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Under what circumstances does the wave function collapse?

  1. Aug 29, 2014 #1
    The hypothesis that a conscious observer collapses the wave function has been discarded, right? The real reason is that the particle you use to measure the other disrupts the wave function, forcing it to choose an eigenvalue.

    So since we are able to remove the conscious observer as the cause, I'm confused how to interpret everyday reality. Wouldn't almost all particles be forced to constantly choose an eigenvalue because they are so closely interacting with everything else (like my foot and the floor)? This would lead me to speculate that only the rare particle that is isolated enough, like in the vacuum of space or in the laboratory, would be in a superposition of eigenvalues and the majority of matter around us is constantly at a defined eigenvalue due to the wave function always collapsing.
     
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  3. Aug 29, 2014 #2

    atyy

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    1) There is everyday reality. But everything to do with the wave function - including collapse - is not necessarily real, and just a tool to calculate the probabilities of the outcomes of measurements. I especially recommend the textbook of Landau and Lifshitz or Weinberg on this point, in which they stress that to use quantum mechanics, we need to put a commonsense cut between the macroscopic measuring apparatus which registers a particular outcome, and the quantum world which is not necessarily real. Because the cut requires commonsense, it is acceptable if unfashionable to say that a "conscious observer" or an "observer with commonsense" is needed to collapse the wave function. Here is an example of treating the wave function and collapse as not necessarily real: http://arxiv.org/abs/1007.3977.

    2) As a variant of the above, one can use the formalism of continuous measurements: http://arxiv.org/abs/quant-ph/0611067.

    In the past, some people have said that maybe quantum mechanics means that there is no commonsense reality that extends throughout all spacetime, since part of it is always described by the wave function, which is not real. This was partly due to an erroneous proof by von Neumann, which claimed that there cannot be "hidden variables" that describe reality underlying quantum mechanics. However, Bohmian Mechanics provides one sort of counterexample, so there is no problem with simply saying there is commonsense reality throughout all spacetime, but we just do not know the true underlying degrees of freedom, and we use quantum mechanics as an effective theory, until it is falsified experimentally and experimental data guides us to discovering the degrees of freedom.

    An alternative approach to reality is to say that all possible outcomes occur. This is the Many-Worlds approach. It is not yet known if this is without technical problems. One should also say that the Bohmian approach looks very ugly if special relativity is exact.
     
    Last edited: Aug 29, 2014
  4. Aug 29, 2014 #3

    bhobba

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    Well actually wave-function collapse is not part of the formalism of QM - its only part of some interpretations, and even then only for so called filtering type observations.

    The real reason is decoherence:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

    In so far as we have an explanation that is - it doesn't explain the so called problem of outcomes without going into exactly what that is.

    The modern view would be an observation occurs once decoherence happens, which causes apparent collapse. The classical common-sense world we see around us is explained by the fact everything is observed by the environment.

    Some issues do remain however I will be happy to go into if you are interested.

    Thanks
    Bill
     
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  5. Aug 29, 2014 #4
    So is this to say that the majority of matter that we humans encounter (or are immediately aware of from our limited senses) is in fact being "observed" or collapsed by nature, giving it a definite eigenvalue?
     
  6. Aug 29, 2014 #5

    atyy

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    No, your question seems to assume that the wave function is real. What we observe is real. The wave function is not necessarily real, and its collapse is not necessarily real. As an example: in relativity different observers have different notions of simultaneity, and collapse occurs simultaneously across all space for each observer, which means different observers do not agree on collapse. So collapse is not an invariant event. The probabilities of measurement outcomes are invariant, even though each observer collapses the wave function along a different slice of spacetime. http://arxiv.org/abs/1007.3977

    If you want an example of the sort of reality that may underlie the quantum world, you can take a look a Bohmian mechanics.

    I recommend reading Bell's 'Against Measurement' http://www.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf.
     
    Last edited: Aug 29, 2014
  7. Aug 29, 2014 #6

    Ken G

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    Another point to bear in mind is that when a conscious observer does a measurement, they are making a choice. They are choosing an apparatus that effects docoherence in a very particular way, and there is no reason to think that reality itself yields the same type of decoherence. For example, a person doing a measurement might want to know the momentum of a particle, so would intentionally couple the quantum system to an apparatus that decoheres the different momentum states, or they might want to know the location, so they intentionally couple to an apparatus that decoheres position states. Nature doesn't need to make those choices, it just lets the particle interact with its environment in ways that might create very complicated decoherences that put the particle into neither an eigenstate of location or of momentum. So I think it is rather incorrect to hold that nature is constantly putting particles into eigenstates, without the participation of physicists.
     
  8. Aug 29, 2014 #7

    fluidistic

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    Could you please suggest us a lecture with respect to this statement? I am very interested. Thanks.
     
  9. Aug 29, 2014 #8

    bhobba

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    Its to do with the axiomatic basis of QM.

    You will find a correct explanation of those axioms (there are only two and they do not contain collapse) and their consequences in Ballentine - Quantum Mechanics - A Modern Development:
    https://www.amazon.com/Quantum-Mechanics-A-Modern-Development/dp/9810241054

    To elaborate further see a recent post I did (post 137)
    https://www.physicsforums.com/showthread.php?t=763139&page=8

    Its a strange but true fact one can derive all of QM from a single axiom (which is what I show in the post above where I derive Ballentines two axioms).

    The axiom is:
    An observation/measurement with possible outcomes i = 1, 2, 3 ..... is described by a POVM Ei such that the probability of outcome i is determined by Ei, and only by Ei, in particular it does not depend on what POVM it is part of.

    As you can see their is nothing about collapse in the axiom. That axiom implies the existence of this thing called a state, but its simply an aid to calculating the probability's of outcomes.

    Most of the time when you observe something its destroyed by the observation so speaking of the state collapsing in that case is meaningless. Only in the case where its not destroyed does continuity imply the state of the system is an eigenvector of the observable after the observation, but, really all you have done is prepared the system in a different way - these are called filtering type observations. The modern view is to associate a state with a preparation procedure so naturally it changes if you prepare a system differently. Since, from the formalism, the state is simply an aid to calculating the probability of observation outcomes the fact it changes when you prepare a system differently doesn't mean anything - naturally it will change.

    Its the same if you throw a dice. Before throwing the dice each side had a 1/6th chance of coming up. Throw it and one side gets a probability of one, the rest zero. Nothing collapsed - the knowledge about the dice simply changed.

    Ballentine also carefully explains this as well.

    This is from the formalism. Interpretations have their own take.

    Thanks
    Bill
     
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  10. Aug 29, 2014 #9

    atyy

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    Well, bhobba and I disagree on this. My opinon is that Ballentine is misleading and does not get it correct. In fact as far as I can tell, bhobba does get it correct because he explicitly postulates the equivalence of proper and improper mixed states, which is equivalent to collapse. But Ballentine has no such postulate, as far as I can tell.

    One can get away without collapse, but the interpretation is usually quite weird. For example, if one denies the existence of successive measurements. This strategy also denies the existence of Bell tests, because a measurement that is simultaneous in one frame, will be successive in another frame. Nonetheless, since the cut is subjective, there is nothing to prevent one from denying the reality of distant measurements. Milburn and Wiseman's book has some formulations without collapse https://www.amazon.com/Quantum-Measurement-Control-Howard-Wiseman/dp/0521804426.

    Some people also say that the Bohmian interpretation and Many-Worlds do not have collapse, which is correct if they are exactly equivalent to quantum mechanics. I would not say that, because the Bohmian interpretation as a solution of the measurement problem implicitly goes beyond quantum mechanics. Many-Worlds would be exactly equivalent to quantum mechanics, but I think its correctness is still debated.
     
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  11. Aug 29, 2014 #10

    bhobba

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    Mate we all know you have a different view of Ballentine than most around here.

    I have zero problem with you explaining your view - in fact I think its great for people to get different takes.

    Thanks
    Bill
     
  12. Aug 29, 2014 #11

    bhobba

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    Yea Ken - all true mate.

    But I think it also needs to be pointed out nature is making observations all the time without any kind of concious intervention eg a dust particle is decohered by a few stray photons from the CBMR to give it an effective definite position.

    Thanks
    Bill
     
  13. Aug 29, 2014 #12

    stevendaryl

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    To me, the case where "collapse" seems appropriate is a case of a composite system such as the typical EPR thought experiment: We prepare two spin-1/2 in an entangled state, with opposite spins. Alice measures the spin of one particle in direction [itex]\vec{a}[/itex] to be spin-up. Later, Bob measures the spin of the other particle in some other direction, [itex]\vec{b}[/itex].

    After Alice measures spin-up for her particle, but before Bob measures his particle's spin, she can predict the probability of Bob measuring spin-up by assuming that Bob's particle is in a definite state that is spin-down in direction [itex]\vec{a}[/itex]. So from Alice's point of view, Bob's chances of measuring spin-up in direction [itex]\vec{b}[/itex] changed discontinuously when she performed the measurement of her particle.

    It seems strange to view that as a matter of "filtering".

    Of course, one way to view this that avoids the issue of collapse is to say this is a composite measurement: Alice measures spin of one particle at angle [itex]\vec{a}[/itex], and Bob measures spin of another particle at angle [itex]\vec{b}[/itex]. QM gives the probability distribution for possible outcomes of this composite measurement. However, if the measurements are not simultaneous (suppose, as I said, Alice does hers first), then Alice can consider the situation between her measurement and Bob's measurement, and ask: What is the state of affairs in this in-between time? She can certainly characterize the state by a probability distribution on Bob's outcomes, and this distribution will be different before and after Alice's measurement. To me, that change is what is meant by "collapse".
     
  14. Aug 29, 2014 #13

    bhobba

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    It all depends on how you view it.

    The formalism is neutral on that.

    In EPR all you have done is disentangle an entangled state - and that still is simply a different preparation procedure.

    Thanks
    Bill
     
  15. Aug 29, 2014 #14

    atyy

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    Yes, but if one defines a rule of preparation with given probabilities (in other words, a rule of collapse), then a measurement is defined.

    For example, in section 6.2.2 of http://arxiv.org/abs/0810.3536 they state that a measurement model defines an instrument which defines an observable. Here we only need that an instrument defines an observable, since defining an instrument defines a rule of collapse or preparation from measurement (Eq 6.8).

    A similar statement is found in http://arxiv.org/abs/0706.3526, where an instrument as something that is a rule of collapse is defined in Eq 3 and 4, followed by the comment "The above equation shows that every instrument defines a unique POVM."

    At this point, the state evolution changes from deterministic to probabilistic, and there is a new rule of evolution. It is true that one need not view collapse as non-unitary, but there is a new rule here which Ballentine is missing, or misleadingly suggesting that unitary deterministic evolution is sufficient.
     
    Last edited: Aug 30, 2014
  16. Aug 30, 2014 #15

    Ken G

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    But why would nature conspire to give it a definite position and not, for example, a definite momentum? I think nature is decohering every which way from Sunday, and we just pick out the examples where it decoheres around a position basis for what we call a position measurement, and where it decoheres around a momentum basis for what we call a momentum measurement. Most of the time, nature isn't doing either of those, not being a careful physicist after all! So my main point is, just because we have an eigenstate of one type of measurement does not mean the state "has collapsed", since it's still in a superposition state in regard to any measurement that is more of the complementary type. This is just like your point about when measurements are more properly characterized as preparations, but the careful physicist chooses the preparation he/she desires, whereas nature makes no such choices so has preparations that are more of a mish-mash.

    For example, nature might prepare a mishmash of spins, and the careful (and conscious) physicist passes them through a Stern-Gerlach apparatus to separate up and down spins. So we have a preparation/collapse in regard to up and down spins, but we built the Stern-Gerlach for that very purpose. Now, nature might accidentally make what is in effect a Stern-Gerlach apparatus too, but more likely it won't, due to certain entropic considerations. The thinking mind is in some sense locally bucking entropy for some particular purpose, and that is quite important in how measurement/preparations work in physics. So I think we could rightly claim that collapse, in the careful sense of preparation that you point out, is indeed a consequence of the functioning of the conscious mind of the physicist. Not in some magico-mystical way that requires some strange explanation, but in the very clear choices that physicists make when they do physics, which requires no explanation that goes beyond neuroscientific understanding of how we figure stuff out.
     
  17. Aug 30, 2014 #16

    bhobba

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  18. Aug 30, 2014 #17
    Wrong.

    "A poll was conducted at a quantum mechanics conference in 2011 using 33 participants (including physicists, mathematicians, and philosophers). Researchers found that 6% of participants indicated that they believed the observer "plays a distinguished physical role (e.g., wave-function collapse by consciousness)".

    https://en.wikipedia.org/wiki/Von_Neumann–Wigner_interpretation#Reception
     
  19. Aug 30, 2014 #18

    bhobba

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    Again you are interpreting it a particular way not implied by the formalism.

    The formalism is this - a systems state is defined as synonymous with a preparation procedure. It evolves according to Schrodinger's equation until subject to another preparation procedure - or destroyed. Naturally, since its the definition of a state, it will change when subject to a different preparation procedure.

    I mentioned before throwing a dice. It has one side up - so that has probability one and the rest zero. The reason for that is it has been prepared that way. I throw it and after another side may come up and that side has probability 1 and the rest zero. I have subject it to a different preparation procedure. Has anything collapsed - no.

    In fact the above lies right at the heart of QM.

    Suppose we have a system in 2 states represented by the vectors [0,1] and [1,0]. These states are called pure. These can be randomly presented for observation and you get the vector [p1, p2] where p1 and p2 give the probabilities of observing the pure state. This is like throwing the dice as above. Such states are called mixed. Probability theory is basically the theory of such mixed states. Presenting the states for observation is a preparation procedure. Now consider the matrix A that say after 1 second transforms one pure state to another with rows [0, 1] and [1, 0]. But what happens when A is applied for half a second. Well that would be a matrix U^2 = A. You can work this out and low and behold U is complex. Apply it to a pure state and you get a complex vector. This is something new. Its not a mixed state - but you are forced to it if you want continuous transformations between pure states.

    QM is basically the theory that makes sense of such complex states. Its done by the Born Rule - and Gleason strongly suggests its the right way to go - but doesn't prove it. But the situation is the same. A state is simply some preparation procedure. The very foundation of QM is that the state can change continuously - in fact that, as explained above, is what separates it from standard probability theory which can't.

    IMHO this collapse thing masks the real issue - it's the problem of outcomes - but that should really be another thread.

    Thanks
    Bill
     
    Last edited: Aug 30, 2014
  20. Aug 30, 2014 #19

    bhobba

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    You consider 6% a mainstream interpretation?

    The issue isn't that such an interpretation exists, its that the vast majority, by your number 96%, dismiss it. They can do that because, unlike what some bits of junk, such as - What The Bleep Do We Know Anyway, will tell you, plenty of interpretations exist that don't require it, and views that do are very much in the minority. That's one reason that book is a load of bollocks - it uses a minority view to justify new age mysticism. I in fact view it as a strong reason to reject such views - it can be used as justification for mystical nonsense.

    Thanks
    Bill
     
    Last edited: Aug 30, 2014
  21. Aug 30, 2014 #20

    Ken G

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    I feel that might be begging the question though, because we agree that it is the preparation type of "collapse" that we are really interested in, so it's not the position per se that is important, it is what the position tells us about the state we are trying to understand. So for a spectrometer, the position in the CCD tells us the momentum of the original state we are studying, and in a Stern-Gerlach, the position tells us the spin of the original state. So while at the end of day we are always decohering position (indeed, we are probably reading a dial, so that's already a position), what matters is what is the meaning of the dial reading-- and that's what needs to be selected carefully by the conscious intelligence. Nature can do this accidentally, like how raindrops can break sunlight into the spectrum of a rainbow (which is a momentum preparation that will correlate with decohered position), but it is rare as required by entropy considerations. Decoherence that reliably or consistently conveys information about the original system (which is what happenstance position decoherences of the type Schlosshauer must be talking about do not do) is what we really mean by collapse, and that is what requires the careful conscious choices made by an intelligent scientist.
    I absolutely love this, you have pointed this out before and I don't think I have ever seen any more valuable insight into the nature of quantum mechanics. I think it is also consistent with the view that "collapse" is really just a manifestation of the choices we make when we decide how we are going to frame nature. We choose a die because it gives discrete outcomes, and so we spend our time studying dice. Then we ponder how weird it is that nature gives us discrete outcomes out of continuous phenomena? It's not mystical to say that consciousness is responsible for collapse, it is simply recognizing the role of how we think, of what types of experiments we consider to be telling us something.
     
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