On the origin of probability in quantum mechanics
http://arxiv.org/abs/1110.0549
I give a brief introduction to many worlds or "no wavefunction collapse" quantum mechanics, suitable for non-specialists. I then discuss the origin of probability in such formulations, distinguishing between objective and subjective notions of probability.
Here's what I say in the conclusion.
Decoherence does not resolve the collapse question, contrary to what many physicists think. Rather, it illuminates the process of measurement and reveals that pure Schrodinger evolution (without collapse) can produce the quantum phenomena we observe. This of course raises the question: do we need collapse? If the conventional interpretation was always ill-defined (again, see Bell for an honest appraisal [1]; Everett referred to it as a "philosophical monstrosity''), why not remove the collapse or von Neumann projection postulates entirely from quantum mechanics?
The origin of probability is the real difficulty within many worlds interpretations. The problem is subtle and experts are divided as to whether it has been resolved satisfactorily. Because the wave function evolves entirely deterministically in many worlds, all probabilities are necessarily subjective and the interpretation does not require true randomness, thereby preserving Einstein's requirement that outcomes have causes.
[1] J.S. Bell's famous article Against Measurement.
