What causes wavefunction collapse?

[Delta Kilo]

The cat is an observer, yet from perspective of outside observer, it does not collapse the wave function of the radioactive atom. It merely goes into an entangled superposition state with it.

That is provided the cat is perfectly isolated from the outside environment, which is not usually the case and which would be extremely hard/impossible to achieve for macroscopic objects of reasonable size.
Penrose makes an argument that any macroscopic object in superposition would necesserily interact with other objects through gravity. And we don't have http://en.wikipedia.org/wiki/The_First_Men_in_the_Moon" to make the box out of.

 

[K^2]

Delta Kilo, the argument that there is no way to isolate the system is not particularly appealing, since we can always improve the isolation and reduce time scales.

But my point is that there is no conflict in this. We can't compare the views of the cat and the views of the outside observer and think they don't match. The merging of two views, requires them to be communicated to the same observer sort of like we need to parallelltransport vectors from one tangent space to another one before we can even define the concept of angles.

That's fine, but again, you are looking at collapse as a matter-of-perspective thing, and that's basically MWI.

 

[JordanL]

K^2: What I meant was that "realism" as discussed in EPR is, to my understanding, defined as predictability without measurement, (or at least without precise, definitive measurement).

It's my understanding that this is why Einstein was never much of a fan of QM in the first place, because he felt it pointed to an inherent unrealism of our universe.

But if I am misinformed, please correct me. That's why I'm here. :)

 

[DrChinese]

K^2: What I meant was that "realism" as discussed in EPR is, to my understanding, defined as predictability without measurement, (or at least without precise, definitive measurement).

The EPR elements of reality, yes that is as good a definition as you can get. If you can predict, without disturbing the system, the outcome of a measurement, then there should be a physical element of reality associated with the observable. A lot of people prefer other definitions, for reasons that elude me. Kinda like slicing hairs to me. But the EPR definition is strong enough to have survived anyway.