On Thursday, June 13, 2019 at 8:42:43 PM UTC-5, Brent wrote: > > > > 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]> wrote: >> >>> On 11 Jun 2019, at 08:14, Bruce Kellett <[email protected]> wrote: >>> >> >> >>> On Tue, Jun 11, 2019 at 3:53 PM Bruno Marchal <[email protected]> wrote: >>> >>> On 10 Jun 2019, at 08:54, Bruce Kellett <[email protected]> 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. >> >> Bruce >> > > The 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. > > Brent >
I think you are thinking of the MWI, not so much the multiverse. Quantum mechanics is time reversal invariant, so rewinding the dynamics is not the issue. The issue is whether there are causal propagations in both directions. In other words we generally think there is a causal direction from past to future. In QFT and the path integral we compute the amplitudes of time ordered fields. We do not though think of there being both forwards and backwards propagators, retarded and advanced gauge potentials and the rest. If there are retarded and advanced potentials, then we can write a potential as a vector sum of these. This would mean a spacelike propagation of information. Thus if you were to say nonlocality is constructed this way it would ultimately mean there is classical information that would obey the Bell inequalities. With cosmology things are a bit stranger. For one we really do not know if these other cosmologies or pocket worlds are not just off-shell parts of the amplitude, where there may only be one on shell set of states which is this pocket world. This might be the case if the quantum description is that of a pure state. The multiverses would be real if there is a statistical mixture. Either that or this pocket world is one state in a coherent set of states that would mean all the other pocket cosmologies are identical to this one we observe. Remember, coherent quantum states define a symplectic submanifold in Hilbert space and are "classical-like." LC -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/2a699369-c44a-47f5-88d7-3599443e74e1%40googlegroups.com.
