I wonder if someone with a good understanding of QM can answer this question, which has been troubling me. Let us imagine the case of a single particle, and let us imagine we know its position at time 0. Now my understanding of the evolution of the wave function for the position of the particle, according to a Deutsch-ian version of MWI, is roughly as follows:
The position wave function of the particle represents the distribution of universes in which the particle is at a certain position. As an observer, I don't know which of those universes I am in, until I make a measurement of the particle's position. There is one eigenvalue for each universe (or better, branch in the multiverse), and the probability of that eigenvalue is a measure of the underlying universe 'count', or proportion. So far so good. The explanation seems coherent, even if we haven't explained the distribution of those universes. However, I am puzzled by the case of spins. Consider a set-up in which a photon is polarized in the z direction, so that we know that the particle will, with probability 1, pass through another polarizer also oriented in the z direction. However what of the situation where the second polarizer is oriented at 45 degrees to the first one? In that case, the probability is 0.5 that the photon will pass through. If it does, then obviously the probability is 1 that it will also pass through a third polarizer also oriented at the same angle. So what is going on in the multiverse in this scenario? Clearly, prior to the photon hitting the 45 degree polarizer, it can't be the case that half the universes have photons polarized at 45 degrees to the z axis, because in fact 100% of them are polarized in the z direction. Yet after the hitting the polarizer, half do. So in this case the discontinuity between quantum state and measurement, which MWI saves us from in the case of a continuous variable like position, seems to persist. What is going on at the point of the photon's interaction with the polarizer in an MWI account? Clearly, the multiverse differentiates into two branches corresponding to the two spin eigenvalues, each with measure 0.5. But does MWI have anything to say about the the weirdness of the jump between the z polarization and the 45-degree polarization? -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
