Causality is generally accepted to be a fundamental physical principle. But quantum mechanics is acausal (e.g. there is no 'why' as to the result of a measurement of the position of a particle in an infinite 1D square potential well). In which case what is a distinction between causality and determinism, that has a meaningful non-acausal definition of causality.

It seems to me that "causality = determinism + quantum mechanics". It seems that it doesn't provide any new concepts, it's just a flawed attempt to reconcile determinism with the arbitrary probabilistic nature of QM, which are ultimately unreconcilable. Does anyone agree with this idea? If not, I would like someone to try and convince me that I'm wrong.

I am fully aware that a lot of threads barking up this tree, and I have tried reading many of them, but none seem to crystallize my problem very well.


Let's clarify the concepts:

  • Determinism: for a given experimental set up, every experiment will yield the same results. QM are not deterministic, if you measure a superposition of states, you could get one or the other, with certain probability, and there is nothing you can do about it.

  • Causality: causes happen before its effects. QM is causal, because if you have a system in superposition and do a measurement on it, you destroy the decoherence and make it collapse. But this happens as you measure, not before. You cannot change the outcome of the experiment you did yesterday.

And let me throw another one:

  • Teleology: everything happens for a reason. Giraffes evolved to have a long neck so they could eat from the high branches. The electron jumped to a lower level so it would release a photon and you could read. This is a matter of pure philosophy, not science.

Your "there is no 'why' as to the result of a measurement of the position of a particle in an infinite 1D square potential well" falls into the teleological approach, not on the causality. Causality tells you that I pressed the button on the machine before the measurement, but it is completely irrelevant why I did it.

  • $\begingroup$ That literally blew my mind. Thanks for being so explicit about the difference between a cause and a reason. I was in the mindset that in qm one can say that a particle has a position, a la stochastic classical mechanics. I think I have to keep reminding myself that it's a ket in some crazy Hilbert space, and that it's meaningless to speak of the particle having a position before you operate on it with a bra and a position operator. Please correct me if any of my interpretation sounds wrong. I understand quantum mechanics a lot better now. Thanks so much for your reply. $\endgroup$ – Dennis Addison Apr 18 '14 at 16:20
  • $\begingroup$ An electron knows nothing about bras and kets. It just exists. Where is it? You cannot know until you measure, and to do that you have (for example) to knock it with a light beam. But before that, the particle is sort of spread over an area. $\endgroup$ – Davidmh Apr 18 '14 at 16:54
  • $\begingroup$ Bras, kets, and hilbert spaces are just tools for a mathematical description. If you want to know the position for a given wavefunction, you apply the position operator, and get a probability distribution, but everything remains the same. If you do the actual measurement, you get a single value, but then you have destroyed the experiment. $\endgroup$ – Davidmh Apr 18 '14 at 16:56

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