On 06/08/2010 11:08 PM, [email protected] wrote: > The QM problem here is that a "wave function" is NOT a physical reality. It > is a > mathematical equation which we use to *describe* the state of a system *to the > best of our knowledge at the time*. When we make a real observation of the > real > physical system, our *knowledge* about it changes , and hence we need to use a > different equation. The wave function is said to "collapse" but all that > collapse really tells us is that we now know more about the system than we did > previously ... >
I don't think that's quite right. You've described the "hidden gears" model of QM and my impression is a superposition of states is more than just simple state which we don't happen to know at present. For a possibly overly simplistic example, consider a single photon in a beam of non-polarized light. Let it encounter, and pass through, a vertical linear polarizer. We have now measured a single parameter of its state -- and, if it got through the polarizer, we have found that its polarization is vertical (/exactly/ vertical). Before we sampled it, its polarization was described by a superposition of states, with all polarization angles being *equally* *likely*. Yet, since half the time a nonpolarized photon will get through the polarizer, after we sample it we would conclude that there was actually a 50% chance that it was vertically polarized. Next consider a beam of incoherent unpolarized light passing though a polarizer. Note the before/after difference: Before the beam encounters the polarizer, *all* polarization angles are equally likely for each photon in the beam. Yet, after it passes through a polarizer, we find that HALF the photons in the beam (the ones which passed through the filter) are -- and, apparently, WERE -- *vertically* polarized. By the act of *measuring* the polarization, we seem to have retroactively changed the beam from a collection of randomly polarized photons to a 50/50 mix of vertically and horizontally polarized photons.
