
#19
Apr1211, 10:19 AM

P: 1,115

"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 supercomplicated 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. 



#20
Apr1211, 11:22 AM

P: 789

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. 



#21
Apr1211, 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. 



#22
Apr1211, 01:26 PM

P: 348





#23
Apr1311, 03:19 AM

P: 1,115





#24
Apr1311, 03:21 AM

P: 1,115





#25
Apr1311, 09:09 AM

Sci Advisor
PF Gold
P: 5,147





#26
Apr1311, 10:27 AM

P: 661





#27
Apr1311, 10:44 AM

P: 315

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. 



#28
Apr1311, 10:44 AM

P: 789

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. 



#29
Apr1311, 11:12 AM

Sci Advisor
PF Gold
P: 5,147

http://www.optics.rochester.edu/~str...ester/UR19.pdf 



#30
Apr1311, 11:58 AM

P: 1,115





#31
Apr1311, 12:16 PM

P: 661

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! 



#32
Apr1311, 12:38 PM

P: 1,115





#33
Apr1311, 08:48 PM

P: 789

Also, how do you explain the Wigner's friend variation of the Schroedinger Cat paradox? 



#34
Apr1311, 10:11 PM

Sci Advisor
P: 1,867

"Wigner's friend" was a philosophical musing that Wigner published in a popularscientific 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. 



#35
Apr1411, 09:03 AM

Sci Advisor
PF Gold
P: 5,147

2. What's to explain? There is no experiment. 



#36
Apr1411, 10:22 AM

P: 789




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