way of thinking about Many Worlds is that there's only one universe, in which: after measuring a particle in a superposition of two states, you are now in a superposition of two states. Each of the superimposed you states thinks your instrument measured a single clear result.

The wavefunction of the universe goes on propagating as usual, including both of the superimposed yous, but within each of the superimposed states, that you thinks that it has witnessed wavefunction collapse.

[ravi-delia]

It annoys me that you really have to dig into Many Worlds for a while before someone says outright what you're saying here. I guess it's because to someone already familiar with QM, decoherence is more about explaining why the newly different portions of the wavefunction don't interact than that there's a superposition at all, but to the layman the takeaway should really be exactly what you wrote.

[akvadrako]

The main problem with the Copenhagen interpretation is it's too vague. Some say it up involves collapse as a physical process, some say it doesn't. Whatever collapse is in Copenhagen, it is fundamental.

In many worlds, collapse is a subjective and approximate phenomena, not something fundamental.