(Transferred as a separate question from comments in Scott Aaronson’s gravitational decoherence question) Reversing gravitational decoherence
The modern answer seems to be that they never occur, and that therefore nothing causes them. This leads on (or back) to Everett and MWI. A closely related idea is that there is only one collapse, just as there is only one wave function, but it covers the whole universe, all time and all space.
An older answer, at least sometimes called Copenhagen, is that collapses are merely Bayesian updating, and hence do not occur as physical processes.
A third answer, associated with Wigner and von Neumann himself, is that the observer causes collapses by the act of observation.
GRW suggest that collapses occur gradually, due to a nonlinear modification of the Schrodinger equation.
I do not find these answers satisfactory. See related question, “Can you count collapses?”
I am interested in answers that do not modify QM, (unlike GRW), but (like GRW) nevertheless regard collapse as a real physical process that can be observed, and counted.
Can someone provide references to papers that discuss collapses that meet this criterion?
Are there other answers to the collapse question not mentioned above?