On Fri, Jun 14, 2019 at 11:32 AM Lawrence Crowell <
[email protected]> wrote:
> On Thursday, June 13, 2019 at 7:20:27 PM UTC-5, Bruce wrote:
>>
>>
>> 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.
>
That would be a useful result because it would put these retrocausal models
to rest permanently. But how do you prove this?
The retrocausal argument takes the form given by Price in 1996 ('Time's
Arrow and Archimedes' Point, p.246-7). Price notes that all that you need
is that the production of the particle pairs is governed by the following
constraint: "In those directions G and H (if any) in which the spins are
going to be measured, the probability that the particles have opposite spin
is cos^2(alpha/2), where alpha is the angle between G and H." Price notes
that such a condition explicitly violates Bell's independence assumption.
My problem with this has been that such a condition does not specify any
plausible dynamics that could operate in this way. And if there were such a
dynamical mechanism, it would violate the rotational symmetry of the
singlet state. But I don't see that this would inevitable lead to classical
probabilities, and correlations that obey the Bell inequalities.
Bruce
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