quote:Originally posted by Destineer: This can't be right. One of the assumptions of scattering theory is that a lone particle can interact with a single other particle without collapse.
I did say 'can be', rather than 'is necessarily'.
quote:The idea that any interaction is a measurement is patently false. After all, electrons are constantly interacting with the atoms they orbit even though they have to be in a superposition of position states in order to occupy energy levels.
Well, yes, but the nucleus doesn't interact with the position information of the electron, it interacts with the momentum information. While an electron can be in a superposition of energy states, it will rapidly make up its mind and choose one of them. Now, I do not hold this as holy writ, but I suggest that only the relevant quantum numbers will collapse in an interaction.
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quote:You are more-or-less correct about the two-slit experiment; the wave behaviour is caused by the slits, right enough, but it doesn't make sense to ask whether it exists without an observer. This is a bit like the old tree falling in a forest with nobody to hear; the proper answer is "your question is meaningless", or as the Buddhists very succinctly put it, "Mu!" Again, though, the term 'observer' is a little misleading, since it seems to imply consciousness. Anything capable of interacting with the photon coming out of the slit is a perfectly valid observer.
Does that answer your question?
If the first quoted paragraph means that the detector is the observer, then yes, it does. Thanks.
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quote:Well, yes, but the nucleus doesn't interact with the position information of the electron, it interacts with the momentum information.
I see what you're saying here, but there is a very clear sense in which the nucleus is interacting with the electrons's position. After all, the electron interacts with the nucleus via a potential V(x), so the strength of the interaction depends pretty straightforwardly on position.
You are right insofar as any time-dependent solution of the Schrodinger equation for an atom will quickly evolve into something very close to the lowest energy level. This isn't the same as an energy measurement taking place, however.
quote:Would someone explain to me what is meant by "collapse of wave function"? We never discussed this particular thing in any of my college classes, and I have never heard of it elsewhere. The uncertainty principle and the concept of subatomic particles existing in more than one place at the same time according to a pattern of probability I understand.
The collapse is what happens when we measure a quantum system (whatever that means). A system that was previously in a mixture of different states (different positions, for instance, like you mentioned yourself) seems to take on a single state all of a sudden when we observe it. So when we look for the particle, we always find it in a particular place even though it acts like it was spread out in a wave before we looked. That's all we mean by collapse.
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quote:The collapse is what happens when we measure a quantum system (whatever that means). A system that was previously in a mixture of different states (different positions, for instance, like you mentioned yourself) seems to take on a single state all of a sudden when we observe it. So when we look for the particle, we always find it in a particular place even though it acts like it was spread out in a wave before we looked. That's all we mean by collapse.
I don't know why, but that makes me laugh (in a good way.) I had an image in my mind of naughty little particles breaking the "natural" laws of reality by existing in multiple places at once, havin' a big ol' party. Then you shine a flashlight on them and they suddenly look all meek and behave. It made me chuckle.
Here is my question--how can we know a particle acts in this way without observing it? Isn't *that* a paradox? Are we able to observe it without *really* observing it? I assume that no one really believes that the unobserved cat is in multiple states of dead and alive, do they?
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Bev, have you heard of the two-slit experiment? It's the clearest example I've seen for non-physicists.
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It doesn't really answer that question, but it does tell you how we can know a particle acts in this way without observing it. Because the mere presence of the second slit produces the wave interference pattern, even if individual particles are fired one at a time, we know that the particle must be interfering with itself somehow.
Anyway, the collapse happens when the particle passes the detector. when a detector is set up in the slits, the wave interference pattern disappears.
Question for those in the know: does there have to be a detector in both slits, or does a single detector in one slit cause the interference pattern to go away.
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quote:Here is my question--how can we know a particle acts in this way without observing it? Isn't *that* a paradox?
That's what opens the way, in principle, for approaches like the Bohm theory I was talking about earlier. But it was shown by Bell's inequality that any theory in which the particle acts like a particle all the time will be very strange, involving things like faster-than-light causation.
quote:I assume that no one really believes that the unobserved cat is in multiple states of dead and alive, do they?
Wigner did, but he was a pretty big weirdo. Like KoM and myself, most people think that whatever the correct theory of measurement turns out to be, it will have to entail that the cat is an observer.
quote:Question for those in the know: does there have to be a detector in both slits, or does a single detector in one slit cause the interference pattern to go away.
The latter. If you don't detect it going through slit 1, you can tell it must have gone through slit 2.
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quote:Anyway, the collapse happens when the particle passes the detector. when a detector is set up in the slits, the wave interference pattern disappears.
Huh. This I didn't know (or didn't remember). What are the theories on why this happens? What exactly is the detector? Does it bombard the electron with photons? That is freaky weird.
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