On 6/13/2019 6:32 PM, Lawrence Crowell wrote:
On Thursday, June 13, 2019 at 7:20:27 PM UTC-5, Bruce wrote: On Thu, Jun 13, 2019 at 11:23 PM Bruno Marchal <[email protected] <javascript:>> wrote: On 11 Jun 2019, at 08:14, Bruce Kellett <[email protected] <javascript:>> wrote: On Tue, Jun 11, 2019 at 3:53 PM Bruno Marchal <[email protected] <javascript:>> wrote:On 10 Jun 2019, at 08:54, Bruce Kellett <[email protected] <javascript:>> wrote:If retrocausality is right, then QM itself is certainly wrong. In the EPR situation, the singlet state is rotationally symmetric in standard QM, and this cannot be the case if that state is dependent on the future polariser settings. Conversely, if QM is right, retrocausality is impossible.If QM with collapse is right, I would understand and agree. That is why Deutsch see the “retrocausality” has a semantic variant of the many-worlds interpretations, but I have not entirely figure out if this makes sense It makes no sense at all! Deutsch has gone completely off the rails over quantum mechanics. He is essentially abandoning the theory as it currently stands. The argument from symmetry is, to my mind, a total killer of any retrocausal explanation -- retrocausality must destroy the very symmetry that is at the heart of the QM predictions for the singlet state, Collapse and many worlds are all irrelevant to this argument.It would be nice if you could elaborate on this. The basis of retrocausality is the observation that there is no problem with non-local influences in QM if the initial state is allowed to depend on the final state, namely, on the settings of the polarisers in the EPR experiment. The QM representation of the singlet state is rotationally symmetric (about the propagation axis). This symmetry is central to the derivation of the correlations that violate the Bell inequalities. If the initial state is made to depend on the final polarizer settings, then the rotational symmetry is lost. So the basis for the original correlation predictions is lost, and the theory becomes incoherent. As it currently stands, the formalism of QM does not allow the singlet state to depend on the final polariser settings, so standard QM is inconsistent with retrocausality. It might be possible to restore the required rotational symmetry in a wider context (taking the remote polarisers into account), but QM does not do this. Retrocausality is a different theory, it is not QM. And that different theory has not been coherently worked out. The rotational symmetry of the initial singlet state is independent of whether you have a collapse model, or have Many Worlds. The difference between these two only comes into play when you include the final measurements. So it is the retrocausal model that requires collapse -- retrocausality cannot work coherently in a many worlds setting. BruceThe dependency of the initial and final states means the probabilities are classical and will obey the Bell inequality. This is a pretty iron clad result and I am not sure why some people persist in thinking they can get around it.
If you consider a multiverse view in which there are an ensemble of results (whose correlations violate Bell's inequality) and then you just "play the multiverse movie backwards" will not the many multiverse results interfere and re-cohere to produce the singlet state? The multiverse is non-local and so can violate Bell's inequality. I agree with Bruce that this doesn't provide a mechanism, but given the time symmetry of Schoedinger's equation I don't see that it's a different theory.
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