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What collapses a wave function?

  1. Jul 11, 2008 #1
    what is it that actually collapses a wave function, an observer? what constitutes an observer? also is it true that everything has a wave function, because if it does who collapsed the universes wave function
    some may say wave function collapse only works on the quantum level but the universe was sub atomic sive at the time of the BIG BANG.

    can resistance in space-time also collapse wavefunctions

    if a person collapses a wave function by looking (observing) where does light come into the question. for the person to actually make the observation the light has to travel from the wavefunction to tge persons eye, what if the light is intercepted by another persons eye.

    also how can light wave function collapse?
    Last edited by a moderator: Jul 12, 2008
  2. jcsd
  3. Jul 11, 2008 #2
    It collapses?
  4. Jul 11, 2008 #3
    Is this a joke?
  5. Jul 12, 2008 #4


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    "Observer" is a bit misleading; any form of interaction that measures the wave function will collapse it, even a photon interacting with the system (no observer necessary).
  6. Jul 12, 2008 #5


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    Wave functions collapses when interactions are taking place, just as DaveC426913 says. Remember that humans observe things by performing scattering experiments etc. (That is how our sight works, light is scattered from the objects, and our eyes detect the diffraction patterns)

    And ALL answers will be written here in this forum, we will not mail you, youl will get mails when someone is answering here.
  7. Jul 12, 2008 #6
    Dave and Malawi are mistaken in their answers. The existence or nonexistence of wavefunction collapse in QM is entirely dependent on one's interpretation of QM. Before one specifies what interpretation of QM one is working with, one cannot give a meaningful answer to the question of "what collapses the wavefunction". In particular, use of the word "measurement" is too vague even in textbook QM.

    But your questions QUANTUMQ are quite valid. For starters, I recommend reading John Bell's papers from his book "Speakable and Unspeakable in QM".
  8. Jul 12, 2008 #7
    In particular, read Bell's paper "Against Measurement".
  9. Jul 13, 2008 #8


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    No they are not. The worst you can claim is that they are premature in that they make an assumption.

    I do believe that, in asking the question, the OP has declared which interpretation he's interested in.
  10. Jul 13, 2008 #9
    Yes they are ultimately mistaken, I would claim (I have specific views on this matter). After you choose an interpretation within which to assess the question of whether the wavefunction collapses, you have to then justify that said interpretation is self-consistent in its treatment of "measurement processes". You also have to justify that said interpretation is not less fundamental than another interpretation, or that it cannot be derived from another interpretation. If it can, then the answer it gives to the question of collapse cannot be taken as a valid statement about what is mostly likely actually happening with a wavefunction in the physical world, as QuanutmQ is inquiring about.

    What is the OP? In any case, I have not seen any indication of a preferred interpretation to work with, in the question asked.
  11. Jul 13, 2008 #10
    May be you should have a look to decoherence theory (see introduction of http://arxiv.org/PS_cache/gr-qc/pdf/0111/0111105v1.pdf ).

    Decoherence is a general effect of a quantum system coupled with its environment (described as a heat bath).
    Throught decoherence, coherences decrease exponentially with time. The decoherence time depends with the scale of the system. For small quantum system (say hydrogen atom) decoherence time is very long. For large scale system (earth/moon) decoherence time is very small.
    The exponential decrease has been confirmed experimentally for mesoscopic system.

    I'm not a specialist (the following should be confirmed by other posts) and I hope the following won't be too wrong !

    Applied to measurment, coupling the detector to a heat bath yields to exponentially decreasing coherence between the detector and the quantum system under measurement. This behavior has been tested experimentally at mesoscopic scale and intends to explain/decscribe the decorrelatoin between the detector and the quantum system under measurement.

    As is well know, quantum interacting systems become "intricated" under time evolution. However, after measurement, measured quantum system and detector (considered as quantum system) should be decorrelated (in Schrödinger's cat paradaox, the cat is not in a superposition of dead and alive state).
    The decoherence theory does not intend to explain which value the detector will measure, but the decoherence between the detecor and the quantum system under measurement. This is the quantum/classical transition.

    I may add the following personnal note that my help for measurment theory: every detector can be concieved as a macroscopic system close to a phase transition (Wilson chamber for example, photo-multiplicator, etc...). Interaction with a small quantum system yields to a small perturbation of the detector that implies throught time evolution a large number of degrees of freedom of the detector (because of its state close to a phase transition implying a large correlation length) : This results in a phase transition in the detector. Coupling the detector to a heat source seems then "natural" (not had'oc) since it is a fundamental component for a detector to be... a detector.

    Hope this help.
    The main element of the Schrödinger's cat paradox is now explained. The cat has 1/2 chance to stay alive, 1/2 to die and is never into superposition state due to decoherence; this is not different for a coin-tossing game !

    The intention of the observer is of no use (however, if you read this message, it is abviously intentionnaly, and I write this intentionnaly... Observer acts on the world, but, observers can't replace god as Bohr could have answer to Einstein !! ;-D ).

    The explanation for collapsing part of the question should be the following : after interaction between the detector and the system under measurment, taking decoherence intoaccount, the resulting state is a statistical (not quantum like) set of state where the detector indicate a value, and the measured system in the coresponding eigenstate (this is done throught density matrix). So collapse may have occur during decoherence process (I can't say more).
  12. Jul 13, 2008 #11


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    There's already a problem with this. Why are using a reference on decoherence that is coupled to "gravitational waves", something that is still not detected and still very much being determined? Aren't there other more well-established references in peer-reviewed journals that have better illustrations of decoherence without invoking unverified phenomena? Look in the General Physics forum in the Noteworthy Papers thread on at least a couple of them.

    Furthermore, for this forum, only published references are allowed, not preprints such as those in arXiv. If you know of the publication citation for this work, that is what you must also include when making such references.

    Please review the https://www.physicsforums.com/showthread.php?t=5374" in case you have missed it. Pay particular attention to our policy on personal, unpublished theories.

    Last edited by a moderator: Apr 23, 2017
  13. Jul 13, 2008 #12

    George Jones

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    With respect to part of ZapperZ's post: if an arxiv paper has been published in a journal, often the journal reference is given in tne arxiv entry. This is true for the paper above, so


    is a better link. Anyone that wants to look at the full paper can use the Download section at the top right, or they can look in the appropriate journal.
  14. Jul 13, 2008 #13
    I appologize for including my personnal views. I won't include them again.

    I wanted to prevent natural questions on the fact that explaining classical statistical behavior by introducing heat bath (and so classical statistical behavior) could seem unsatifactory on theoretical ground.

    About the link, I've not spent a long time to find it. I know Serge Reynaud as a physicist who worked on the experimental verification of decoherence. Please see references inside the article if you are interested more deeply by the subject. Search for gravitaiotnnal source of heat bath (the subject of the article I linked) is a developpment of the verified decoherence theory. I will abandon direct link to article.
  15. Jul 13, 2008 #14
    There's really no clear answer to this, as you can see from prior posts. You're question itself already implies that there are elements of physical reality associated with both the wave function and a process of collapse. In it's most rarified, non-interpretational sense, there are a small number of rules of quantum mechanics, that when followed, predict the outcome of expreimental measuments. But in assuming elements of physical reality attributed to both wave functions, and a collapse process, one immediately obtains a contradiction upon introducing more than one observer.
  16. Jul 13, 2008 #15
    How can a photon collapse a wavefunction when there is no observer present? Particles are continually interacting with each other. What if one was to choose the sytem as the photon and the thing it interacts with? The only way I can understand collapse of the wavefunction is to assume that it is the existence of a conscious observer that causes the collapse.
  17. Jul 13, 2008 #16
    This may seem like a small point, but I would be interested in a really clear physical example of a situation where the wave function collapses. I often follow these discussions and they tend to hover around generalities. Schroedinger's Cat is not the best example from my point of view because I can't imagine what the wave function of a cat looks like. I'm interested in an example where you could basically write the equation for the wave function, at least in principle, and then consider how it might collapse. Because I think sometimes there is a reasonable mechanism to explain these things.
  18. Jul 13, 2008 #17
    The wave function could not dissapear if its not the particle it self is annihilated. In a detection this is not the case. I think Marty has put a good question here.

    Consider a particle in a long box or a plane wave. At t=0 put on the strong and local detection interaction at x=xc,


    Now, does collapse mean that the original wave extending over the long box is going to be localized closely to xc after a short time [tex]\tau[/tex]? I think it is interesting to study this example. Perhaps I do it soon. Perturbation theory is not good enough.
  19. Jul 13, 2008 #18


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    This is why 'observer' is a poor choice of words; it doesn't require consciousness; it merely requires an interaction.
    Precisely. And wavefunctions are continually collapsing. It requires some very careful (and only theoretically possible) tinkering to get systems not to collapse. And that tinkering increases in difficulty geometrically with the number of particles, which is why is virtually impossibnle with anyting but a couple of particles at a time.

    Note though, that while the photon may have collapsed the system, there is still a system "wrapped around this one" so to speak. This outer system that includes two things: the original system and the photon. Until that system is collapsed (by another interaction) there are two superposed states this outer system could be in: one where the photon interacted collapsing the sytem to state (a) and one where the photon interacted and collapsed the system to (b).
  20. Jul 13, 2008 #19
    You are still assuming that wavefunction collapse is an objective process that actually happens - but you have NOT established that yet. In particular, it is not clear what theoretical framework you are using to talk about an "interaction". And it is misleading to answer his question in that way.
  21. Jul 13, 2008 #20


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    Feel free to contribute. I am answering the OP's question in kind, without miring him in a university course in QM.
  22. Jul 14, 2008 #21


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    Maaneli: Both my and Dave's answer are witihin the Copenhagen interpretation of QM, the paradigm of QM, that is why we are giving the answers that we do. It is not missleading, it is just that we answer within the most accepted interpretation framework - the paradigm. Of course there are more interpretations of QM, but I think the OP (original poster) also asked within the framework of Copenhagen interpretation, hence - our answers will be within that framework too.
  23. Jul 14, 2008 #22


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    I think it's good to first ask what a wavefunction supposedly represents. I think the best view is to see it as relational information. Ie. it aims to describe the observers information, about the system.

    It's doesn't "solve all problems", but it's good reading and a good start, check out C.Rovelli's
    "Relational Quantum Mechanics"
    -- http://arxiv.org/abs/quant-ph/9609002

    I'd say from the instrinsic view: new information. Does it have to be more complicated?
    To ask, where does new information come from and why, I ask myself who is asking.

    Are You asking

    1) why a second observer (you are the first observer) perceives a collapse of the wavefunction of a system?

    Note that in this case, you are avoiding the problem. Your wavefunction doesn't necessarily need to collapse. But you are not observing the system, you are observing a second observer observing another system.


    2) are you considering that this second observers asks why it sees a collapse?

    I think the second question is never asked like that, I see it as an extrinsic question. I think problems appear when we try to project questions to other observers, not considering wether they are constructible withing the intrinsic view.

    Like others has said already, measurements and observations are "interactions". So any part or subsystem of the universe interacting with the rest of the universe can be seen as observers.

    ( But there are open philosophical question here that can be debated. For example, if the suggested interpretation of "relative information" is to be consistent a natural question is where this information is encoded, and if this information is encoded in the system that makes up the observer itself, then one seems to reach a conclusion that the complexity of the observer itself, limits the _amount of information_ it can relate to. But if you asks these questions, I think there are yet no established answers that everybody agree upon. )

  24. Jul 14, 2008 #23
    Please could you precise what you mean when saying that "interaction" causes collapse (within the most accepted interpretation framework) ?

    QM postulates doesn't say anything about the source of collapse (except that it is measurement, not interaction that causes collapse). Moreover, since the problem of the description of the colapse arised early in the history of QM, it was admited that the linear evolution operator couldn't permit to describe the collapse (giving rise to the Schrödinger's cat). It was also admitted that the evolution during collapse was something non-linear and very complex, but I've not heard of a work that accomplished this program (even recently).

    "non measurement" experiments are interesting when thinking about what is the "source" of collapse (assuming this makes sens).
    For example, consider the Young's experiment (see Double-slit experiment on wiki). Put a photo-detector on one slit. Then ask what is the photon distribution on the screen when the photons have not been detected by the photo-detector ?
    The answer is that the distribution is the same as the one obtained with only the second slit. This means that collapse has occured, cancelling the component of the wave function from the slit where we put the photo-detector.
    Is-it then possible to consider that the photon has interacted with the photo-detector while the photo-detector hasn't measured anything (the photo detector has not changed...) ? What is the source of collapse in this case ?
    Last edited: Jul 14, 2008
  25. Jul 14, 2008 #24
    Are there any experiments which show that a wave function can collapse with an observer not present, due to physical interactions?
  26. Jul 14, 2008 #25
    Well, you have two questions :

    1/ Are there any experiments showing that a wave function can collapse with an observer not present ?

    There are no reported experiment where there is no observer. The question itself is not consistent. (This sounds like a philosophical question about the status of reallity).

    Note that it doesn't mean that collapse occurs or doesn't occur without observer. Only that there is no observer (so no result to be described in Physics !).

    2/ Are there any experiments showing that a wave function can collapse due to physical interactions ?
    Please have a look to decoherence theory which is an attempt to describe measurment process (however results apply in more general context).
    Keep in mind the problem of non-measurement (I don't know any paper discussing this point in the framwork of decoherence);
    Then you will have a good knowledge of the state of the art I think.

    Is the interaction an element of the causes of collapse ? The general accepted opinion is that "it should" (not "yes it is!").

    The scheme is still not clear enough today and is still subject to controversy. There are severals cases that still have to be clarified such as non-measurement that I focused in previous post; see also the 2004 beautiful book of Penrose(and refs therein) where he reports several experiments where measurement causes some apparently ("all happens as if...") backward in time effects. Those are elaborated experiment with correlated photons.

    I can only here indicates what exists, and open questions.
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