Title: What does it mean to say a wave-function collapses? - Quora
Headings: What does it mean to say a wave-function collapses?
What does it mean to say a wave-function collapses?
Let us talk about spin of an electron:
Clifford Chafin
Doug Mason
Doug Morris
D. B. G. Tilak
Anonymous
Priyam Chaudhuri
Stephon Henry

Content: It's important to realize that in Bohr and Heisenberg's interpretation of quantum mechanics (Copenhagen), the wavefunction is something that represents epistemic knowledge about a system, it was never intended to represent the objective "state" of a system. Von Neumann also emphasized that when the "collapse" occurs is arbitrary, and doesn't refer to any physical process. You can use many different wavefunctions to represent a given system, some of which involve superpositions and some of which don't, as long as they make the same predictions in the end. Of course, in quantum mechanics there is a minimum amount of uncertainty required for some observables whereas in classical physics there is not. In 1956, a precocious student of John Wheeler's, Hugh Everett, proposed that perhaps there is a "universal wavefinction" that provides a God's eye view, such that any specific wavefunction used by a particular observer just contains a subset of all the information contained in the universal wave function. If that's true, then this universal wavefunction could be regarded as objectively real, rather than epistemic. And it is clear that if you accept the wavefunction as real, as Everett did, it never collapses. As interesting as this proposal is, it's still regarded as controversial among most physicists. Some believe in it, others do not, and still others shrug the question off as metaphysical and essentially meaningless. But it doesn't affect the question of whether wavefunction collapse is a physical process--just about everyone on both sides agrees that it is not.