Bohmian Mechanics Explained

Here’s a nice set of short videos about the de Broglie-Bohm theory which is a causal, deterministic, interpretation of Quantum Mechanics that can be thought of as a hidden variable theory. It’s a common misconception that hidden-variable theories are ruled out by experimental evidence (which is probably why they tend not to be discussed very much at undergraduate level) but this is not the case. What is true is that hidden variables have to be non-local, which many physicists consider too weird for comfort – but who’s to say that the Universe isn’t even weirder than we thought?

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10 Responses to “Bohmian Mechanics Explained”

  1. I was very influenced by Sidney Coleman’s view of quantum mechanics, that he elaborates on in the fantastic recording lecture “Quantum mechanics in your face”. He points out that trying to interpret classical mechanics in light of quantum mechanics is completely the wrong way around – we really should be interpreting classical mechanics in light of quantum mechanics.

    So fond as I am of Bohmian mechanics (particularly as it is a great counter-example to a common misconception) it always seems as the perfect example of what not to do when trying to understand the way the world works.

  2. Actually, here’s the lecture:

    [video src="http://media.physics.harvard.edu/video/index.php?id=SidneyColeman_QMIYF.flv" /]

    It’s well worth watching.

  3. I misread the title as ‘Bohemian Mechanics explained’, and was very disappointed not to hear something about the exploits of Central European engineers…..
    M
    x

  4. Anton Garrett Says:

    Bohmian mechanics, at least as it was in Bohm’s day (I’ve read him but not seen the YouTube clips through), was not really anything other than an interpretation of quantum mechanics, because it never predicted differently. Even if you posit that some of the elements of its formalism can in principle be observed, it can’t be right because although this theory is nonlocal, it is not acausal – and we know that the order of measurement on the two particles in a Bell experiment is not Lorentz-invariant, so that which signals to which upon being measured cannot be answered – ie, acausality.

    Respect to Bohm for defying the Copenhagen view that quantum mechanics should not be interpreted, though. It is embarrassing that we can predict properties of the electron to one part in a billion but we cannot predict its motion in an inhomogeneous magnetic field in apparatus nearly 100 years old (Stern-Gerlach). So we need hidden variables to do that? So let’s look for them!

  5. Can you have a non-local theory that doesn’t violate causality?

    • Anton Garrett Says:

      Classical electrodynamics with the retarded potential. (Nonlocality is just a modern word for what used to be called action-at-a-distance.)

      • That’s how I’d heard plain old locality referred to before, but that definition also makes sense.

        My understanding was that Bell’s assumption of locality (and the kind discussed in the EPR Paradox, etc.) was that no [i]instantaneous[/i] action-at-a-distance was taking place (or at least no faster than light communication). It’s no coincidence Bohm’s theory is acausal – it must be, to reproduce the predictions of quantum mechanics.

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