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Cause and effect in QM

  1. Nov 9, 2009 #1
    It is my understanding that in classical mechanics, cause and effect are universally accepted.

    Is it the same in QM? Is causality sound in QM?
  2. jcsd
  3. Nov 9, 2009 #2
    Yes, of course. In a wave mechanics there is causality.
  4. Nov 9, 2009 #3
    no, of course :)
    Causality emerges on the macroscopic level
    On the QM level the notion of 'event' is not clearly defined.
  5. Nov 9, 2009 #4
    Hi I'm italian and my english is terrible. However, causality in QM is accepted. What is not allowed is the Determinism. Determinism is a philosophy based on Causality: all is caused. This is true in Classic Physic as in QM. But the problem of Determinism is that in this philosophy every event can be expected exactly with all probabilities (probability=1). Instead, the QM does not allow us to expect an event with probabilty=1 (certain) but with 0<probabilty<1 (uncertain: not statistic, probabilistic). This means that we can't know exactly what will be, but we can express mathematically this uncertainty through probabilities.
    Causality does not depend by our knowledge of reality, as the old Copenhagen interpretation said. Today, the phenomenon of Quantum Decoherence led us to think that not the measurement, but the interactions between a Quantum and the other (and the environment) is the cause of the eigeinstates: it's not our knowledge or our consciousness to produce reality, but the actions, and so also our actions. The caratheristics of the single Quantum (not all the carathereistics, only the Dynamic ones, position, speed, and so on) are in Superpositions: all the possible features that a Quantum can assume are coexisting until an interaction makes this coherent states, decoherent. After the interaction, the possibilities fall down into a static certain state, an "eigeinstate", that is the result. During a mesaurement, we necessarily interact with the Quantum, and we can observe its states becoming decoherent in the "decoherence time".

    Yes, it allows Causality, in the philosophycal sense: all is caused.
    No, it is not Deterministic: we can't expect exactly nothing, but probalistically.
    Last edited: Nov 9, 2009
  6. Nov 9, 2009 #5
    One-point measurement is probabilistic but the ensemble of measurement data is determined with the wave function. In CM we also collect many data points to obtain a deterministic average. One point is a too poor experiment; it says nothing about our system.
  7. Nov 9, 2009 #6
    AFAIK there is no causality in QM, for example it seems to be generally accepted that there is no actual cause for the decay of an unstable nuclei.

    To me it is an evidence for the incompleteness of QM.
  8. Nov 10, 2009 #7
    You are absolutely right. QM cannot predict actual events. It can only predict probabilities. In this sense QM is not a "complete theory". But there is a respectable school of thought which says that these quantum events (like decays) are fundamentally random and no theory will be ever capable to predict them. Time will tell.

  9. Nov 10, 2009 #8


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    Here is yet another response:

    If you see that the ontology of QM is different than than of CM, we have not only causality but also determinism.

    In CM the ontological elements are postion and velocity of the system. One set of such, deterministically causates all futures sets. (initial conditions and Newtons laws)

    In QM the ontological elements are information about XXX. On set of such, deterministically causates all future information about XXX. (initial information and the schrödinger equation)

    So there are determinate laws of evolution QM, exactly as in CM.

    "information about XXX" is represented by the quantum state vector, and QM with the schrödinger eq defines how this information about XXX, evolves.

    Of course the real issues IMO is
    - what exactly is information?
    - And what about the "information" contained in the evolution laws? Why does only some information evolve, and other parts are eternal? If "infomation is in some sense fundamental", then it appears very ad hoc or even incoherent (to me at least) to make such a distinction.
    - And what is the logic behind "information implying it's own evolution"?

    Of course some sod these problem are existing also in CM, but this inconsistency becomes more apparent in QM, since it has higher standards and exposes these things better.

    So CM -> QM seems to be a step in the right, direction, but it seems there are yet more steps to take.

  10. Nov 10, 2009 #9
    Thanks everyone for some insightful answers.

    After reading I realised I hadn't really completely stated why question. I am interested in whether or not for all effects that occur, there is always a cause or whether some effects are causeless.

    The example of the radioactive decay as gives me the impression that there are unknown causes for certain effects. There seems to be the idea that QM can only talk about the likelihood of an effect taking place but is not able to say much about the cause.

    I'm going to have to read the remarks through again and see if I can get my head around it.
  11. Nov 10, 2009 #10
    QM answers perfectly all physical questions. Concerning the radioactive decay or other single events - it is not what one seeks from physics. Information on some object consists of many "points" like a high quality image consists of many pixels. This is what QM describes. One, single point on a screen in a double-slit experiment is useless for getting information about the whole interference picture. It is just one element of the ensemble of elements representing information about the system. One single decay is useless to determine the half-life of the radioactive nuclei in a sample. One single point is so poor that one cannot even prove its origin without making sure that it belongs to the studied object. And any making sure includes many-many single points. That is how determinism appears.
    Last edited: Nov 10, 2009
  12. Nov 10, 2009 #11
    We agree that QM does not tell us anything (or tells us very little) about single events. However, I don't think that we can dismiss these single events as being irrelevant for physics. As Schroedinger taught us, a single radioactive atom can kill a cat. This does not seem insignificant to me. So, I would prefer the point of view that there is a degree of randomness (or unpredictability, or lack of a cause-effect relationship) in Nature, which cannot be explained by our existing theories.
  13. Nov 10, 2009 #12
    you can chose any DETERMINISTIC interpretation of QM then
  14. Nov 10, 2009 #13
    Can any deterministic interpretation of QM predict when a single chosen radioactive atom will decay? I believe not. So, in my opinion, these interpretations do not add anything of practical significance to the basic formalism of QM.
  15. Nov 10, 2009 #14
    One single event is a part of ensemble. I do not dismiss it. I collect them. I just point out that a single event it is not all information. In CM, when you observe a body with your eyes, you get plenty of points, you average them and attribute the average number to the body geometrical center. One photon is not sufficient to get information about a classical body either. You always need plenty of them to get certainty, determinism.
  16. Nov 10, 2009 #15
    In practical sense - no, you're right.
    But no theories (including future theories) will allow doing that
  17. Nov 10, 2009 #16


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    I can relate to both these points and I think a possible solution is even to answer the personal question I wrote in the other post:

    "And what about the "information" contained in the evolution laws? Why does only some information evolve, and other parts are eternal? If "infomation is in some sense fundamental", then it appears very ad hoc or even incoherent (to me at least) to make such a distinction."

    My objection here is exactly the apparent incoherence that at one level (individual QM events) there is apparently a lack of causality, and at another level (ensemble or statistical levle) there is PERFECT causation(determinism).

    This is why my personal view is that even the rules of causation (the laws of physics ~ the hamiltonian) must be treated on the same footing, and thus we should talk about information about laws; this is exactly what one gets in the inference approach, where "information about law" is respected. Thus we get a link between the two apparent unrelated levels (single event level) and (perfect statistics), and I think the reality is in fact somewhere in between, which means the perfect causation at probabiltiy level is actually an idealisation. But the symptoms are apparent only in extreme domains.

    So if we constrains also causal laws to the operational perspective, then I argue that the complexity bounds of the observer, actually puts a physical limit on what causal relations that are physically inferrable or measureable.

    Then at least we reach a coherence in the framwork, where ALL information is subject to operational constraints. No "laws of inference" will escape as meta laws.

    Then the explanation if why the causality is lacking between individual events is because it's not possible to infere - with any level of confidence - anything from a single data point. The more data we have, the more confident do we get. But the limiting case of PERFECT confidence is also unphysical - THIS is not respect in corrent physics abstractions, and its' why I find it incoherent from this choice of analysis.

    Edit: This is IMO also the root of alot of infinities. But assuming infininte confidence in some things (while this is not really true) it's no surprise that odd things happens like "infinite probabilities" etc. When we "count" possibilities, the standard procedure ignores WEIGHTING the possibilities with the limited confidence in the inference system (causation rules) that is used. To connect to Bob's quite different "reformulation" - this is how I would like to perform a "reformulation. But ignoring the fact that some inferences are not perfect, we thereby assign them a unphysical weight and thus of course when we try to sum the result it diverges.

    Last edited: Nov 10, 2009
  18. Nov 10, 2009 #17
    That's exactly my point. We will never know when a single atom chooses to decay. God does play dice.
  19. Nov 10, 2009 #18


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    I fully agree with your point that one event is not all information. But I think it is also clear that no real observer EVER gets ALL information. In particular would the confidence in a picture depend on the "resolution of your eyes".

    There is no infinite-resolution reference to hang on to, because even if it did, a finite obserer could not extract all it's information.

    So I think this means that even the ensembles are not well defined operationally. This is why the determinism that exists at infinite ensemble level is an unphysical abstraction. Do you disagree?

  20. Nov 10, 2009 #19
    I'm a philosopher, and I will express the question in philosophical terms.
    We don't know it, but it doesn't mean that God does play dice, that the single event is ontologically random (not epistemologically). If we really "DON'T KNOW IT", we can't say that an event is caused or not, because we simply don't know it.
    It depends by the interpretation you choose, but all the modern interpretation led to the same result, all of them make the same probabilistic prediction.

    I think that macroscopical causality must be exlained by a microscopical causality: if the quantum world is random, why the macroscopic level is causal (not Deterministic, causal)?.
  21. Nov 10, 2009 #20
    But we usually do not care much about the precise moment. It is like baking an apple pie. The longer you wait, the more done it is.

    By the way, the reason of decay is tunneling. No tunneling, no decay. More generally, it is instability of a compound system. In a usual world all atoms suffer excitations and emissions - in this way the thermal equilibrium is established between the compound systems at T > 0.
    Last edited: Nov 10, 2009
  22. Nov 10, 2009 #21
    Well, sometimes we do care. If the radioactive atom is placed near Geiger counter which can break an ampule with poisson, which can kill my own cat, I am very interested. I would like to know when this event happens, so I can save my pet. QM cannot answer this question. QM can only predict probability in an ensemble of 100 cats, but it is silent about the fate of my own cat. So QM is not a "complete theory" as far as I and my cat are concerned.
  23. Nov 10, 2009 #22
    Ehh... no...

    We (the observers, "frogs") will never know - correct.
    God ("birds view observers") - does not play dice.

    This is true in BM and MWI for example.
    Observers can not predict the events
    But God does not play dice
  24. Nov 10, 2009 #23


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    Welcome to PhysicsForums, Galactor! This question can easily drift into semantics. Given your question as I think you intend, I would say:

    There are NO causes for any quantum behavior. This is especially obvious with radioactive decay, but the same can be said for emission of any particle of light. Or for that matter anything which has a potential for occurance that is less than 100% - which would apply to almost all subatomic events within a given time frame.

    Not, that said, let me add a few caveats.

    a) Orthodox quantum field theory has not identified any causes for any probabilistic behavior. As far as anyone knows, there are none.

    b) There are interpretations of QM - such as Bohmian types - which appear to be fully deterministic at least in principle. However, in practice there is no more ability to identify causes than in orthodox QM.

    c) As has been mentioned, the wave function itself appears deterministic. However, it would - in my opinion - be difficult to describe this as fundamental causal as there is wave function collapse which is not deterministic. I do not see how you could describe the observed results of subatomic experiments as "causal" based on wave dynamics.

    d) In my opinion, only a minority of physicists believe there is an underlying cause for subatomic events.
  25. Nov 10, 2009 #24
    Of course, God knows everything. He knows in advance how his die will fall. So, he is laughing at us poor mortals trying to comprehend his wisdom. He lives in a sterile boring world where there is no place for mystery and chance. We -- frogs -- are condemned to face the mysterious unpredictability of the quantum world. We are allowed to know only part of the truth. The rest is available only to such supreme beings as God and birds (perhaps also Dmitry67?).
  26. Nov 10, 2009 #25
    i just wanted to say that "God does not play dice" is not applicable to BM and MWI
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