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Schrödinger local and deterministic?

  1. Mar 19, 2010 #1
    There have been many QM Interpretation thread, but I haven't found this question answered:

    Taking aside the fact that a complex probability amplitude is not something we can picture, is the Schrödinger equation local and deterministic at once?
     
  2. jcsd
  3. Mar 19, 2010 #2
    Yes, the Schrodinger equation and the evolution of the wavefunction that follows from it is local and unitary. Unitary means that the time evolution of the wavefunction is unique and completely determined by the initial conditions. It is therefore deterministic.

    There is, however, a major practical obstruction that prevents us from actually calculating this time evolution for any macroscopic system. This is partically because it is practically impossible to determine the initial state of a macroscopic system. But even if we did know this state or if we are somehow able to finetune it, the time-evolution itself is a many-body problem which is, again, computationably intractable.

    We therefore always need to resort to some form of approximation, e.g. a statistical description of the system or ignoring a large number of degrees of freedom. Such a statistical description automatically introduces a degree of 'uncertainty' which manifests itself as a non-determinstic description of the system.

    So even if you put the whole measurement problem aside, you still end up with a non-deterministic description of macroscopic systems due to practical limitations.
     
  4. Mar 19, 2010 #3
    So how does that compare to the saying "QM can't be local and deterministic" by Bell's theorem and similar ones?
     
  5. Mar 19, 2010 #4

    jtbell

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    Bell's theorem plus the results of experiments testing it (insofar as one accepts those results and/or the validity of Bell's theorem with respect to those experiments, which is the source of the vigorous arguments here about the subject :smile:) support the statement that "QM can't be local and realistic", which is not the same thing as deterministic.
     
  6. Mar 19, 2010 #5
    Yes sure. And I do not wish to start yet another Bell discussion.

    But what's wrong about saying the Schrödinger equation is local and deterministic?
    Mathematically it does look so.
     
  7. Mar 19, 2010 #6
    I believe you said it, "...the fact that a complex probability amplitude is not something we can picture..." is the reason. Well, if it can only exist as math, it's not physics, just math. So, from the perspective of a mathematician... it is as you say. From the perspective of a Physicist... it is too, but it's not useful if it can't be made to do work. Hence all of the rest... so I'd say to answer your question: To avoid confusion.
     
  8. Mar 19, 2010 #7
    I guessed so. Now I'm trying to get some ideas to understand how determinism gets lost.... :)
     
  9. Mar 19, 2010 #8
    See, that's not too hard, because Determinism is lost when we have to calculate positions, velocities, etc... as probabilites. It all comes from the Heisenberg Uncertainty Principle (HUP), now backed up by the CMB surveys.
     
  10. Mar 21, 2010 #9
    I don't agree that the UP supports an external indeterminism in events; The UP does highlight however our lack of knowledge on a system. Just because there is a lack of knowledge from our behalf should not suggest that the universe is not deterministic.
     
  11. Mar 21, 2010 #10
    The CMB would beg to differ, barring a superdeterministic uneven distribution of "stuff" at 360K years post-BB...
     
  12. Mar 21, 2010 #11
    Calculating probabilities is fine. The problem comes in when someone tries to make a theory that works with probabilities alone.

    So if ppl wouldn't try to squeeze QM into basic probability theories, then QM would be local, deterministic and even linear?

    Maybe some sophisticated ingredient can make even the probabilities logical again.
     
  13. Mar 21, 2010 #12
    Time to start building the AI's that can find that... maybe they'll even be nice enough to try and explain it to us! :wink:
     
  14. Mar 21, 2010 #13
    No i beg to differ, because the UP is in light of what we can know - its a limitation of knowledge which does not impede determinism.
     
  15. Mar 21, 2010 #14
    Ok... then how is it that something which is a limitation on KNOWLEDGE managed to effect the (should-have-been-EVEN) distribution of "stuff" in the early universe? The HUP explains that nicely, as does SUPERdeterminism. The HUP + Determinism = Horse****.
     
  16. Mar 22, 2010 #15
    Wasn't the fundamental problem that the equation didn't properly model the interaction between particle and wave as de Broglie envisioned? (It rather just models "some wave" of unknown origin and constitution)
     
  17. Mar 22, 2010 #16
    Yeah... sadly yes... and the Bohmian interpretation replaces that issue with a Pilot wave of "unknown origin and constitution" as you put it so well. Welcome to QM... I need some aspirin. :wink:

    EDIT: Hence us left with 50-50 chances, or worse, 50-50-1! Never good when you get 101% in a physical theory...
     
  18. Mar 22, 2010 #17
    You do realize that particles are simply statistical averages right? Physics in general is a statistical theory at best yes? It's statistical because we don't have all the knowledge on a quanum system, but this is because of our lack of knowledge, not because there needs to be an indeterministic world externally of our limited knowledges.
     
  19. Mar 22, 2010 #18

    DrChinese

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    You keep saying this, but this is generally rejected as a viewpoint. The HUP is not about lack of knowledge, although at one time that was a common belief. It is generally held that particles have attributes only within the context of a measurement.
     
  20. Mar 22, 2010 #19
    To paraphrase DrChinese in my own words, representing my own opinion, "No, I don't realize that, because observational data has shown the HUP is a physical law, not merely a statistal event horizon for observers."
     
  21. Mar 22, 2010 #20
    Classical determinism: Repeating the same experiment many times always has the same result. Classical mechanics allows us to determine that result.
    Quantum determinism: Repeating the same experiment many times yields a unique probability distribution of all possible results. Quantum mechanics allows us to determine that probability distribution.
    Quantum mechanics does not predict the experimental result; it is not deterministic in the classical sense.

    Locality is a property of the space-time of classical physics. It is classical in nature. The wavefunction (probability amplitude) is defined in a Hilbert space. It seems to me that locality is an issue only if the wavefunction propagates in space-time, as many believe.

    In the classical sense, quantum mechanics is neither deterministic nor local.
     
  22. Mar 23, 2010 #21
    Hello Gerenuk,
    The answer to your question is simple and i am surprised that nobody has given it yet.

    The quantum theory relies on two processes. One deterministic process, called U, like Unitary, governed by Schrödinger's equation, and a probabilistic process, called R, like Reduction, governed by Born's rule.

    Both are needed for the theory to actually work. The loss of determinism occurs inside the R process, which has nothing to do with Schrodinger's equation.
     
  23. Mar 23, 2010 #22
    I'd be very, very careful with such a statement ;-)
    Usually the guys crying out "it's so easy!", don't have the slightest clue what the problem is about.
    This observation doesn't apply here, but it is one thing to remember :)

    To my knowledge the R process is ill-defined, so it's hard to use it for arguments. I mean when is an observation an observation? Why don't we consider the human being as quantum objects and thus have U processes only?
    And how does this R process lose locality or determinism?
    For me it's very important not to just know a keyword, but to really understand where mathematically either locality or determinism is lost. Or why at all some people say it is lost, whereas all the theory seems to be based on local and deterministic concepts?
     
  24. Mar 23, 2010 #23

    SpectraCat

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    Well, that is no longer agreed upon I think, since decoherence is now a well-established experimental and theoretical phenomenon that shows it is possible to have very rapid processes that proceed in a unitary fashion according to the TDSE, yet produce observations that are consistent with the original "collapse" (or reduction) theories. In fact, you will see the phrase "there is no collapse" thrown around a lot on this forum.

    I would say that it is very much an open question whether or not there is in fact a loss or determinism as you claim.
     
  25. Mar 23, 2010 #24
    Because the R process makes experimental predictions that the U process doesn't. Example, that YOU will get this or that result when you measure a given system in a given way. If you keep the U process only and use it to built a many world interpretation, you loose the definition of "you", and the above experimental prediction is no more defined.

    The R process lacks determinism in its axiomatic definition, and says nothing about locality.

    Later, Bell, CHSH, GHZ, and Mermin (excuse me if I forget some), have shown that locality and determinism could not coexist. In a larger context, we can say that locality, determinism and realism can't coexist in quantum mechanics.

    Some people however have suggested workarounds. Mark Rubin, for example, in his article about local realism in the Heisenberg picture of operators in the MWI, or JesseM in this forum, with his idea about pasting parallel universe when their future light-cones meet (which is more or less the same idea, as far as I understand). These ideas deserve to be developed. I'm working on JesseM's idea in my spare time.

    They are lost when you violate Bell's inequality in an EPR-like experiment. No modelization of the experiment have been given yet that
    1) Describe what happens in terms of realistic objects
    2) Predicts the violation of the inequality by means of the above description

    Consistent yes, but with not as much predictive power. They do not predict the violation of the inequality without completing decoherence with the last part of the R process, which consists in picking one of the possible results out of many, in an undeterministic way.

    I don't disagree, but Gerenuk's question was simple, and I gave the simple answer, from which we can go on and start further discussions :smile:
     
  26. Mar 23, 2010 #25
    I don't think this R process idea is a satisfactory explanation. And the many attempts for interpretations probably share the same view. It's not well defined when someone is measuring and when he isn't and what reality means.

    I do not want to discuss their work. I'll go through it later, but I know they all make their own hidden assumptions. Anyway:

    If I let the universe run for a long time governed by the Schrödinger equation, and if I make one measurement in the end, then everything was a local and deterministic U process to the very end and I can extract probabilities from this l&d process? Right?

    And then someone else comes along and says, I'm only a stupid quantum process and he is the real observer and waiting for an even longer time than me before he does the measurement. So in his theory everything was l&d an even longer time?!

    It seem everything is l&d at all times. (unless you insist on removing the wavefunction and introduce real probabilities)
     
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