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A Copenhagen Bug

by IllyaKuryakin
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Q-reeus
#19
Apr12-11, 10:19 AM
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Quote Quote by unusualname View Post
All a bit complicated, easier to just accept the universe has evolved along a definite path, but that path is probabilistically defined so we can't know what it is without "looking"...
Admittedly even that entry was somewhat long, but a passage towards the end is relevant:
"To say a wave function is collapsed, you must have a wave function in the first place. A mixture is not a wave function, it is a mixture of wave functions. Classically, it is the mixture that matters, not the quantum mechanics of the wave functions-- the evolution of a mixture is a classical evolution, what the individual wave functions are doing gets lost (like a thermodynamic treatment of an ideal gas where we are not a whit for what any given particle is actually doing, only the generic possibilities for what they are allowed to do). When a cat is a super-complicated statistical average of a bunch of possible individual wavefunctions, then it is a classical object, not a quantum mechanical one."
How much more a star! May be wrong, but likely the OP got his que from that famous exchange between Einstein and Bohr where Einstein is quoted "I like to think that the moon is there even if I am not looking at it." An hyperbole, but combine that with the fact Bohr's Copenhagen viewpoint has almost unanimously considered to have won against the EPR argument, and hey presto - the Moon depends for it's existence on being observed. Or not.
The past has already happened, it cannot be changed.
There have been a few recent threads where 'backward causation' as apparently confirmed in delicate experiments was discussed, but I would agree it has no relevance to 'the world at large'.
Rap
#20
Apr12-11, 11:22 AM
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Quote Quote by Q-reeus View Post
I gained a lot from a (very long) thread on Schrodinger's cat here: http://www.physicsforums.com/showthr...=468101&page=6, in particular #96 helped to clear up some of the misconceptions that abound. Just substitute 'star' for 'cat'.
Yes - a good thread, although not completely resolved. I am still scratching my head about some of those points.

The wave function is not a physical entity with totally objective existence. Its a mathematical tool. It's an encoding of what has been measured, and along with QM theory which allows us to predict probabilities of future observations. The wave function collapses when our knowledge (as the result of measurement) changes. 200 years ago there were no wave functions, because QM had not been developed. To a duck observing a star there is no wave function, because the duck does not understand things in terms of QM.

All of the apparent paradoxes are resolved by this interpretation, which is, I believe, the Copenhagen interpretation. But after the above thread, its clear that things are not this simple.
unusualname
#21
Apr12-11, 12:45 PM
P: 661
The point is that our ability to observe might not be that big a deal, the moon is there but we can't show this by calculations on paper, these calculations can only predict probabilities (assuming a complete physical theory which includes quantum gravity) we have to "look" to see if the moon is there. If no one "looks" that doesn't mean the moon is not there it just means that there are (unlikely) probabilistic evolutionary states of the universe where the moon suddenly disappears, they are so unlikely that they have an average expectation time in excess of googleplexes of the order of the age of the universe, so we discount them for scientific purposes which involves what we can reasonably observe in this universe.

What we are doing physically when we "observe" the universe might be pretty mundane (literally!), especially if we take into account that what created us (evolution) couldn't even manage radio communication technology.

The modern interpretation relies on decoherence to explain away the macroscopic superpositions, but similar mathematics can explain why a "feynman path" might actually be the ontological evolution.
Upisoft
#22
Apr12-11, 01:26 PM
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Quote Quote by IllyaKuryakin View Post
If a tree falls in the woods, and no one was there to hear it, does it make a sound? QM says no.
Actually it says yes. The woods is still there to make the observation/measurement.
Q-reeus
#23
Apr13-11, 03:19 AM
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Quote Quote by Rap View Post
...The wave function is not a physical entity with totally objective existence...
I tend to agree Rap, but try telling that to a MWI advocate!
Q-reeus
#24
Apr13-11, 03:21 AM
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Quote Quote by unusualname View Post
..If no one "looks" that doesn't mean the moon is not there it just means that there are (unlikely) probabilistic evolutionary states of the universe where the moon suddenly disappears, they are so unlikely that they have an average expectation time in excess of googleplexes of the order of the age of the universe,...
Amazed that however tiny, there is a finite probability for such an event. Mind explaining what that implies re COEM (conservation of energy/momentum) - is it violated via an extremely unlikely HUP fluctuation, or do we expect some compensatory shuffling of matter elsewhere that maintains an exact overall COEM at all times? Some folks think COEM is only statistical in QM, others believe it is always obeyed.
DrChinese
#25
Apr13-11, 09:09 AM
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Quote Quote by Rap View Post
The wave function is not a physical entity with totally objective existence. Its a mathematical tool.
Are you sure about that? You say that, but there is plenty of evidence that would say that it is objectively real. After all, you can manipulate probabilities through spacetime as if they are physically real in any sense of the word "real". In fact, I would say that the evidence supports the idea that the wave function is more "real" than the unmeasured properties of the same particle.
unusualname
#26
Apr13-11, 10:27 AM
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Quote Quote by Q-reeus View Post
Amazed that however tiny, there is a finite probability for such an event. Mind explaining what that implies re COEM (conservation of energy/momentum) - is it violated via an extremely unlikely HUP fluctuation, or do we expect some compensatory shuffling of matter elsewhere that maintains an exact overall COEM at all times? Some folks think COEM is only statistical in QM, others believe it is always obeyed.
I think COEM would not be violated, certainly not globally (ie in the entire universe). Just like COEM is not violated when a single atom quantum tunnels across a potential barrier, now imagine the probability of every single atom in the moon (or any macroscopic part of it) suddenly quantum tunnelling outside the solar system, this is vanishing small, and we ignore it in scientific models, just like we ignore the poincare recurrence theorem when doing usual statistical physics.
dm4b
#27
Apr13-11, 10:44 AM
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Quote Quote by DrChinese View Post
Are you sure about that? You say that, but there is plenty of evidence that would say that it is objectively real.

You piqued my interest - what is some of that evidence you are thinking of? I guess I always leaned towards giving the wave function some "reality", but never really had a solid ground for doing so.
Rap
#28
Apr13-11, 10:44 AM
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Quote Quote by DrChinese View Post
Are you sure about that? You say that, but there is plenty of evidence that would say that it is objectively real. After all, you can manipulate probabilities through spacetime as if they are physically real in any sense of the word "real". In fact, I would say that the evidence supports the idea that the wave function is more "real" than the unmeasured properties of the same particle.
There was a very good discussion on this at http://www.physicsforums.com/showthr...=468101&page=6, but its a long thread.

I am, of course, not absolutely sure, its one viewpoint in the many attempts to deal with the interpretation of waveform collapse. I find it to be the best. It disposes of many of the problems of waveform collapse, wondering when and how the collapse occurs. In particular, referring to the above thread, it deals with the problem of "Wigner's friend". Wigner's friend is a scientist inside the box with the cat and dealing with what he observes, using wave functions to describe the state of the cat. How does the observer outside the box deal with this? Is Wigner's friend in a superposition of states, some of which involve calculations of the wave function for a dead cat, some of which involve QM calculations for a live cat?

What is it like for the scientist inside the box to be in a superposition of states? Once you realize that the wave function is a calculational tool, all of this makes sense. The wave function that Wigner's friend uses is different from the wave function that the outside observer uses, because they have access to different information. It is a good demonstration of the subjectivity of the wave function. Only when all observers have access to the same information will they agree on the wave function. This is always implicitly assumed, but is not necessary, as this example demonstrates.
DrChinese
#29
Apr13-11, 11:12 AM
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Quote Quote by Rap View Post
...I am, of course, not absolutely sure, its one viewpoint in the many attempts to deal with the interpretation of waveform collapse. I find it to be the best. It disposes of many of the problems of waveform collapse, wondering when and how the collapse occurs. In particular, referring to the above thread, it deals with the problem of "Wigner's friend". Wigner's friend is a scientist inside the box with the cat and dealing with what he observes, using wave functions to describe the state of the cat. How does the observer outside the box deal with this? Is Wigner's friend in a superposition of states, some of which involve calculations of the wave function for a dead cat, some of which involve QM calculations for a live cat?

What is it like for the scientist inside the box to be in a superposition of states? Once you realize that the wave function is a calculational tool, all of this makes sense. The wave function that Wigner's friend uses is different from the wave function that the outside observer uses, because they have access to different information. It is a good demonstration of the subjectivity of the wave function. Only when all observers have access to the same information will they agree on the wave function. This is always implicitly assumed, but is not necessary, as this example demonstrates.
If the wave function were real, then you could manipulate it physically. And there is every evidence that you can. You can use a polarizing beam splitter to separate the H and V portions, then do tricks to them, and later recombine to restore the original beam. The recombined beam having attributes that the separate beam components would not have. That seems like more than a mathematical device. (However, this does little to explain the phenomenon of collapse itself.) See Eberly:

http://www.optics.rochester.edu/~str...ester/UR19.pdf
Q-reeus
#30
Apr13-11, 11:58 AM
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Quote Quote by unusualname View Post
I think COEM would not be violated, certainly not globally (ie in the entire universe). Just like COEM is not violated when a single atom quantum tunnels across a potential barrier, now imagine the probability of every single atom in the moon (or any macroscopic part of it) suddenly quantum tunnelling outside the solar system, this is vanishing small, and we ignore it in scientific models, just like we ignore the poincare recurrence theorem when doing usual statistical physics.
Clearly in the context of 'massively moving Moon' this is purely an academic exercise, but my view of tunneling as per Schrodinger eq'n is that while say a bound atomic electron can have a finite probability of being found at any spacial location, it does so at the 'expense' of the rest of the atom's energy/momentum. In other words the overall system COEM places an absolute restraint on what is possible. This is not so - given enough time, anything is possible?
unusualname
#31
Apr13-11, 12:16 PM
P: 661
Quote Quote by Q-reeus View Post
Clearly in the context of 'massively moving Moon' this is purely an academic exercise, but my view of tunneling as per Schrodinger eq'n is that while say a bound atomic electron can have a finite probability of being found at any spacial location, it does so at the 'expense' of the rest of the atom's energy/momentum. In other words the overall system COEM places an absolute restraint on what is possible. This is not so - given enough time, anything is possible?
The "overall system" has to really extend to the entire universe, even if you're talking about a single atom, since it's not possible to isolate any single part of the universe for an idealised laboratory experiment. In practice we can get excellent approximations to isolated systems, but they are always approximations.

Eventually this discussion will lead to statements about supposed absolute laws and how applicable they are, but it will be fruitless since we do not really know the exact correct form of the absolute laws. We do not worry that the second law of thermodynamics contradicts poincare recurrence, since there is no conceivable macroscopic scenario where poincare recurrence can have more than the remotest possibility of being relevant. When the final laws are found it's posiible that we might see that the universe recurs eternally on huge timescales, but it's pointless to worry about those sort of questions before we have established the basic fundamental laws.

We're still struggling to accept the probabilistic nature of QM, over 80 years after it was discovered!
Q-reeus
#32
Apr13-11, 12:38 PM
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Quote Quote by unusualname View Post
...Eventually this discussion will lead to statements about supposed absolute laws and how applicable they are, but it will be fruitless since we do not really know the exact correct form of the absolute laws. We do not worry that the second law of thermodynamics contradicts poincare recurrence, since there is no conceivable macroscopic scenario where poincare recurrence can have more than the remotest possibility of being relevant. When the final laws are found it's posiible that we might see that the universe recurs eternally on huge timescales, but it's pointless to worry about those sort of questions before we have established the basic fundamental laws...
I take the point, but darn, was so hoping to lever this into a heated discussion about the 'Boltzmann Brains Crisis' some ivory tower academics worry so much about!
Rap
#33
Apr13-11, 08:48 PM
P: 789
Quote Quote by DrChinese View Post
If the wave function were real, then you could manipulate it physically. And there is every evidence that you can. You can use a polarizing beam splitter to separate the H and V portions, then do tricks to them, and later recombine to restore the original beam. The recombined beam having attributes that the separate beam components would not have. That seems like more than a mathematical device. (However, this does little to explain the phenomenon of collapse itself.) See Eberly:

http://www.optics.rochester.edu/~str...ester/UR19.pdf
When you say "If the wave function were real, then you could manipulate it physically", do you mean that if it were not you could not?. I don't see a problem with manipulating the wave function by performing certain measurements. The measurements alter your knowledge, which alters the encoding of that knowledge - i.e. it alters the wave function.

Also, how do you explain the Wigner's friend variation of the Schroedinger Cat paradox?
alxm
#34
Apr13-11, 10:11 PM
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Quote Quote by Rap View Post
Also, how do you explain the Wigner's friend variation of the Schroedinger Cat paradox?
The same way Wigner did: They're not in superpositions because they're interacting with the environment and their wave functions have decohered.

"Wigner's friend" was a philosophical musing that Wigner published in a popular-scientific book of his. To whatever extent he took those ideas seriously himself, he later abandoned them.
Because he later not only embraced decoherence as the answer to how the wavefunction "collapse" occurs, but actively contributed to the research on it.
DrChinese
#35
Apr14-11, 09:03 AM
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Quote Quote by Rap View Post
1. When you say "If the wave function were real, then you could manipulate it physically", do you mean that if it were not you could not?. I don't see a problem with manipulating the wave function by performing certain measurements. The measurements alter your knowledge, which alters the encoding of that knowledge - i.e. it alters the wave function.

2. Also, how do you explain the Wigner's friend variation of the Schroedinger Cat paradox?
1. I have 50% probability A and 50% probability B. As separate entities, they may have definite polarizations H (if A) or V (if B). But if I recombine them, I restore a superposition. Yet that state cannot be constructed from an H or V individually. So the thing I am manipulating in each portion is not a photon in and of itself. It is a wave state.

2. What's to explain? There is no experiment.
Rap
#36
Apr14-11, 10:22 AM
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Quote Quote by DrChinese View Post
1. I have 50% probability A and 50% probability B. As separate entities, they may have definite polarizations H (if A) or V (if B). But if I recombine them, I restore a superposition. Yet that state cannot be constructed from an H or V individually. So the thing I am manipulating in each portion is not a photon in and of itself. It is a wave state.
When we all have access to the same information, we all agree on the wave function, and it appears to have an objective reality. Its only when two people have different information, and this is not the result of incomplete measurements by one or the other, that you need two different wave functions to encode knowledge. The only way I know to do this is to make one observer part of the system being observed by the second observer, as exemplified by Wigner's friend.

Quote Quote by DrChinese View Post
2. What's to explain? There is no experiment.
I don't understand - it is a thought experiment. Are you saying thought experiments are invalid? Are you saying that I did not explain it correctly or completely?


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