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Bohmian interpretation and schroedingers cat

  1. Feb 2, 2008 #1
    What happens to the cat in the Bohm interpretation?
    If I've got it right:
    In MWI it splits, dead in one universe, alive in another.
    In CI it's not decided until observed.
    In TI both these absurd concepts are escaped and it's just either dead or alive(I cannot remember the details)

    but in Bohmian mechanics what does happen to the poor cat?

    Btw, was Schroedinger allergic to cats ?:P
  2. jcsd
  3. Feb 2, 2008 #2
    I think both in more modern forms of CI, as well as in Bohmian mechanics, measurements and measurement-like interactions are likely to end superposition in such situations rather quickly.
  4. Feb 2, 2008 #3
    Bohmian kills off observer role, CI brought it in.

    i didn't understand you, what happens to the cat? layman terms
    does it die/not die? and why is it dead/not dead?
  5. Feb 2, 2008 #4
    My understanding is that many physicists who today say they support CI, have replaced 'conscious observation' with measurement-and measurement-like interactions which leave a trace of information. That is, a superposition of positions is ended when the position is measured, or when the objects in superposition have an interaction with other objects that would leave a trace of information about the position.

    I would assume, without knowing the details, that in Bohmian mechanics this is very similar, however here there is a mathematical model for this process that defines precisely the kind of situation which will end superposition (although I'm not sure whether this can be expressed in a way that would be intuitively understandable, rather than only mathematically).

    So for both interpretations, I'd assume that superposition ends before it reaches the cat, and that the cat will be either one or the other in the common sense.
  6. Feb 2, 2008 #5


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    In BM, this is in fact ambiguous. You have to know that BM has TWO dynamics to it:
    one is the unitary evolution of the quantum state, without any "projection" or so, exactly as in MWI. But on the other hand, there is a "particle dynamics" which is Newtonian, and in which a potential appears, which is derived from the wavefunction. Note hence that the wavefunction dynamics has an influence on the particles (through the quantum potential) but that the particles don't have any influence on the quantum dynamics which leads its own life, just as in MWI (and not CI, because there's no "projection" at any point).

    So the QUANTUM states of the cat happen as in MWI, there is a "live" and a "dead" quantum state of the cat which will continue to exist. But the PARTICLES will follow one or the other (as they had statistical distributions in their initial conditions), and the *particle cat* will end up, depending on initial conditions, in the live or dead state.
  7. Feb 2, 2008 #6
    Ok so basically BM says that the cat is either or? and the opposite cancles out and don't exist?
    Last edited: Feb 2, 2008
  8. Feb 2, 2008 #7
    I don't see the ambiguity, since there is only one particle. The elements of the "wavefunction dynamics" corresponding to the other possibility will become 'empty channels', and not influence the particle anymore. There are no other cats in those empty channels.
  9. Feb 2, 2008 #8

    Bohmian doesn't suggest any "branches" where the cat's state would survive.
  10. Feb 2, 2008 #9


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    Bohmian mechanics doesn't involve any superpositions at all, the whole idea is that all particles have definite positions at all times, and there is a faster-than-light "pilot wave" coordinating their actions to account for the type of correlations seen in entanglement. You can read more about this interpretation here:

    Last edited: Feb 2, 2008
  11. Feb 2, 2008 #10
    I think in addition to the guiding quantum potential and the guided particle, one needs to understand the process of measurement in BM (which I don't, I've read the term "von Neumann" measurement), in order to have a good picture of what happens here, and to understand BM in general, since the position of the particle is not directly measurable. It seems to me that the common descriptions of BM (including my little attempts) completely miss an important element there.
  12. Feb 2, 2008 #11
    Hmm, yes, I was more thinking of "something that can cause interference patterns" than of "superposition". That's why I mentioned that one needs to understand how measurement and measurement-like interactions work in BM. Not all questions of how BM replaces CI-collapse might be answered by the concept of guided particles alone.
  13. Feb 3, 2008 #12


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    This is correct, but these terms in the wavefunction continue to exist, and this wavefunction, in BM, is "just as real" as the particles: it is the "other half" of the universe, which consists on one hand of a particle dynamics, and on the other, of a wavefunction dynamics. The particle dynamics depends on the wavefunction dynamics, but not in the other way. I know that in BM, one has tendency to give emphasis to the particle dynamics, but unfortunately, it is not autonomous (from the particle states alone, one cannot predict nor the particle evolution, nor the wavefunction). So the wavefunction is a part of reality in BM (is a genuine real "field" in hilbertspace), just as much as the particles are.

    So the "ghosts of the cat" continues to exist in the "wavefunction part" of reality, but only one "particle cat" will exist in the "particle part" of reality.
  14. Feb 3, 2008 #13
    Oh, the cat has multiple ghosts? :) What difference does that make?
  15. Feb 3, 2008 #14
    vanesch that makes no sense, so your saying that BM is MWI without actually other real universes? just "ghost universes" ?

    That must on the microscopical level, not macroscopic.
  16. Feb 3, 2008 #15


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    Let us not forget (what is often done in BM), that we need TWO dynamical elements in BM: the wavefunction dynamics, and the particle dynamics. What happens is that the wavefunction dynamics is the unitary quantum mechanics one, just as in MWI. But ON TOP of that, there is the particle dynamics, and in BM, one often gives the impression that that is all there is. A bit as if in classical electrodynamics, we had charged particles, and the EM field, but we pretended that there were in fact only the particles, and not the EM field. But the analogy doesn't work entirely, because in this example, the particles DO influence the EM field, while this is not the case in BM.

    So it is as if we have, say, an electron, and a coulomb field around that electron. Afterwards, the electron interacts with something, and the electron goes LEFT in that interaction, but we now have two coulomb fields: one centered on the electron going left, and one going right, but with no electron in it. Of course, that's not how things work in classical EM: the EM field "goes with" the particle. But not so in BM: the wavefunction evolves independently of what happens to the particles.

    So we have in BM reality:

    "particle world": { cat particles with a statistical uncertainty on the initial position}

    "wavefunction world": |cat - in - box state>

    After the famous experiment, we'd have:

    "particle world": {cat particles in a living cat} (randomly choosen because of initial condition uncertainty)

    "wavefunction world": |living cat> |stuff> + |dead cat> |otherstuff>

    So there are now two terms in the wavefunction: the first term is "centered on" the particles, while the second term is "living on its own" independent of the particles.

    It is true that we can, concerning the *particle dynamics*, just as well forget about the second term, it will not influence much the dynamics of the particles anymore. But it still exists in the "wavefunction world". So it looks a bit like our "coulomb field going to the right, without an electron in its center" (which, again, cannot happen in classical EM, it is just an image of what goes on here).
  17. Feb 3, 2008 #16


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    Well, the superposition still holds for the "pilot wave" dynamics, because it is the same as the usual quantum dynamics...
  18. Feb 3, 2008 #17
    Yes, Bohm speaks of the classical potential and the quantum potential, and the classical force and the quantum force acting on the particle. He frequently uses the term "quantum field", and says the particle is alway accompanied by its field. However the field is merely a guiding principle, not a second part of the particle's (or the cat's) embodiment.

    Here are a few quotes from a chapter in "The Undivided Universe" about the cat paradox:

    "For the wave function corresponding to the live cat has no effect on the quantum potential acting on the dead cat or vice versa."

    "Thus if we consider the system of firing device, plus gun, bullet and powder, it is clear that there is no overlap between the wavefunction [wavefunction-unfired] and [wavefunction-fired] (...). [Edit added:] Therefore the particles constituting this system will be either in the state corresponding to the firing or non-firing of the gun."

    "In other words, once the electron has in effect been 'detected', everything proceeds in essentially the same way as it does in classical physics."

    Yes, those are called "empty channels" and become irrelevant.
    Last edited: Feb 3, 2008
  19. Feb 3, 2008 #18


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    They become irrelevant for the *particle* dynamics, but they continue to exist in the wavefunction dynamics. What you seem to miss, is that the universe in Bohmian mechanics, contrary to what Bohmians often try to claim, doesn't consist just of a "particle universe", because it cannot explain the entire dynamics (you cannot reconstruct the wavefunction dynamics just from the particles). The Bohmian universe consists of a double universe: the particle universe (the one they like), and the wavefunction universe (identical to MWI). They need the wavefunction universe to have the particle dynamics right, so it "exists". And it is in this universe that the "ghost" branches continue to evolve, even though they won't act upon the particle universe anymore (due to a similar mechanism as in decoherence).
  20. Feb 3, 2008 #19
    It is perhaps like the forces of gravitation (be it mediated by fields, waves or particles) extending beyond where there is matter in the universe, if that area didn't extend as well, without anything to act upon.

    Or perhaps even like curved space extending beyond where there is anything to act upon.
  21. Feb 4, 2008 #20
    In Bohmian mechanics the cat has a definite fate (either alive or dead), depending on the initial position of the particles involved, but there is in principle no way to know those initial positions. So, apart from this initial epistemic uncertainty, the mechanics is totally deterministic and there is neither a "collapse" nor a branching of worlds. The "ghost" world does not really exist, it just represents an unrealized possibility. It is only the "particle cat" that exists and its fate, based on the initial particle positions, was determined from the beginning. But presumably only God knows that fate.
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