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ASEI
2006-Jan-11, 03:19 AM
Quantum physicists and people who can actually do the math:

I have a question about quantum scale objects. I've been trying to come up with a workable conceptual model for some time now. Apparently, observation does not allow me to visualize quantum objects as having, even in theory, certain properties which are well defined in the macroscopic world (position and momentum to name two). Apparently the macroscopic properties somehow average out of the quantum ones. So the particle model that underlies most of classical physics (object as point entity) is out.

The wave model appears to be more apt, but waves are usually considered to be continuous, and these quantum objects are discreet.

So, how close is this conceptual model to what really happens?:

Quantum objects, due to inherent uncertainty, not just uncertainty in initial conditions, don't travel along any single definite path and therefore they spread out in a wave-like manner. When they intersect with another object, there is a probability that they will interact at some point in their mutual fields. When they do, an event is generated, the original particle is gone, and the possibility that it can interact along any other part of it's wave is also gone. Hence the wave collapses. New particles propogate from the event?

HenrikOlsen
2006-Jan-11, 03:59 AM
It looks as good as most analogies, sounds like it explains things, but all details are wrong. :)
The problem is that unless you actually do the math, and use that as your concept, any attempt at conceptualising it will be limited by never having experienced anything acting remotely like that.

Ken G
2006-Jan-11, 05:00 AM
So the particle model that underlies most of classical physics (object as point entity) is out.

The wave model appears to be more apt, but waves are usually considered to be continuous, and these quantum objects are discreet.

Let's clarify this part first. The particle model is alive and well in quantum mechanics, it is far from "out". But the difference is, entities show up as particles, but they move about like waves. That is, if you want to know where they are likely to show up, you need wave mechanics, but when they do show up, they are seen to be pointlike particles. The same is actually true at all scales, even bowling balls, but when the wavelengths are ridiculously short we get our macro concepts of motion.


Quantum objects, due to inherent uncertainty, not just uncertainty in initial conditions, don't travel along any single definite path and therefore they spread out in a wave-like manner. When they intersect with another object, there is a probability that they will interact at some point in their mutual fields. When they do, an event is generated, the original particle is gone, and the possibility that it can interact along any other part of it's wave is also gone. Hence the wave collapses.
Yeah, that sounds pretty close to me, but there is one important correction. Interaction with another quantum entity, such as another particle or a conservative field of some kind, does not collapse the wave function. You just end up with a more complicated superposition of all the things that might have happened. The same can be said for encountering a slit, which is why you get diffraction patterns. In order to collapse the superposition into a mixed state of statistically distinct results that don't interfere with each other, you need to do an actual measurement, i.e., you need to couple the system to macroscopic entities that bring along a huge baggage of uncontrolled noise contributions. These noise contributions are really what collapses the wave function, because all hope of reconstructing the superposition of phase information is lost. When phase information is lost, you have collapsed the wave function-- you can then treat all the possibilities statistically instead of quantum mechanically.

Bad jcsd
2006-Jan-11, 01:46 PM
The collapse of the wavefuncion is completely independent of the unitary evolution of the wavefunction. Collapse occurs when a measurement occurs, but measuremnt cannot be expalined in terms of unitary evoltuion.

Decoheremce is not collapse as whilst any measuremnt made on our macroscopic system will not show our 'orignal' system in a superpostion of states, the system as a whole is still in a superpostion of states.

So in conclusion, collapse cannot be explained by how the wavefunction of one object interacts with that of another.

Ken G
2006-Jan-11, 04:55 PM
That's because when you are collapsing one wavefunction, the object doing the collapsing is macroscopic and has no discernable wave function. This is the whole point, a macro system has too much noise in it to keep track of a coherent wave function, and so to couple to such a thing is to lose all coherences and become a mixed state. A mixed state is the statistical analog of a collapsed wave function, and when our consciousness interacts with the mixed state, it becomes a definite state. This final bit is the only part that is not describable in detail, it's the "roll of the dice".

Bad jcsd
2006-Jan-11, 05:24 PM
That's because when you are collapsing one wavefunction, the object doing the collapsing is macroscopic and has no discernable wave function. This is the whole point, a macro system has too much noise in it to keep track of a coherent wave function, and so to couple to such a thing is to lose all coherences and become a mixed state. A mixed state is the statistical analog of a collapsed wave function, and when our consciousness interacts with the mixed state, it becomes a definite state. This final bit is the only part that is not describable in detail, it's the "roll of the dice".

My point is that matmetically going from a pure state to a mixed is still unitary whereas collapse is a projection which is not unitary so the wavefunction (of the system as a whole-which does in principle exist and infact it's an assumption of decoherence that it exists) can't be said to have truly collapsed, there's still superpostion even though there are no intereference effects.

Ken G
2006-Jan-12, 04:00 PM
I see what you mean, though I still think that a wave function of a macro system that includes noise is a useless concept. Our inability to describe the noise implies that we should just replace the wave function with a classical description.

hhEb09'1
2006-Jan-12, 08:16 PM
I see what you mean, though I still think that a wave function of a macro system that includes noise is a useless concept. Didn't it produce Schrödinger's Cat? :)

As well as the whole menagerie of other popular Schrödinger objects, including the universe itself? or am I thinking of something else?

Ken G
2006-Jan-13, 01:11 AM
Schrodinger's Cat is exactly what I'm talking about, it is a meaningless problem.

HenrikOlsen
2006-Jan-13, 09:02 AM
Schrödinger's Cat is not a meaningless problem, it's a thought experiment he made to tell his colleges that their attempts at getting from their quantum effects to the macroscopic world was handwaving.
That you call it meaningless just shows that they still haven't gone past that point.

Ken G
2006-Jan-13, 02:55 PM
Well, I mean meaningless from the perspective of learning or using quantum mechanics. It must have had use as a metaphysical aid for those pondering the philosophical issues, but as you say, it is an exercise in proving the limitations of quantum mechanics. Unfortunately, it is often described as a fundamental paradox between the quantum and macro worlds, and that is just what it is not. Quantum mechanics is perfectly aware of its own limitations in this regard. Due to the fact that macro objects do not have usable wave functions because you cannot keep track of all the noise and thus you cannot keep track of the quantum coherences, a wave function is a perfectly useless concept for a macro system like a cat. This is the point of the paradox-- to prove the uselessness of a certain concept at the macro level. Its usefulness is to prove the uselessness of something, which is why I consider it support for my position, not a refutation. There may be some situations where, with great care, you can apply QM to macro objects, like in LIGO or some such thing, but I'm sure it's very tricky.

hhEb09'1
2006-Jan-14, 08:15 PM
It must have had use as a metaphysical aid for those pondering the philosophical issues, but as you say, it is an exercise in proving the limitations of quantum mechanics. Unfortunately, it is often described as a fundamental paradox between the quantum and macro worlds, and that is just what it is not. Quantum mechanics is perfectly aware of its own limitations in this regard. Part of that awareness came from similar exercises, though. I wouldn't call the concept of Schrodinger's Cat useless, no matter how much fun I make of it. :)

Ken G
2006-Jan-14, 08:21 PM
What I am saying is that many people report the Shroedinger's cat as if it was true, that you could have a cat in a superposition state of dead and alive if you coupled it to a quantum state. But a cat cannot be so coupled, because noise would enter in impossible ways to disentangle. The whole proposition is bogus, and merely proves that wave functions are not useful ideas for a cat.