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Quantum theory and causality

  1. Jul 27, 2009 #1
    Hello all.

    I am merely an inquirer into physics; not even an amateur.

    The question that has puzzled me with regard to quantum theory/physics/mechanics, is the status of causality.

    What is the status of causality in quantum theory?

    I would be grateful if anyone could explain this to me.

    Best regards, Borean
  2. jcsd
  3. Jul 27, 2009 #2
    Everything is OK with it: the wave function equation is causal, its solution is determined with the initial conditions, just like in classical mechanics.

    Another thing is a probabilistic outcome of the measurement process. The measurement "points" are spread out (remember an interference picture). But it is an intrinsic property of quantum world. The average (inclusive) things are well predicted with the wave function.
  4. Jul 27, 2009 #3
  5. Jul 27, 2009 #4
    Quantum theory has two components. There is the wave function (or state vector) which is assumed to evolve in a causal way before it collapses, and then there is the collapse process which is not causal.
    There is a standard mathematical machinery that describes these processes and which has been proven to work in all attempted experiments. This mathematical formulation gives good predictions but the results are of a probabilistic nature.
    While the mathematical formulation is usually not challenged due to its proven success, its interpretation remains ope to debate.
    You can use the math to predict (to the extent allowed by the theory) the results of the experiments. But when you start asking: What does this really mean? you'll get mainly two different answers:
    (1) You should not ask this question. Just keep using the math (because it works).
    (2) It is valid and potentially useful to try to gain a better understanding of how the different elements in the mathematical apparatus of quantum mechanics correspond (or not) to elements of reality.
  6. Jul 27, 2009 #5
    It only means that quantum world is different from classical one.

    The main thing - the wave function evolution - is causal.
  7. Jul 27, 2009 #6
    So that means that the math, in itself, is neutral to whether or not there is causality or not behind our observations?

    Or does the math specifically force us to abandon causality?

    Since we can still predict this supposedly "acausal" wave collapse process probabilistically, wouldn't it be incorrect to say that it is "acausal", and more correct to say that it is not fully determinable?

    Causality really just means one thing depending on another, even if we don't know what depends on what, and even if we can't explain every causative event causally, and even if we don't have the observational abillity to give a causative account of everything in the universe.

    That is, causality is not = determinism.
    Last edited: Jul 27, 2009
  8. Jul 27, 2009 #7
    There is no wave function collapse while measurement as there is no probability collapse. The notion of probability belongs (covers) all (big amount of) measurements.
  9. Jul 27, 2009 #8

    But is the "wave function collapse" (or anything else in quantum theory) explicitly acausal or is it simply undeterminable what it's "cause" is?
  10. Jul 27, 2009 #9
    The cause is an interaction with a measurement device. If we speak of photon absorption by a photographic plate, it cannot be predicted because the plate consists of many atoms whose sizes are much smaller than the photon wave-length. Who knows which one is the most ready to absorb a photon? So such events are not predictable but only their sum.
  11. Jul 27, 2009 #10
    Ah, I see.

    So there are some limitations as to our observatory power.

    But my question still remains; are there any explicitly acausal things in the universe, according to quantum theory? Can things happen without causes?

    And remember, indeterminacy does not mean acausality. If things happen without us knowing the cause, that does not mean they have none. What I am asking is rather if quantum theory positively affirms acausality as such.
  12. Jul 27, 2009 #11
    No, as far as I know, there is no such an affirmation. There is just huge difference in classical mechanical and quantum mechanical causalities, but there is no acausality in QM.

    I would say that the classical mechanical determinism emerges from the inclusive QM picture when many-many events are added together and averaged. The average value is unique.
  13. Jul 27, 2009 #12
    Interesting. What about something like atom/nuclear decay? Isn't that acausal?

    The discussion I linked earlier says that it is.

    Let me quote:
    But I suspect what he really means is that it is indeterminable, to us.

    Can anyone clarify this?
  14. Jul 27, 2009 #13
    It is the same song. One cannot predict any single quantum event, it is a nature feature. But you can:

    1) predict whether it will happen or not. The radioactive decay law says it will happen for sure.

    2) predict the average number of decayed or remaining nuclei N(t) in a sample of many-many atoms.

    Any single event, taken separately (observed only one time), does not even say what has happened. You need statistics to create notions, language of phenomenon description.

    The radioactive decay has a clear cause - the potential barrier transparency (for simplicity).
    Last edited: Jul 27, 2009
  15. Jul 27, 2009 #14
    So does that mean that single quantum events don't have any cause -are acausal- or simply that they are indeterminable/unpredictable in isolation but still have a cause?
  16. Jul 27, 2009 #15
    They are are indeterminable/unpredictable but have a certain cause to happen.

    Look at the determinism as at an average (unique) picture obtained from many spread, occasional points.
  17. Jul 27, 2009 #16


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    QM is acausal, as explained by the following experiments.

    Shine light through a polarizing filter. Some fraction will pass through it, which will then be polarized.
    Now place a second polarizing filter after the first, and aligned with it. Most of the light which gets through the first filter will also pass through the second (100% in theory, but polarizers are not perfect).
    Rotate the second polarizer to that it is aligned at 90 deg to the first. Then, none of the light will get through it.
    Now align the polarization of the second filter at 45 deg to the first and about 1/2 the light will get through.
    However, you can reduce the intensity of light so far that there is only a single photon passing through the apparatus at any one time. Given that the photon passes through the first filter, it is not possible for "half the photon" to pass through the second. Instead, QM says that the photon either passes through the second filter or it doesn't. The outcome is random, with each possibility happening with a probability of 50%.

    When it is said that the wavefunction is deterministic, this means that we can calculate the 50% probability precisely (the probabilities themselves are deterministic), but the outcome of the experiment is still random.

    According to the standard interpretation of QM, the outcome is fundamentally random and it is theoretically impossible to predict the outcome.
    Of course, you can always suggest that maybe there is some more fundamental theory than QM with additional underlying variables, the knowledge of which would allow you to accurately predict the outcome of such experiments. However, there are good reasons for believing that this is not the case (e.g., Bell's theorem), although it doesn't convince absolutely everyone.
    Last edited: Jul 27, 2009
  18. Jul 27, 2009 #17
    QM is causal only to the extent that the wave function can accurately predict probabilities. But that is not what we normally understand as strict causality. Lets say we have a wave packet that represents a particle. We can predict how the shape and the position of the wave packet is going to evolve. But when we try to find the location of the particle, we can't say with certainty were we are going to find it. In this respect QM is acausal.
    You can send a photon through a half-silvered mirror and it'll have 50% chance of being reflected and 50% chance of being transmitted. You can put detectors that will find out if it was reflected or transmitted. There is no element in the theory that allows you to know beforehand which outcome you are going to get. This is acausal.
    There have been discusions about the possibility of some hidden variable determining the outcome of apparently random events. I think it was determined that if there were hidden variables they could not be local.
    Nobody has ever been able to exploit the concept of a hidden variable to predict the result of an otherwise random event.
    The idea of random (probabilistic) events does not need to be that unacceptable. When you travel through space, you cas find objects scattered around without any "causal" relationship to the adjacent objects. By looking at a region of space you can't predict what you'll find in adjacent regions. Why can't it be the same with time?.
    If you believe that there is "one world" (that is one past, one present and one future) you could picture this world as represented in a strip of film. You will find some connection between adjacent frames in the film. Usually you'll see that objects "move" smoothly, etc. But it doesn't necessarily have to be that way. You could imagine a film where a particle is at a totally random location in each frame. In this case you would be assuming that the particle does have a particular location which is unpredictable. But I think there are experiments that show that even assuming that the particle moves in an acausal way is not enough. You have to assume that it does not have a particular location until you measure it.
    Other way of looking at it is to say that the particle is in all places at once (a superposition). How you think of this would depend on your particular interpretation of quantum mechanics.
  19. Jul 27, 2009 #18
    It's random, but that does not mean acausal.

    We need to distinguish between determinacy and causality.

    Indeterminacy does not mean acausality; it means indeterminacy.

    If our ancestors millennia ago had no idea why the sun rose, no matter how hard they tried explaining it, that would not imply that the universe is acausal, but simply indeterminable.

    Even if we couldn't determine why the sun rises today, we would still not have come to the conclusion of acausality; still only indeterminacy.
    That is not necessary to affirm indeterminacy of isolated events in the universe.

    Simply because, there is nothing that says any theory which has anything less than total information about the universe will ever be able to predict/determine the total amount of events in the universe, in isolation.
    But Bells theorem does nothing to suggest acausality over a fundamental indeterminacy.
  20. Jul 27, 2009 #19
    Again, that is indeterminacy, not acausality.
  21. Jul 27, 2009 #20
    As soon as QM works with probabilities, it is clear that the events are occasional, spread, with non-trivial distributions. The physics task is to catch this property of nature and QM does it. The laws of ensembles (distributions) are described well, without hidden parameters.
  22. Jul 27, 2009 #21


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    Ok, I'm not really sure about the definitions here. Yes, QM is random. Is this the same thing as acausal? What caused the photon to pass through the polarizers rather than be absorbed? I don't know - QM just says that it is just a random event with given probabilities. If you consider that to be enough to constitute a cause, then it is causal.
  23. Jul 27, 2009 #22
    The polariser presence means introducing an interaction (absorption). Any interaction has many channels, choices, paths, etc. No wonder sometimes photons are absorbed, sometimes not.
  24. Jul 27, 2009 #23


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    I think Borean's only disagreement with my post was over the meaning of the word acausal, and that we agree about what QM is saying. Which is fine by me.

    And I think Bob_for_short is agreeing with my post, but I'm not sure. QM tells us the probabilities for each outcome. If that is really the best that can be done (and I expect that it is), then we can't really expect anything more of it.
  25. Jul 27, 2009 #24


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    If the polariser is working properly, then
    - a photon with polarization aligned with the polariser axis will pass through it.
    - a photon with polarization at 90 deg to it will be absorbed.
    That's all you need to be able to conclude that the probability is 50% in my experiment, regardless of how the photon actually interacts with the polarizer. Furthermore, it allows you to say that the probability is cos2(a) for an angle a between the polariser axes.
  26. Jul 27, 2009 #25
    Can anyone tell me what is the cause of the phenomenon known as quantum tunnelling?

    And additionally, the cause of the phenomenon of vacuum fluctuations or virtual particles?
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