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I Does quantum mechanics obey causality?

  1. Aug 5, 2016 #1

    n01

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    My intuition tells me that it does not given that physical phenomena don't obey the principle of sufficient reason under quantum mechanics (a dogma many still hold certain). A lucid definition of the PoSR can be found here. Meaning, that some events are non-localized and the distinction between localized phenomena and global phenomena gets significantly blurred.

    I don't see where else to post this as it's more of a meta-physical question.
     
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  3. Aug 5, 2016 #2

    mfb

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    Quantum field theory obeys causality. The commutator between spacelike separated points in spacetime vanishes, which means they don't influence each other.
     
  4. Aug 6, 2016 #3

    bhobba

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    Its associated with the cluster decomposition property:
    https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

    Normal QM being based on the Galilean transformations doesn't. To fully understand the connection see the following beautiful book by the great theoretical/mathematical physicist Lev landau:
    https://www.amazon.com/Mechanics-Third-Course-Theoretical-Physics/dp/0750628960

    I don't know why but that is the only book I know that gives the details, admittedly in Landau's usual terse style. Its very important but other books simply do not delve into it.

    You will find QM dynamics developed from Galilean Relativity in Chapter 3 of Ballentine. Its the reason for Schrodinger's equation etc so is foundational and inescapable in ordinary QM. One must go to QFT to rectify it.

    Thanks
    Bill
     
    Last edited by a moderator: May 8, 2017
  5. Aug 6, 2016 #4
    Yes there is causality just sometime it is reversed
     
  6. Aug 6, 2016 #5

    bhobba

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    Its actually a deep physical question going to the heart of modern physics and its foundations from symmetry. It was quite possibly the deepest discovery of 20th century physics that symmetry is of such importance to physics like in the 19th century mathematicians discovered its importance to mathematics and especially geometry. I suspect there is a deep secret there waiting to be uncovered - but only time will tell.

    Thanks
    Bill
     
    Last edited: Aug 6, 2016
  7. Aug 6, 2016 #6

    bhobba

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    There is no causality in standard QM as can be seen by the form of the Hamiltonian obeying the Galilean transformations.

    You are thinking of the retro-causal transnational interpretation. Its simply that - an interpretation.

    Thanks
    Bill
     
  8. Aug 6, 2016 #7
    You have right.
    One of the possible right interpretation.
     
  9. Aug 6, 2016 #8

    n01

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    Yes,
    I'm keenly interested on this from a quasi-computational perspective, and in some sense it's a tautology. So, let me elaborate if I don't start sounding metaphysical. Given that every physical law is either computable or non computable, then within such a system there will arise situations or "state of affairs" that could not be explained within the system itself. This is basically Godel's Incompleteness theorem stated in a nutshell.

    My hunch is that QM and the logical conclusions derived at by Godel are in some deep sense intertwined and manifest in reality (I mean, how can they not be... unless we're talking about higher dimensions; but, even then those higher dimensions would require another higher dimension to maintain deterministic causality of each sub-dimension).

    Hope that doesn't sound too outlandish as it does to me.
     
    Last edited: Aug 6, 2016
  10. Aug 6, 2016 #9
    I take Godel a little more loosely, as in the system being used can't encompass all of reality, not that any given reality can't be modeled completely by a complete system. In my humble opinion, it is due to the introduction of supposed "infinite" degrees of freedom into a "finite" system which leads to the inconclusive result...
     
  11. Aug 7, 2016 #10

    Paul Colby

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    Is this a definition of causality or merely a requirement of such? There are those amongst us that might claim entanglement experiments demonstrate superluminal "cause". I am very much not one of them.
     
  12. Aug 8, 2016 #11

    mfb

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    "they don't influence each other" is what I would consider as definition of locality - causality then arises if you consider the evolution in one time-direction.

    There are non-local interpretations of quantum mechanics - but you don't have to follow those, there are also local interpretations.
     
  13. Aug 8, 2016 #12

    stevendaryl

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    People keep saying this, but to me, it's only addressing half of the quantum formalism. In quantum mechanics, you apply mathematics in two different ways:
    1. The state and/or the field operators evolve with time.
    2. You use the state to compute probabilities of outcomes.
    The issue with whether QM is causal was never about #1, it was about #2. When you observe an outcome, does the state change? If so, how? The fact that field operators at a spacelike separation commute (or anticommute) doesn't have any obvious relevance to the question of whether Von Neumann's collapse happens, and whether that collapse (if it does happen) violates the no-FTL rule.

    #1 would be sufficient to prove that causality is never violated, if that were the only process involved in QM (as is the case in many-worlds). But that conclusion doesn't follow from QFT, it requires an interpretation of QFT.
     
  14. Aug 8, 2016 #13

    mfb

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    If there is a causal interpretation (and there is: MWI for example), and a non-causal interpretation, I would call the underlying theory causal. If it would be non-causal, then causal interpretations would be impossible.
     
  15. Aug 8, 2016 #14

    Paul Colby

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    IMO, the entire reason "Does QM obey causality" is a question is due to a lack of a well defined theoretical or operational definition of the term "causality". The appearance of probability in a theory complicates this question.
     
  16. Aug 8, 2016 #15

    DrChinese

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    There is no physical evidence that at the quantum mechanical level there is any cause which specifically determines an outcome to a measurement (excepting repeated measurements when the systems is already in a known state). I doubt anyone here would dispute that statement.

    It is a metaphysical question (as you say), because there are interpretations in which there are such causes. However, those interpretations cannot be discerned from non-causal interpretations by virtue of physical evidence. It is by inference and personal preference alone that such can be made.
     
  17. Aug 9, 2016 #16

    n01

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    If any of you are interested in entertaining this thought and possibly looking at it from a more specific example of the origins of causality in QM...

    I have always been puzzled by what exactly determines a wave function collapse? Or stated another way, what keeps quantum systems evolving as opposed to simply collapsing on itself?
     
  18. Aug 9, 2016 #17

    vanhees71

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    There is no wave-function collapse. So you don't bother you with it :-).
     
  19. Aug 9, 2016 #18

    n01

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    Well, yes according to the MWI; but, in that case what determines that our universe is the way it is as opposed to other worlds?
     
  20. Aug 9, 2016 #19

    vanhees71

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    I don't believe in the religion of MWI. I'm a proponent of the minimal statistical interpretation. "Collapse" is just the interaction of the measured object with the measurement device, which usually is macroscopic, and we coarse grain over many microscopic degrees of freedom which leads to the classical behavior of the relevant observables of measurement apparatus. Thus, in general we don't know the state of the total quantum system (measured system + measurement apparatus), and tracing over the measurement apparatus leads to a mixture for the state of the observed system.
     
  21. Aug 9, 2016 #20
    Yes. Quantum mechanics is probabilistic, but it's not random. The probabilities are deterministic.
     
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