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Lets say a scientist were to perform the double slit experiment. He sends in one electron through the slits and then that he measures the random location on his measurement device after the wavefunction collapses. Now if one were to ask what was the specific cause/reason that electron was measured at that specific location, you would have to say there was none, at least with the Copenhagen interpretation. So then this goes against the Principle of Sufficient Reason since there is no reason for that the electron to be measured at that specific location.

The principle of sufficient reason being that everything must have a cause and/or a reason.

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    This is a good question. Please make it better, though, by starting with an explanation of what the principle of sufficient reason is, and maybe an example of how it was used. Feb 24, 2022 at 9:54
  • Specifically Ch.8 Quantum Mechanics. Feb 24, 2022 at 10:28
  • The indeterministic interpretations of quantum mechanics (Copenhagen and the like) that declare probabilistic predictions complete as to what there is to be known prior to the collapse do contradict PSR. They are the dominant ones among physicists today, so in that sense PSR is disproved by QM as interpreted. But empirical theories cannot prove or disprove metaphysical doctrines in the strict logical sense. There do exist alternative minority interpretations of QM compatible with PSR, such as Bohmian mechanics.
    – Conifold
    Feb 24, 2022 at 11:45
  • @Conifold how dominant are they really? I'd be curious to know if there's a study about how many of the top interpretations are seen by experts. You didn't mention Many Worlds, which is another deterministic interpretation that would leave PSR in-tact.
    – TKoL
    Feb 24, 2022 at 17:49

3 Answers 3

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The philosopher Robert Nozick said it does. Only a weakened form of the PSR can survive.

Paraphrasing: if there is no reason for something, then there is at least a reason for there being no reason

The mathematician John Conway who wrote on quantum foundations also explicitly stated the original PSR contradicts quantum mechanics as such.

What QM actually is is still hotly contested. So there are many interpretations where the PSR would still hold. I don’t know of other cases of explicit denial of the PSR beyond above but I’m sure there are some. But limiting ourselves a bit, it is clear textbook QM and the original PSR conflict.

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  • Who contests QM, and what do they contest about it?
    – RonJohn
    Feb 24, 2022 at 23:49
  • 3
    @RonJohn I think broadly there are those who think the wavefunction is complete (operational, textbook, MWI, Bohm, Copenhagen) vs incomplete (hidden variables) and on other axes, "real"/ontic vs epistemic
    – J Kusin
    Feb 25, 2022 at 2:06
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Arguably not, if you allow chance to be a cause, which Aristotle noted in his Metaphysics, that some philosophers argued for. This thread of thought has been forgotten since the success of Newtonian physics, which does not allow such causes, and is strictly deterministic.

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  • If chance can be a cause, then the PSR says nothing, because you could simply say that any event happened by chance. Why are the laws of physics as they are? Chance. Why did the universe begin? Chance. To say something happened randomly is to say that we have no specific understanding of how it happened.
    – causative
    Feb 24, 2022 at 22:19
  • @causative: No, not true. You can say that any event happens by chance, but that does not mean that is true. Like me responding to your comment wasn't by chance. Feb 24, 2022 at 22:38
  • @causative: "to say something happened randomly is to say that we have no specific understanding of how it happened" is clearly wrong. Some evebts are like and one can probe for deeper reasons whilst other events have no deeper reason, that is the way they are - this is what Aristotle meant vy "chance can be a cause" and that this can be the case is shown by something like radioactivity. Feb 24, 2022 at 22:41
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    In the Bayesian sense, probability represents our lack of knowledge about an outcome. If you know something will or won't happen, the probability you assign is 1.0 or 0.0. If you don't know - if you're maximally uncertain - you assign an 0.5 probability. To say something happens by chance is to say that we can't predict it. So it negates the assumption of the PSR that we can understand how one event leads to another. Also, if the PSR allows us to explain things by "chance," then it allows our explanations to terminate in a brute fact - the fact that a random variable came out a certain way.
    – causative
    Feb 24, 2022 at 23:25
  • @causative: There's a difference between chance which is due to a lack of information and chance where there is no further information to be had. The latter is chance as cause. Bringing in 'Bayesian' theory does not help in the latter case, simply because there is no further information. Have a look at QM or radioactivity to see a situation where chance is a cause, where chance ontonologically speaking, is unavoidable. Feb 25, 2022 at 6:25
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Contrary to a popular misconception - maybe many people are not aware of - Quantum Mechanics (at least according to the classical interpretation, see e.g. Bell Inequality) IS deterministic, because its evolution operators are unitary.

What is unknown, however, is whether General Relativity is deterministic or not. See e.g. cosmic censorship in black holes.

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  • But is the collapse of the wavefunction deterministic? Feb 24, 2022 at 12:06
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    @AmeetSharma: "The collapse of the wavefunction" is merely an interpretation - a mental trick for making sense of it with a brain not accustomed to QM thinking - not part of the theory. Feb 24, 2022 at 17:21
  • @Steve: QM is deterministic as a quantum mechanical evolution from the quantum mechanical initial conditions, but those are completely non-localized not just in space and time but in what you might call "choice of worlds" in a MWI context. Feb 24, 2022 at 17:23
  • This depends on interpretation, so I don't think it is justified to state QM is deterministic without the MWI qualifier. Yes the wavefunction evolves deterministically, but in the Copenhagen Interpretation the collapse of the wavefunction is clearly indeterministic.
    – user289980
    Feb 24, 2022 at 22:24

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