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The causal principle states that everything must have a cause. William Lane Craig argues that even in the cases of subatomic events in QM, there are certain necessary conditions that produce an effect, even if they don’t sufficiently produce an effect.

My concern is deeper than this. The question I have is how can we rule out the notion of measurements affecting a system in such a way that it only appears to create effects that don’t have sufficient causes? If this was true, is this the same or different from superdeterminism? If this was true, would it be a local or non-local hidden variable theory?

Interestingly, I found the same concern in an SEP article here. As quoted,

It is true that, given Heisenberg’s principle of uncertainty, we cannot precisely predict individual subatomic events. What is debated is whether this inability to predict is due to the absence of sufficient causal conditions, or whether it is merely a result of the fact that any attempt to precisely measure these events alters their status. The very introduction of the observer into the arena so affects what is observed that it gives the appearance that effects occur without sufficient or determining causes. However, we have no way of knowing what is happening without introducing observers into the situation and the changes they bring. In the above example, we simply are unable to discern the intermediate states of the electron’s existence apart from introducing conditions of observation. When Heisenberg’s indeterminacy is understood as describing not simply the events themselves but these events relative to our knowledge of the events, the Causal Principle still holds and can still be applied to the initial singularity, although we cannot expect to achieve any kind of determinate predictability about what occurs in specific cases on the sub-atomic level given the cause.

  • You write "local or non-local hidden variable" similar you did in your previous question on Bell's theorem. Do you mix up the term with "local, hidden-variable theory"? Note: The theory is a local theory, not the variable a local variable. – Jo Wehler Nov 08 '23 at 13:27
  • @JoWehler Yes, that is what I meant. I edited it, thanks for the correction. –  Nov 08 '23 at 13:29
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    How can we rule out Cartesian demon affecting our minds in such a way that it only appears to create effects we take to observe? The answer is we can't. Such concerns are called skeptical concerns and the response to all of them is the same: idle hypotheticals do not warrant consideration. Until we have tangible reasons to consider skeptical scenarios they are to be dismissed out of hand. The SEP quote brings up Heisenberg's early interpretation of the uncertainty principle that he walked away from and physicists now consider obsolete. – Conifold Nov 08 '23 at 13:31
  • @Conifold Quantum mechanics is to my knowledge the only field where measurements do, regardless of whatever your interpretation is, impact the system in a significant way. Even in the Copenhagen interpretation that posits inherent randomness and thus a clear violation of the causal principle, the concept of a wave function collapse exists in that the measurement changes the system. As such, it seems that there is more tangible reason here to not rule out what was proposed compared to other more skeptical theses such as a Cartesian demon. –  Nov 08 '23 at 13:54
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    @thinkingman Actually, there are many cases where a measurement can affect the measured variable, well beyond QM. For instance, when measuring the temperature of a solution, one has to dip a thermometer into it. Doing so might have a (tiny) effect on the temperature to be measured. – Olivier5 Nov 08 '23 at 15:48
  • That would just add to my point then @Olivier5. The more measurements affect a system, the more we should hesitate ruling out what I said in my post. –  Nov 08 '23 at 16:22
  • @thinkingman We should hesitate before concluding anything, really, including determinism. The world is far more messy than Laplace thought. – Olivier5 Nov 08 '23 at 16:27
  • Regardless of your interpretation, measurements impact classical systems just the same, by the law of action and reaction. Measurement is an interaction and always changes the system. The upshot of Bell's inequalities is that quantum indeterminism cannot be eliminated even with that impact taken into account. PSR had good inductive basis in classical mechanics, albeit not unquestionable. With it removed, it would take something more tangible than notalgia to revive it. If there is a concrete theory along those lines that, at least, reproduces QM predictions, let's see it. Until then, no luck. – Conifold Nov 09 '23 at 01:21
  • @Conifold I suppose you can’t rule out quantum indeterminism but you also can’t rule out measurements affecting the system in such a way as that it only appears indeterministic, correct? That was the root of my question. I suppose you earlier already said you can’t rule it out but that it’s just a skeptical hypothesis. But don’t you think the impact on a system by measurement is more likely to be paramount on the micro scale than the macro scale of the world? Of course this is just an intuition but still –  Nov 09 '23 at 04:40
  • Both CM and QM describe the impact of measurement on the system and how it changes as a result. You cannot "rule out" some "deeply hidden" additional impact that makes CM indeterministic or QM deterministic. Concrete theories either postulated some absolutely undetectable entities (like Bohm or MWI) or revised QM in ways that contradict experiments (like objective collapse). Most physicists looked at them and took a pass. The rest, including superdeterminism, are Cartesian demons that no one took seriously enough to even develop into theories. – Conifold Nov 09 '23 at 08:22

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