A quantum system consists of some states. It is in all of the states but beCAUSE of the act of the measurement, the wavefunction collapses into one state. I am really confused because I can repeat the same measurement and get different results at each time! The probability to get a specific outcome is considered to be a function of the wavefunction, but this is not what I mean. This is OK, but when it comes to predicting the outcome of each measurement, it becomes acausal in my opinion, but I am not sure. I have tried to bring my confusion from my mind to stack exchange and this is the result: If performing exactly similar observations yields to different outcomes, so it means that the cause of each outcome (=observation) is not enough to determine the outcome. So, what else contributes in determining the outcome to be what it is after each measurement? There is no other cause (that we don't know) because if there was any, it would be that hidden variable that realists like Einstein believed to exist and we are assuming the orthodox interpretation here.Therefore, the only other thing that can contribute in determining the outcome is the system itself. For instance, the electron decides to which state to go! This means that the rest of this causality must be filled by the system itself, so the system must change itself. This is impossible because of the same reason that I cannot pull myself up the ground by pulling my hair upward! (or in mechanics, the third law of Newton explains why something can't change itself by exerting a net force on itself!).

MY QUESTION: So, here is my question: I assumed causality and orthodox viewpoint and arrived at either 1- rejecting orthodox by accepting realism or 2- accepting that systems can cause themselves to be changed. None is logical as the first is in contradiction to my assumption and second is in contrast with the third law of Newton. So, the two assumptions I made are incompatible, that is, they cannot coexist. So, the Copenhagen interpretation and causality cannot be both true, and so the orthodox viewpoint is acausal. Now, is this argument true? If no, why and if yes, which one do you accept to be true: the causality or the Copenhagen interpretation of QM?

  • I see you've asked an apparently related question at physics.stackexchange.com/questions/520423 But it seems to me that question might be better suited for this forum, whereas this question might be better suited for that one.
    – user19423
    Dec 19, 2019 at 16:21
  • Yeah that's right. There are many physics questions so most are not answered properly, including mine. So, I asked my question also here; maybe I would find my answer! Dec 19, 2019 at 17:05
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    @JohnForkosh So the OP's questions are entangled?
    – user4894
    Dec 19, 2019 at 21:54
  • "There is no other cause... Therefore, the only other thing that can contribute in determining the outcome is the system itself" does not follow. There needs to be no other cause contributing, electron does not "decide" anything, it just happens, uncaused. The principle of sufficient cause can simply be false, this is called indeterminism. And indeterminism is perfectly compatible with the momentum conservation law, which is what "the third law of Newton" amounts to, see Quantum Explanation of Newton's Third Law of Motion.
    – Conifold
    Dec 20, 2019 at 1:20
  • @conifold, thanks for the comment. There is a problem about your statement I think. This "happens uncaused ", does it mean that no cause was behind the choice of the electron to spin up but not down? If yes, then it is exactly what I tried to show. If no, please tell me why and I think it would be the very last question in that case! Dec 20, 2019 at 7:39

1 Answer 1


Excuse me for being a little nasty, but your profile says A-level student, whereas I think you just made an F-level remark in your premise

      "I can repeat the same measurement and get different results at each time"

Wanna try again, i.e., repeat your statement and try getting a different result:)?

When you >>first<< make a measurement, then the outcome is indeed stochastic. But as a result of that measurement process, not only do you get a value for the measured observable, but the system also collapses (typically from a mixed state) into a corresponding eigenstate of that observable.

So that means if you immediately >>repeat<< the same measurement, with no intervening disturbance to the system, then the same outcome is guaranteed with probability 1.

  • I agree that if you measure again simultaneously, the next outcome is predictable with certainty. However, that is another assumption you are making which I didn't. In other words, I did not assume in my argument that the measurements are simultaneously after each other. Dec 19, 2019 at 17:10
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    @seyedsepehrmousavi What I said is, "no intervening disturbance", and >>that's<< the fundamental requirement; the interval between measurements could conceivably be a million years, as long as you can guarantee there's >>no intervening disturbance<<. And if there is a disturbance, then that's the >>cause<< of any difference between measurement outcomes. But anyway, everyone here and in physics.se told you you're wrong, which you seem disinclined to accept. So go write a paper and submit it to Physical Review, or elsewhere. If you're indeed right, then it's a great and important discovery.
    – user19423
    Dec 20, 2019 at 1:33
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    So if the act of measurement has finished, and my system is isolated and so there is no disturbance, is the next outcome the same as the previous one? If yes, my question is solved! Dec 20, 2019 at 7:48
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    @seyedsepehrmousavi Guess what?... Yes, so your question is indeed solved. The observable corresponding to a measurement apparatus is represented by a Hermitian operator. So once the measured system's state is an eigenfunction of that operator, then repeated applications of that same operator to that state (i.e., repeated measurements) leave the state unchanged, whereby you repeatedly measure exactly the same eigenvalue outcome. Ask for further clarification back on one of those related physics.se posts you started. That's where this question belongs.
    – user19423
    Dec 20, 2019 at 8:36
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    Thank you so much, John. I really appreciate your help. I will try to understand what is a hermitian operator and eigenfunction so that I can understand QM more deeply. Meanwhile, I would ask my question from you and other kind people at physics.se for clarifications. Thanks! Dec 20, 2019 at 13:51

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