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Causality in quantum mechanics

  1. Jul 15, 2011 #1
    Can someone please elaborate these lines:

    "Causality applies only to a system which is
    left undisturbed. If a system is small, we cannot observe it without
    producing a serious disturbance and hence we cannot expect to find
    any causal connexion between the results of our observations. "

    (Reference:Dirac-Principles of quantum mechanics)
  2. jcsd
  3. Jul 15, 2011 #2


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    This is an old-fashioned description, and is not in popular usage any longer. Uncertainty is not a result of "disturbance" in the sense implied here.
  4. Jul 15, 2011 #3
    I just want to understand what causality has to do with a system being disturbed or not.
  5. Jul 15, 2011 #4


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    Causality is not lost because an observer "pokes" a system in the process of making a measurement.

    It is sometimes said that the wave function of a closed system evolves deterministically; however, nothing in that eliminates the element of chance in the outcome of any measurement.
  6. Jun 28, 2012 #5
    My view on the issue is that causality with reference to the classical Newtonian cause and effect is indeed violated in the quantic scale. One cannot predict the outcome of an action, not even if such an outcome occurs in the first place.

    I don't understand why Dirac's view is considered anachronistic. Or maybe is just a matter of term's confusion. I think the concepts of Cause&Effect, Causality and Determinism got tangled up a little bit here.
  7. Jun 29, 2012 #6


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    To the extent that Dirac's statement is seen as saying "disturbance causes uncertainty, and uncertainty implies indeterminism"... that is old fashioned.

    More like "disturbance triggers collapse, collapse triggers indeterminism, indeterminism shows up as uncertainty" where these things are essentially part and parcel of a single idea anyway.

    My essential point being: the observer does not "inject" some kind of unknown/uncertain force into the system under observation, which then acts to produce an uncertain outcome. For if that were true, entangled particle pairs could not produce the observed statistics (perfect correlations, for example). Because the outcomes would reflect the uncertainty injected by the observers (Alice, Bob) and that doesn't happen. The only variable is the context, i.e. the *difference* between Alice and Bob's settings... which is known.
  8. Jun 29, 2012 #7
    It occurs to me that one interesting aspect of the interpretation that "non local" or ftl signalling is used to explain the observed results of entangled photons experiments is that cause and effect are not clearly defined in any claimed entangled ftl interaction. For example, lets us say we have a pair of polarisers, A and B that are far apart with filter A slightly nearer than filter B. We can posit that when entangled photon A passes through filter A that it causes the state of entangled photon B to change instantaneously before photon B passes through filter B. Now if we switch to a different reference frame we can find a frame where photon B appears to pass through filter B before photon A passes through filter A. In this new reference frame, the event photon B passing through filter B appears to be the cause and photon A changing state is the effect which is a complete reversal of the cause and effect sequence observed in the original frame. The interesting aspect is that for any normal cause and effect relationship between non quantum systems, the cause is very distinct from the effect and and any ftl link between cause and effect would be very noticeable, while in the entangled photon case, the reversal is completely indistinguishable. Could it be that nature only forbids an ftl link between cause and effect if the reversal of the temporal order is clearly distinguishable?

    Just to elaborate a little, in SR, for any ftl transmission in a given reference frame, we can always find a reference frame where the transmission goes backwards in time and this gives rise to paradoxical situations. In the entangled photons case, the fact that cause and effect are indistinguishable, means that any ftl link can always be interpreted as a forward in time link in any reference frame and thus those paradoxical situations are avoided.
    Last edited: Jun 29, 2012
  9. Jun 29, 2012 #8


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    Keep in mind that there are also published experiments in which particles are entangled AFTER they have already been detected. You can even entangle (conceptually at least, experiment has yet to be performed) particles that never even existed at the same point in time (in any reference frame).
  10. Jun 29, 2012 #9
    I'm not challenging the conclusion of your thought experiment, however the very possibility of this experiment is a big question to me. Keeping in line with special relativity, is it even possible to find a reference frame such that we will observe that B passes through the polariser earlier than A ? If yes, can you please give an example?
  11. Jun 29, 2012 #10
    By the OP:

    I'm reading the quote a bit differently than the above posters. I read the quote to imply that two different measurements might not be causally connected due to the disturbance caused by each measurement. That makes sense.

    However, the causality of the system itself has not been disturbed...so I do agree with posts such as:

  12. Jun 29, 2012 #11
    The injected force is not unknown but it does trigger collapse to the system, doesn't it? If the observation itself effects the outcome of an action in an unpredictable way, I really don't understand why it shouldn't be linked directly to the its uncertainity.

    Once again I think there is a confusion with the terminology. Does "system causality" implies the deterministic behaviour of our equations? But are the equations themselves not of probabilistic nature to begin with?
    If cause and effect cannot be discerned in this level either due to the principles of quantomechanics or due to our apparent involvement during the measuring process, then we need new definitions.
  13. Jun 29, 2012 #12
    nice post yuiop. at the edge....

    how is cause and effect indistinguishable in QM?

    cause ---> if we try to observe/detect the position of a photon,
    effect ---> we collapse it's wave function.

    in entangled photons

    effect --> the entanglement is broken when
    cause --> one of the particles is detected/measured

    in what sense is cause and effect indistinguishable in QM?
    Last edited: Jun 30, 2012
  14. Jun 30, 2012 #13

    Some will agree some will disagree. You can read extensive discussions in these forums debating those perspectives. Different people interpret the mathematics differently. The language is sometimes confusing, but more often it seems different people have different interpretations of the same terminology AND different interpretations of the mathematics AND different interpretations of the physical causalities.

    [One great discussion is about Heisenberg Uncertainty Principle that went on for several hundreds of posts. I think this is it: https://www.physicsforums.com/showthread.php?t=516224 ..... what is it about position and momentum that forbids knowing both quantities at once? ...And a professional paper by Ballentine is also dissected there...be prepared for several days reading and thinking!!!!]

    Check out these descriptions: [I am posting these for perspective,as ONE set of descriptions, not because everybody will agree with them. In these forums, no matter what you say about QM many 'experts' will disagree and often times disagree with each other. ]

    Albert Messiah, in Quantum Mechanics, says:

    [so a system seems different before and after a measurement. ]

    [the dynamical state is completely defined yet the measurement variables are probabistic]

    [Sounds like a measurement screws up a system in multiple ways: One we can minimize, one we can't.]
  15. Jul 1, 2012 #14
    is the detection/measurement partial?

    once the particles are detected the entanglement is broken, on that factor/state/property

    is the entanglement (after detection) being done on some other property/axis?
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