Reply To: Retrocausality is intrinsic to quantum mechanics

Ken WhartonKen Wharton

Hi Daniel,

As you know, while I agree that your arguments are indeed indicative of a hidden, underlying retrocausal account of entanglement experiments, I was one of the people who pushed back on your claim that “retrocausality is intrinsic to quantum mechanics”. As I see it, standard QM has gone out of its way to avoid the need for retrocausality. (Too far, in my opinion!)

Reading these other two papers you cite, it looks like you probably would have also gotten push-back from both Peres and Ma et al. on this point, but for a different reason. The issue Peres raises is that “being entangled” should be thought of as a state of knowledge, not a state of reality. Ma et al. also raise this ontic/epistemic issue, and they note that the apparent retrocausation disappears if you don’t think of “entanglement” as a state of reality. If “being entangled” is not a state of reality, their argument effectively goes, then it can hardly be used to judge causation.

I think you have a potential case to make against this argument, starting with the question: If entanglement is not a state of reality, then what is? That question puts you right into the hidden-variable ontological framework in which Bell inequalities are surprising, and arguably indicative of retrocausality in their own right.

Still, I didn’t read this as your argument. I read that you were implying that *standard* QM is retrocausal. But for this to go through, there has to be something ontological about the past that is different for different future decisions. In standard QM, the full wavefunction is the implied ontology, and no matter what future measurements are made, the standard wavefunction *before* those measurements is always the same. So there’s no “variable” on which retrocausality could even potentially act.

I understand that you’re looking at Alice and Bob’s measurement outcomes, not the wavefunction, but in standard QM those outcomes result from the wavefunction, which is in turn unchanged by anything a future Jim might do. In the standard QM account, Jim’s remaining piece of the wavefunction tells him how those earlier outcomes were correlated (if Jim makes the proper measurement to extract the information). Standard QM implies that Jim’s not *causing* an earlier correlation, he’s being *informed* about it, at least if he chooses to make the right measurement.

Like you, I think there’s a more natural retrocausal resolution of these sorts of experiments (esp. given the similarity to the case where Jim measures first). But without some hidden past variable that can be “caused”, I guess I just don’t see how *standard* QM has any room for retrocausation.



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