One nice thing about a smooth transition between the states measured at two different times is that it looks the same for both directions of time–in keeping with the “Time-Symmetric Theories” theme.

Actually, upon reconsidering what I just wrote, it occurs to me that weak measurements may actually give some information about what happens between the two “strong” measurements at t1 and t2, contrary to what I just said. I’m not certain that that’s the case–I’ll have to think more about weak measurements–but if it is, then the following items…[Read more]

Well, I don’t think so, but if I’ve misunderstood something I’d appreciate your enlightening me. The point is that if I make a measurement (or prepare a state) at time t1 and again at t2 > t1, with no intervening measurement, then I have no experimental evidence for *how* the system got into the state I observe at t2. The Copenhagen inter…[Read more]

Hello Ruth et al.,

Another way to answer Michael’s question, which I believe is equivalent to Ruth’s answer, is that not everything that is real is observable. The obvious example (from a realist perspective) is the wavefunction itself.

Hi Ken,

I had originally been trying to devise a variational principle (VP) with the desired characteristics essentially as a mathematical exercise, deferring the question of how to justify it physically. The Lagrangian viewpoint tells me that I should use the extremization of the action as my VP, so my current approach is to start with a given…[Read more]