Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Determinism, realism, hidden variables

  1. Jun 12, 2016 #1

    I'm still confused about those three concepts. They sound pretty much the same to me, but this does not comply with Wikipedia's table of comparison of QM interpretations, where pretty much all combinations seem to be present.

    Actually, is the "Wavefunction real?" column even about "realism" in the EPR sense, or is this something different?
  2. jcsd
  3. Jun 12, 2016 #2
    I would say that "realism" and "hidden variables" are basically the same thing. "Determinism" is even stronger, since it is generally interpreted as that everything in the universe is predetermined from the outset. A hidden variable on the other hand can be randomly generated at the source, and Bell's theorem will still hold (Bell's theorem tends to be the only context where these concepts are used).
  4. Jun 12, 2016 #3


    User Avatar
    Gold Member

    You can see in this table that everytime you have hidden variables the theory is deterministic. At the exception of stochastic interpretation (maybe the values of the hidden variables could have a random origin)
    But in all the cases if you knew the values of the hidden variables an algorithm would give you the result of the measurement.
    QM with no hidden variables gives the correct probabilities. If you add hidden variables that give the same correct probabilities what does it add if it does not give the outcome?
  5. Jun 12, 2016 #4
    What about counterfactual definiteness, is this the same as well? Its definition sounds quite a bit different to me ("the ability to speak meaningfully of the definiteness of the results of measurements that have not been performed"), but on the correspondent Wikipedia page they seem use it as a replacement for what is usually called realism in the Bell/EPR context.
  6. Jun 13, 2016 #5
    No, there can be realistic theories which interpret the wave function as epistemic, thus, describing only our incomplete knowledge of reality. Once the reality may be described by something different, like the configuration itself, this interpretation may be realistic.

    The problem to distinguish between a deterministic theory and a realistic one with some random process guiding the evolution of reality I do not understand.

    Counterfactual definiteness is, instead, the assumption that some particular variables have predefined values. Say, two criminals want to event a cover story to hide what they have done. They are separated, and asked separately. They give consistent answers. So the police assumes that the answers where predetermined. And they may think that these answers describe reality even if they have no independent witness accounts which tell anything about these claimed facts - this would be counterfactual definiteness. They will probably stop to think so if they observe that these criminals have had a hidden communication channel. But believing in this case in counterfactual definiteness has not much to do with believing in the existence of some reality, or with beliefs about fatalism vs. freedom of choice.
  7. Jun 13, 2016 #6
    This is a very common error in the presentation of Bell's theorem. Counterfactual definiteness of the spin values is not assumed in Bell's theorem, but derived using the EPR argument: We see that whenever we measure at A and B the same direction, we obtain identical results. That means, we can predict, with certainty, the result at B for a direction a if we measure this direction at A. The EPR criterion of reality assumes that in this case, if this measurement does not disturb in any way the other measurement (where we use Einstein causality to show this), then that means that the result at B for a direction a has to be predefined even if we do not measure it. This is the counterfactual definiteness. But it is derived, and nothing general, but a result about this particular situation, and depending on the very nontrivial assumption of Einstein causality.
  8. Jun 13, 2016 #7
    It clarifies that all the mysticism justified on the base of quantum theory is nonsense. Like the rejection of realism, or of basic principles of causality like Reichenbach's common cause principle.
  9. Jun 13, 2016 #8
    I do not see any "mysticism" in established quantum theory at all, unless one insists that the universe must satisfy some misguided notion of classicality based purely on subjective human experience, and hence on what may or may not appeal to us philosophically and aesthetically. The only thing "mystic" here is the concept of ethereal, undetectable, unmeasurable "hidden variables" - you might as well postulate invisible pink unicorns, for all the scientific value it has.

    That's just my personal opinion, from an interested amateur outside of professional academia.
  10. Jun 13, 2016 #9


    User Avatar
    Science Advisor

    The idea of Bohmian hidden variables in QM is analogous to the idea of dark matter in astrophysics. Neither of them can be directly observed, yet both have a strong explanatory value. Let me explain the analogy in a few more entries copy-pasted from one of my conference presentations:

    Bohmian interpretation: deterministic particle trajectories guided by ψ.
    - If it s true, then why trajectories cannot be observed?

    Analogous to dark matter (astrophysics):
    - If dark matter exists, then why it cannot be observed?

    Both questions have a similar answer.

    Indirect detection:
    - sufficient that exists influence on something else (“detector”)

    Direct detection:
    - humans tend not to be absolutely convinced that something exists,
    until they are able to detect the exact place where it exists.
    ⇒ need to know where does the influence comes from!

    Non-dark matter (stars):
    - we observe light from the object
    - light is a wave ⇒ it has direction of propagation
    ⇒ easy to determine where does it come from
    ⇒ observation is direct

    Dark matter:
    - does not produce (or interact with) light
    - observed by static gravitational field produced by dark matter
    - static gravitational field does not have direction of propagation
    ⇒ cannot easily determine where does the field come from
    ⇒ observation is indirect
    ⇒ Indirect detection of dark matter is considered
    less convincing than direct detection of non-dark matter.

    Analogy with Bohmian particles:
    - there is evidence for Bohmian particles (observations can be explained
    by it, but there are also other explanations)
    - non-local quantum potential similar to gravitational static potential
    (does not have direction of propagation)
    ⇒ cannot easily determine where does potential come from
    ⇒ cannot easily determine position of Bohmian particle
    ⇒ evidence for Bohmian particles is only indirect

  11. Jun 13, 2016 #10
    You are making a very good point here - perhaps it is my own opinion that is misguided.
    Probably best to shut up and keep learning so o_O
  12. Jun 13, 2016 #11
    I do not see anything mystical in quantum theory too - we do not understand QT sufficiently good, that's all. But a rejection of the existence of the existence of some objective reality certainly qualifies as mysticism. And to postulate the existence of correlations which do not have a causal explanation is what we know from astrology, and also clearly qualifies as mysticism.

    You can, of course, justifiy any mysticism by rejecting non-mystical views as "misguided notion of classicality based on purely subjective human experience". But this is nothing but cheap polemics. The "hidden variables" of realistic QT interpretations are, by the way, not at all undetectable and unmeasurable, but they are all what we detect and observe - classical configurations. At least I have not yet seen any wave function, I have always only seen quite classical configurations - which are the "hidden variables" of the "hidden variable" interpretations of quantum theory.
  13. Jun 13, 2016 #12


    User Avatar
    Gold Member

    It is not difficult to see in the Young experiment how the DBB trajectories are related to the 2 slits.
    It is harder with a Stern Gerlach to see how the possible trajectories are related to the inital values of the spin.
    No intuitive thing comes to me from this interpretation
  14. Jun 13, 2016 #13


    User Avatar
    Science Advisor

    Perhaps this can help
    To develop intuition, it may be sufficient to look at the figures.
  15. Jun 13, 2016 #14
    This is your interpretation of it, but surely you must see that the standard probabilistic interpretation without hidden variables explains these phenomena just as well. I grant you that the standard interpretation may not appeal to you, but that does not outright invalidate it.
    I am prepared to concede that I must remain open to at least the possibility of some form of hidden variable interpretation as I continue to learn about QT, but I think this has to go both ways - people also need to be open to the idea that perhaps, just perhaps, the universe quite simply is not classical and deterministic at its core. Personally, I see no issue with this.
  16. Jun 13, 2016 #15
    No. It gives rules how to compute probabilities, but explains nothing. I grant you that rules to compute probabilities are important, from an instrumental point of view, even without such explanations. But there is no reason to name something an explanation which explicitly refuses to give an explanation.
    It is, in principle, imaginable that this universe is only a wild dream and what happens does not have any explanations. But up to now it does not look like this, the idea that there is an external reality, which we can understand, that the correlations we observe have causal explanations, has been sufficiently successful during the last centuries of science. And actually there is not even a problem worth to be mentioned to cause doubt, we have nice realistic and causal interpretations.
  17. Jun 13, 2016 #16


    User Avatar
    Science Advisor
    Gold Member

    You will find both sides of the coin expressed: there are plenty of physicists who use these terms interchangeably in conjunction with Bell's Theorem. That is especially true of the terms "realism" and "hidden variables".

    And there are others who tend to see distinctions. There are even papers that explore these differences. Most of those turn on semantics, and there is not a general consensus. So some of it comes down to personal choice. I personally consider both of the following to be fair restatements of Bell's Theorem:

    "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

    "No local realistic theory can ever reproduce all of the predictions of quantum mechanics."

    Further, I don't consider "counterfactual definiteness" to be different in any meaningful manner than the term "realism". Again, not everyone agrees on this. The important thing is for you to understand how Bell's Theorem works first. Then, later, look at the semantics.
  18. Jun 13, 2016 #17
    I don't, and I would say it is clearly false. In particular, dBB theory is clearly realistic, and even deterministic. it specifies what really exists, with evolution equations, and all what described classical reality (the trajectory in configuration space) is part of reality too.

    But does not have any counterfactual definiteness: except for position measurements and eigenstates of the measured operator the "measurement result" depends also on the state of the "measurement device". So, "unperformed measurements" do not have a predetermined result in dBB theory.
  19. Jun 14, 2016 #18
    But what if those probabilistic rules are the explanation ? What if the universe actually does play dice ? Why rule out that possibility, if it fits experiment and observation so well ? You bring up Reichenbach's principle in this context, so, having had only superficial knowledge of it, I did a bit of reading about this - it seems that, firstly, the exact meaning and definition of the principle itself is subject to some debate ( i.e. there seem to be several versions of it ), and, secondly, that there is no general consensus as to whether or not the principle even applies to the case of QT, which exhibits certain differences to classical probability theory. I have found both authors arguing in favour of and against the applicability of the principle to QT. If anything, my impression is that the majority of authors lean more towards non-applicability, or leave the question open to further research. I have found this summary, and the examples given therein, quite interesting :


    This was also an interesting read, though I must admit that a lot of it is over my head :


    The impression I get from all this reading is that the applicability of Reichenbach's principle to QT is somewhat doubtful, but perhaps not quite ruled out as such just yet, depending on which exact definitions of the principle are used. What does seem clear though is that there is no fundamental law of nature that demands ( or even implies ) the existence of a common cause principle for the case of QT - this is of course not an argument to rule out the possibility, but still.

    I think that depends a lot on what one means by "explanation". I think it is perfectly conceivable that the universe functions such that there are not always common causes in the Reichenbachian sense present; but I don't think that this implies any kind of mysticism.
  20. Jun 14, 2016 #19
    @Ilja - I would just like to say that ( while I can't speak for others ), your contributions here are appreciated by me. You are clearly quite knowledgeable in this area, and having the general consensus and opinion challenged in a positive and constructive way, is invaluable for someone like me who is still in the process of learning about all of this. What this did is make me go and do my own further research on certain subjects, and I have learned bits and pieces that I didn't know before. Also, it is always important to take a step back and critically evaluate one's own knowledge and understanding, every so often.

    So for all it is worth, I thank you for bringing this up - I do not need to agree with all your assertions, in order to appreciate your contribution here :smile:
  21. Jun 14, 2016 #20
    Determinism vs. randomness is not the issue at all. In "Bertlmann's socks and the nature of reality" Bell gives some quotes which show that it was not an important issue for Einstein too, despite his side remark about God playing dice. Realism and causality are much more important. Personally I prefer hidden variable theories which have random trajectories, they are simply not that popular like deterministic dBB theory.
    Correct. There are, of course, the relativists who see that once there is a theorem which shows that Reichenbach's principle + Einstein causality => Bell's inequality, that means wrong, one has to reject Reichenbach's principle. And relativists are a clear majority.

    My point of supporting Reichenbach's common cause is that without it we do not have a justification for the need of finding causal explanations of observable correlations. So I see it not that much as a claim about reality but as part of the justification of the scientific method itself.
    I would yet have to see an example of a correlation without any causal explanation which is not mystical. What I have seen as counterexamples were simply too restrictive interpretations of the meaning of "common cause", which excluded things like multiple common causes, microscopic or nonlocal common causes, or the flow of time as a common cause.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Determinism, realism, hidden variables
  1. Hidden Variables (Replies: 9)