Are probabilities always and necessarily positive-definite? I'm asking this because there is a thread, started by Dirac and Feynman, saying the only difference between the classical and quantum cases is that in the former we assume the probabilities are positive-definite.
Thus, speaking of MWI, we could also ask: what is the joint probability of finding ourselves in a universe alpha and of finding ourselves in a universe beta, which is 180 degrees out of phase with the first one (whatever that could mean)? s.
