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Uncertainty principle - epistemic or more?

  1. Sep 21, 2006 #1
    Basically, in my two courses on quantum mechanics that I had already, and probably will be the last in-depth study of the subject, I had a certain interpretation of its foundations.

    Basically, my idea was and is that because we observe a quantum state, the wave collapses, and so, it is our measurement that distorts - the measurement. So, my idea was that the uncertainty principle was based on this fact : we can't measure an undisturbed quantum state, and that's why there's always an inherent uncertainty.

    Now, people have been telling me that I'm wrong, that it has nothing to do with our measurement, and that it's an inherent quality of quantum states to be only expressible with uncertainty.

    Now, in practice this doesn't matter much, but in philosophy it does.
    If the uncertainty is based on an act like our measurement, or a correlation between the measurement and the state, then by knowing all the initial states of all the particles (including the observer's), one can still predict the future. However, if this is an inherent quality of the particles themselves, then one can't possibly do this.

    It has some consequences for the determinism/indeterminism discussion as well.
    However
     
  2. jcsd
  3. Sep 21, 2006 #2

    selfAdjoint

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    If your interpretation is factual, then quantum processes can't go on where there aren't humans, or some consciousnesses, to observe. I know QM was originally built around observation because it was a theory of doing experiments, not a theory of "underlying reality". But it has come to be considered a theory of underlying reality, and that is why the measurement problem is an acute one for philosophy as well as physics.

    There are a bunch of physicsts ('t Hooft, Smolin, and others not so well known, but really professional and not kooks) who are trying to get around the implication of the Bell inequalities and experiments verifying that QM violates them, to define some kind of pre-QM that has observd Qm in the limit but is "realistic" at some scale apart from what we have observed so far.
     
  4. Sep 21, 2006 #3
    Even if the world were completely deterministic, it would be impossible in principle to predict the future of that world from within the world itself, because prediction of the world requires a model of the world, and the model of the world would have to be nested within the actual world - you cannot do this and achieve perfect predictability even assuming determinism (it comes down to information storage).

    Thus the future of a deterministic world (viewed from within that world) would be uncertain in principle.

    Best Regards
     
  5. Sep 22, 2006 #4
    The discussion I was in was about metaphysical claims.

    Though thanks for the input.
     
  6. Oct 23, 2006 #5
    The uncertainty relations, derived from the axioms of the standard mathematical formalism of QM, are statistical-probabilistic in nature (characterizing ensembles of supposedly identical quantum entities). From here we have 2 main interpretations concerning the meaning of HUP:

    1. All we can say is that Heisenberg's inequalities are about relevant statistical ensembles, we cannot extend this to (all) single events (the minimal interpretation of HUP, fully compatible with Born's interpretation of wavefunction).

    2. We can safely extend the inequalities to the level of ALL single events, HUP is inherent in nature (having nothing to do with our current or future incapacity to make better measurements).

    This second interpretation (regarding the meaning of HUP) split further upon the interpretation of QM chosen and the postulated nature of Reality (nature deterministic – in the sense that all events have causes - or not ).

    2.1 For example some supporters of Copenhagen Interpretation (or later refinements) argue that randomness is inherent in nature, that there are uncaused events, that definite trajectories for quantum entities do not exist between measurements etc; they argue for the strongest interpretation of Heisenberg's inequalities, namely that 'there are no quantum states which have both a definite momentum and a definite position' (for such people the principle should be called Heisenberg's Indeterminacy Principle).

    2.2 On the other hand causal interpretations of QM (Bohm's interpretation is one of them), while maintaining 2, interpret HUP as being due to uncontrollable perturbations provoked by the existence of a quanta of action different from zero, given also the size of quantum entities (at least a contextual account can still be defended here). Philosophical determinism is retained (all events have causes, there exist definite trajectories etc), in Bohm's account for example quantum events are postulated to be a special type of deterministic chaotic processes, the final distribution observed in experiments always tending, at limit, to |PSI|2.

    [Of course this second interpretation of HUP is compatible with other views too, like the quasi-deterministic (apart from the real collapse of the wavefunction) Schrodinger's 'wave only' interpretation (at limit, anyway it is closer from 2.2 than from 2.1) or Heisenberg's initial 'interactionist' interpretation of HUP (before being persuaded by Bohr to renounce it).]

    In the case 2.1 things are clear, no better predictions (other than statistical) are possible. If we adopt the second interpretation of Heisenberg's uncertainty relations and philosophical determinism 2.2 (even assuming that we can find non-probabilistic quantum laws + all computations can be finished in short enough time) of course that no physical being will ever be able to make exact predictions of all future events (only a Laplacian demon/angel - inhabiting a non-physical realm and knowing the laws and conditions at limit for all particles in our universe - could possibly do that).

    The interesting case is 1., where the possibility to find later that in some particular cases HUP does not hold is left open (for ex. because there may exist smaller quanta of action)…However I'd say that as much as the smallest quantum of action is greater than zero there will always exist an equivalent of the actual version of HUP which would prevent physical beings (including physical Laplacian ‘demons’ having the capacity to compute basically anything) make exact predictions for all further events (even in the most favorable case presented above).

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    "There must be no barriers for freedom of inquiry. There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors." (J. Robert Oppenheimer)
     
    Last edited: Oct 23, 2006
  7. Oct 24, 2006 #6
    Doesn't the 'uncertainty principle' violate common sense?
    How can you measure the position of something that is moving?
     
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