Seasons

Dear Psimagus:
you write:
It is psi that gives the probability, and this is never a negative value, thus cannot act in true "wave" fashion to exhibit destructive interference.
Psi gives the probability, but it isn't itself the probability. One gets the probability of (e.g.) the particle's being in a given spatial region by integrating Psi*Psi, the square of the modulus of (normalized) Psi, over the region [Psi* being the complex conjugate of Psi (yes, Bev, I am a great fan of complex conjugation)]. This gives a positive real number betweeen zero and one, as desired for a probability. Psi itself, however, is not so restricted. In fact, Psi typically takes on complex values. Therefore, there is no problem with Psi exhibiting characteristically wavish phenomena such as diffraction and interference.

I have been trying to understand a tiny bit of what you are talking about..I looked up QM for dummies and ran into this

To take the two-slit example, we never see electrons dematerialize, or rippling through something, we just find it necessary to think that they do to explain the pattern that we see on the screen. If we deliberately try to observe where the electrons go, we see them as particles somewhere else, but the interference pattern disappears. In effect, the problem is that we cannot say what the particles look like only when they cannot be seen.

Now this is an uncomfortable thought, because all our instincts tell us that particles must be somewhere, even when we cannot see them. But if quantum mechanics can accurately describe all the information we can ever obtain about the outside world, perhaps we are simply being greedy to ask for anything more. The headline "Physics Fails to Describe Events That Cannot Be Observed" is, again, rather lacking in impact.

So ..where is the somewhere else?

Actually, there is another option I could take, which as far as I can see is not inconsistent. I could have it both ways - be a realist and still claim that particles have no trajectories at all. But I would have to treat position values different from the others, and this would seem to be rather ad hoc. Or maybe not - I'm thinking now that I should retreat (for the time being) to agnosticism on this point until I have investigated in more detail the advantages and disadvantages of various options.
If I had to take a stand this moment, though, I would say that being a realist doesn't force me to believe that there is always a fact of the matter concerning the eigenvalues of some complete commuting set of observables.

So, I must be dragged, kicking and screaming, back to that quasi-orgasmic (according to Bev, message 3917) state of working on a problem. Ah, the sacrifices I make!

In fact, here's another problem: the eigenvalues of which complete commuting set of observables constitute the status? It seems that any selection must be arbitrary. Yes, I must clearly take lots of time to think this through.

Wait, no, the realist doesn't have to say which set, just that there is one! And it needn't include position! [whimpers]

A realist doesn't have to deny that our knowledge is limited. In particular, our ability to predict is limited by the probabilistic nature of Quantum Phenomena. [moans]

Oh, Prob123! You've come to join me!

Well, I haven't seen that particular discussion, but often in discussion of the two-slit experiment, the author says, "OK, now what happens if we close one slit? then we'll know that the electron went through the open slit, so we'll know exactly where it was at that moment."

Let me back up for a moment. If the electron were a classical or commonsense particle, it would travel in a straight line, and so what you would expect to see on the screen would be two little rectangles, one from particles that went through slit A, and one for particles that went through slit B. [Shudders with pleasure.] If we saw a single scintillum, it would be easy to tell which slit the particle that made it had gone through. [sighs]

But instead, we get a complicated pattern that looks just like a wave interference pattern, so if we think of classical or common-sense particles, tiny and traveling in straight lines, it seems that most of them must have been radically deflected somehow.

So one might say, "Well, let's close slit A. Then we'll know, when we see a scintillum on the screen, that it went through slit A. Then we do the same for B. The total pattern ought to be just the sum of the patterns for A-only and for B-only."
But no! When you close one slit, you get just what you would expect from a classical particle going through such a slit: vrtually all the hits are in a rectangular region of the scren, from which one could see the source. (We make the slits large enough so there is no significant diffraction effect.) The same thing happens when we close slit A and open slit B. The pattern with both slits open is in no way the sum of the patterns obtained with single slits open! [breathes heavily]

So I would guess that what they mean when they say it goes somewhere else is, that an electron goes somewhere else on the screen, when only one slit is open, than it does when both slits are open.

Is this what you are looking at?

http://infophilia.blogspot.com/2006/11/quantum-mechanics-for-dummies-1-wave.html

Oh...the interference pattern also goes away if you put a detector on one slit, even though the detector is of a kind that allows the electron to pass.

The site I cited on my previous post has a (rather small) picture of the two-slit experiment.

I just looked at, and rather like, the Wikipedia article on the two-slit experiment:

http://en.wikipedia.org/wiki/Double-slit_experiment