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Why does wavefunction collapse occur? 
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#1
Jan2612, 06:57 PM

P: 198

As I understand it, the term "collapse" is a little overexaggerated, but why is it that we measure things as points and not waves even though particles exist as waves?



#2
Jan2612, 08:30 PM

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PF Gold
P: 2,817

First, as Feynman points out, only particles exist, no detector has detected waves  what always appears is particles.
Secondly the issue of wave function collapse is interpretation dependent. It only exists if you think a quantum state has an external existence like say an electric field. You can simply view it as a device to calculate probabilities. You will find a discussion of this in Chapter 9 of Ballentine  Quantum Mechanics. Bottom line here is the assumption it has that kind of existence leads to all sorts of issues so its best not to interpret it that way. What is thought of as waves is simply that states sometimes have wave like solutions  but if you think of a state as a calculational device only then the so called waveparticle duality is rather moot. Look at it this way. Suppose you have a possibly biased dice then you would describe it by 6 positive numbers that add up to one  that would be its state. It doesn't have an existence 'out there'  it simply is a way of describing the likely occurrence of a certain face of the dice lying up. The same with a quantum state. When you observe the outcome of throwing the dice the state does not collapse  you simply make an observation. Looked at this way Shrodenger Cat, and other contrivances, is rather trivial  it has no more a mystery than tossing a dice. For my personal view of QM check out: http://arxiv.org/pdf/quantph/0101012v4.pdf Basically in a stochastic theory you have two choices  a theory where you have continuous transformations of so called pure states  and ones where that is not allowed. The former (basically) leads to QM  the latter standard probability theory. This is not to say QM does not have deep mysteries (eg non local behavior and what properties it has between observations), but by viewing the theory this way they can be kept under control without being bogged down with foundational issues. Thanks Bill 


#3
Jan2712, 01:41 PM

P: 476

Essentially you are asking why particles behave as particles. 


#4
Jan2712, 02:48 PM

P: 226

Why does wavefunction collapse occur?
Ugh. There seems to be no consensus on this subject.



#5
Jan2712, 06:16 PM

P: 198

If particles actually existed as particles and did not oscillate like waves, shouldn't they lose all their energy by traveling real distance over time and accelerating?



#6
Jan2712, 06:55 PM

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PF Gold
P: 2,817

As far as interpretation goes  those that posted there is no consensus are correct  the view I gave is basically the shut up and calculate view  but other interpretations have a different take. Thanks Bill 


#7
Jan2712, 09:22 PM

P: 198




#8
Jan2712, 09:36 PM

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PF Gold
P: 2,817

I am saying they are only ever detected as particles  never as waves so the most reasonable thing to do is model them as particles. But they obey the rules of QM which is described by a quantum state that has, in some circumstances, wavelike solutions. However whether a state has a real existence is open to question  I view it purely as a device for calculating probabilities.
Thanks Bill 


#9
Jan2712, 09:46 PM

P: 198

Unless by "solutions" to you mean somehow working backwards from results? Because I don't think this is just a basic popscience misunderstanding, but at the same time, we don't actually see particles themselves as waves even though they seem to have to travel as waves to end up in the locations they do. 


#10
Jan2712, 10:01 PM

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PF Gold
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Thanks Bill 


#11
Jan2712, 10:13 PM

PF Gold
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#12
Jan2712, 10:22 PM

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PF Gold
P: 2,817

Your second statement is correct  it is not a wave nor classical particle  but a quantum particle which is something entirely different and definitely weird  although with some acquaintance you get used to it and get an idea of why it must be like that  check out: http://arxiv.org/pdf/quantph/0111068v1.pdf 'The usual formulation of quantum theory is very obscure employing complex Hilbert spaces, Hermitean operators and so on. While many of us, as professional quantum theorists, have become very familiar with the theory, we should not mistake this familiarity for a sense that the formulation is physically reasonable. Quantum theory, when stripped of all its incidental structure, is simply a new type of probability theory. Its predecessor, classical probability theory, is very intuitive. It can be developed almost by pure thought alone employing only some very basic intuitions about the nature of the physical world. This prompts the question of whether quantum theory could have been developed in a similar way. Put another way, could a nineteenth century physicist have developed quantum theory without any particular reference to experimental data? In a recent paper I have shown that the basic structure of quantum theory and countably inﬁnite dimensional Hilbert spaces follows from a set of ﬁve reasonable axioms. Four of these axioms are obviously consistent with both classical probability theory and with quantum theory. The remaining axiom states that there exists a continuous reversible transformation between any two pure states. This axiom rules out classical probability theory and gives us quantum theory. The key word in this axiom is the word “continuous”. If it is dropped then we get classical probability theory instead.' Basically QM is necessary in a stochastic theory if you want to model continuous transformations  for the exact meaning of that see the link above. Thanks Bill 


#13
Jan2712, 11:46 PM

P: 275




#14
Jan2712, 11:50 PM

P: 226

@bhobba: I'd very much like to hear how you feel about multiverse concepts. Your descriptions here are very straightforward, "down to earth". I'd also like to see if I have a grip on your view by restating it:
From the ground up. There is field. Fluctuations occur in the field. Excitations are particles. When unviewed particles are doing something they are treated as waves  the wave function being the probability spread (is that misleading of me?) It may be possible they are actually moving like waves. When a particle/wave is interacted with (including measurement/observation) it has a definite particle form. One more thing: what is the field? I understand the mathematical concept of scalar and vector fields  numbers assigned to points in spacetime  but what does the field record in this situation? Energy fluctuations? Or is this not the idea? 


#15
Jan2812, 12:07 AM

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#16
Jan2812, 12:24 AM

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PF Gold
P: 1,776

One way to understand it is to rephrase the loaded terminology: Wave functions don't collapse, they get updated. That corresponds to CI where the wave function is understood as a symbolic representation of possible quantum behavior and not as a direct representation of the physical reality of the quantum.
In that understanding updating the wave function given a change of knowledge is the same as updating say the probability distribution of where to find a lost sailboat given an observation that it was not in sector X. The update is qualitatively the same as with a classical probability distribution though the way of representing probabilities of observations is distinct. 


#17
Jan2812, 12:53 AM

P: 226




#18
Jan2812, 12:54 AM

P: 226




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