Reply To: Retrocausality is intrinsic to quantum mechanics


Hi Daniel,

I agree that retrocausation is a good axiom to use. At least it’s negation – the assumption that the causal arrow of time applies to microscopic degrees of freedom – should not be used.

But I think you’ve overstated your point. From your description, it sounds like there’s a potential causal loop – couldn’t Jim ask Alice and Bob whether they’ve observed a violation of Bell’s inequality, and if they have, decide to jam it? The answer is, as you know, that even if Jim “steers” them into an entangled state, they must know the results of his measurement in order to bin their data in a way that would exhibit violations of the inequality. I think that even in a brief description, you need to mention that, in order to avoid more controversy than necessary.

To me, this is indeed an indication that the wavefunction is epistemic rather than ontic. In a stochastic theory where future as well as past boundary conditions affect the probabilities of microscopic degrees of freedom, and at the same time a low-entropy past condition is imposed, there is a chance that information causality will be a consequence, and that all of QM will follow. In such a theory, there will be a need for something like the wavefunction to represent the known (epistemic) probabilities up to some time, and it will not be surprising that its complexity is exponential in the number of particles (like the Liuoville equation in classical many-body phase space). I do find it surprising that the exponential complexity issue is hardly ever brought up as another indication in this direction.

All the best, Nathan.

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