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About an idea concerning collapse

  1. Mar 12, 2006 #1
    is the idea that collapse cannot be defined as something that occurs, generally known and accepted?

    thx
     
    Last edited: Mar 12, 2006
  2. jcsd
  3. Mar 13, 2006 #2
    hmm just giving this a bump.. is there a reason why no one has replied? =)
     
  4. Mar 13, 2006 #3
    Your statement is ambiguous, so nobody knows what to say.

    How can collapse not be "defined"? As I understood it, it's not particularly hard to "define", it's just an experimental result obtained for quantum mechanics.
     
  5. Mar 13, 2006 #4
    ok. thanks, I'll try and be a bit clearer as to what I'm asking.

    If I were to present proof that rules out the possibility of a superposition of states that collapses into a well defined, real state upon some kind of "measurement" or "observation", would I be doing something new, or not generally known/accepted by the physics community?

    In other words I am saying that collapse can never occur... and so cannot be defined meaningfully...
     
  6. Mar 13, 2006 #5

    vanesch

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    There have been numerous threads on the subject. Within the realm of accepting the empirical predictions of quantum theory, there are several viewpoints (often called interpretations) of what exactly the elements of the formalism mean, and according to the different interpretations, different answers to "does collapse really occur" are given.

    The main contrasting families of interpretations are, on one side: MWI-based and on the other side, Copenhagen-style. I personally am an MWI proponent, and within this viewpoint, there is no objective collapse, but only a subjective one ; the wavefunction is supposed to describe ontological reality, and when an observer's body gets entangled with a system, different "versions" of the observer now exist, each one which experiences only one branch of the wavefunction. So, what appears empirically to be a collapse, in this viewpoint, is nothing else but the observer's body interacting with the system under study and the conscious observation of one of the terms (with a probability given by the Born rule). There's no objective collapse (the wavefunction didn't collapse) in this viewpoint, but one could call the phenomenon by which one consciously is only aware of one of its bodystates, a subjective collapse. There are several variations on this theme. The other terms in the wavefunction (which are supposed to continue to exist happily along) are the "many" worlds of the name MWI: many worlds interpretation, or "relative state" interpretation: what you observe is relative to the state you are entangled with.

    In the Copenhagen family of viewpoints, the wavefunction is only a kind of tool to help you find out what you will observe, but is not a description of reality on the microscopic level - no such description is supposed to be possible. The collapse then corresponds to the transition of the undescribable quantum world to the describable macroscopic, classical world during a special process which is called "measurement". In this viewpoint, there is a real collapse, but not of something that is physical (the wavefunction).

    But note that all these arguments are metaphysical: they are interpretations of what the formula mean ontologically. The empirical predictions are (almost) the same in both viewpoints.
    I say, almost, because there IS a fundamental conceptual difference (although unfortunately, not empirically accessible in practice). In the MWI viewpoint, quantum interference is always, in principle, possible, even on the macroscopic scale, because the different terms keep their existance in the wavefunction. Even though in practice, interference terms on macroscopic scales are suppressed by decoherence phenomena, in principle, one expects to have the potential of quantum interference on macroscopic scales. In the Copenhagen-like views, the collapse happens and is irreversible. The other terms are physically eliminated from the wavefunction, and from that moment on, interference is of course impossible, even in principle. In this viewpoint, MWI is kind of falsifiable, in that, if one has taken into account all decoherence effects, and one predicts a quantum interference experiment, while the interference is not observed, MWI would be definitely falsified.
    So one would think that there is hence an empirically possible (although practically very difficult) distinction to be made between both. Unfortunately, no. As the Copenhagen views do not say exactly WHEN the collapse occurs, no demonstrated quantum interference - no matter how macroscopic - can falsify Copenhagen, because it can always be said that the collapse occured after that.

    So this is why this discussion is metaphysical: with, or without collapse, in MWI or in Copenhagen, one obtains the same predictions for empirical observations by a conscious observer. Formally, you're free to use the collapse, or to drag along the entire wavefunction. As long as you use the Born rule for calculating the probabilities of outcomes, you'll get identical results.
     
  7. Mar 13, 2006 #6
    Regarding MWI I got two questions.

    About how many worlds are we talking in MWI? For every position measurement infinite observer split up? That would be quite many.

    Does MWI define what a measurement is?

    thanks
     
  8. Mar 13, 2006 #7
    No no ratzinger, please don't hijack the thread...

    Ok Vanesch... so in copenhagen/orthodox view the superposition is not a fundamental description of reality??

    This leads to the following:

    If superposition is not a fundamental description of reality, then according to the copenhagen/orthodox interpretation, neither is the HUP.

    So according to orthodox/copenhagen view, the universe is not neccessarily non-deterministic? So the orthodox/copenhagen view tells us that a particle DOES have a definate momentum AND position but that we can never find out because we will always affect one if we measure the other?

    This comes as a big surprise... I was under the strong impression that Einstein objected so strongly to QM precisely because the copenhagen/orthodox view neccessarily implies non-determinism.
     
    Last edited: Mar 13, 2006
  9. Mar 13, 2006 #8

    vanesch

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    I would like to hear about the principle behind your proof. Now, of course, I also prefer the view that this doesn't happen (it makes the formalism, IMO, much "smoother"), but I don't see how you can "prove" (based upon what axiom system ?) that collapse cannot happen.
    Of course, we know already that collapse violates lorentz invariance, locality and all that. So if it is based upon such an idea, that's "old hat", if you like.
    But I don't see how you can disprove the theory, that says that collapse happens each time when *I* learn something (I am then the great cosmological collapser). It's ugly, it violates relativity, it violates unitarity, it violates a lot of things, but it makes correct observational predictions.
     
  10. Mar 13, 2006 #9

    vanesch

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    Copenhagen is weirder than that ! It tells you that the concept of momentum, or position, or whatever other description of an ontology of the particle does not make sense, and that the only thing that does make sense as a logical description of the particle is the measurement outcome, when you actually perform it. At that moment (during a special, undefined process called "measurement") it makes, during an instant, suddenly sense to say that a particle has a position OR a momentum OR whatever you measure. And then it "disappears again in its ontologically meaningless quantum world", and the only thing you're left with is a formula, which is the wavefunction and its evolution, but which is NOT saying what's going on, but is just a tool to calculate the outcomes of the NEXT measurement. But it is postulated that it is fundamentally impossible to say what is actually going on. It is called a positivist viewpoint (logical positivism, http://en.wikipedia.org/wiki/Logical_positivism), in which you do not make any hypothesis about a non-observed ontology, but in which you jump from observation to observation.
    In Copenhagen, on the other hand, there IS an ontology attached to the classical, macroscopic world (in which objects DO NOT have a quantum description, but only a classical description, which is supposed to be exactly true). The quantum objects "hit" the classical world during "measurements". As these "hits" by quantum objects onto classical objects introduce random forces, the classical world gets "noise terms" in its equations, hence becomes stochastical. And a classical apparatus amplifying that noise is a measurement apparatus.

    My personal opinion on this is that this sacrifices too much, namely the idea that the quantum world is fundamentally ontologically undescribable. Hence my preference for MWI.
     
    Last edited: Mar 13, 2006
  11. Mar 13, 2006 #10
    I begin with the assumtion that superposition is a fundamental description of reality. I describe a thought experiment. It is shown from the results and the way the experiment is constructed that collapse cannot occur. i.e that the universe MUST be in EITHER: a determined state with no superposition (a la Dirac/Bohm I suppose..); XOR a determined state superimposed with an effectively infinite number of other definate state, universes and that this superposition will never collpase but merely aquire more and more states (a la MWI I guess..).

    What I'm saying is a new idea perhaps, is that it must be either one or the other, it can't be anything else.

    Although of course that doesn't necessarily mean that Bohmian mechanics or MWI have the correct formalism/concepts but only that they are examples of theories that are compliant with my proposed condition.

    If CI is necessarily non-deterministic then my condition falsifies CI; if CI is NOT neccessarily non-deterministic then my condition implies that a theory which obeys my condition and does not contradict experiment, is a better theory than the CI of QM.
     
  12. Mar 13, 2006 #11
    Ok perfect, you have answered the doubt I had.

    In my words Copenhagen is NOT neccessarily non-deterministic.

    Hence: my condition implies that a theory which postulates either no superposition and thus pure fundamental determinism XOR infinite, perpetual, multi-definate-universe superpostion (and that of course does not contradict experiment) is a better theory than the CI of QM.

    I guess such a proof would be worth publishing if only to help clarify the issue a bit more, and/or add further quality arguments to the discussion.
     
    Last edited: Mar 13, 2006
  13. Mar 13, 2006 #12

    vanesch

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    Well, if you state that you ALREADY assume that macroscopic systems have a quantum state (a vector in Hilbert space: the entire construction of the Hilbert space is nothing else but a consequence of the superposition principle). Then indeed, you end up in MWI - it is also the reason why I find this the formally cleanest way to see quantum theory.
    However, many people find the idea of having macroscopic objects in a superposition such a "ridiculous" idea, that they say that the superposition principle is NOT to be applied to macroscopic systems. And then you create of course a nasty boundary between those systems that do, and those that do not obey superposition, which gives rise to situations a la Copenhagen.

    That said, macroscopic superpositions ARE conceptually weird: it means that your car is in a superposed state of being in front of your house, in the garage, crushed by a truck and for repair, at the same time. Some people have difficulties with that idea. If they wouldn't, they'd accept MWI in a straightforward way.

    So the real question is: does the superposition principle make sense macroscopically ? MWI-ers say that it is universal, and Copenhagen-proponents say: of course not, look around you.
     
  14. Mar 13, 2006 #13
    I disagree. In my post 10 I said you either end up with soemthing conceptually similar to MWI OR you end up with something that is determistic with no superposition at all.

    You might think my second ption is contradicting my initial assumption, but thats not the case. With the second option you merely state the superposition is a superposition of one definate state only... and that all objects are described by a pure definate state wavefunction (which in terms of superposition is adding one state with coefficient equal to one with an infinite number of other states with coefficient equal to zero...)
     
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