I hope you don't mind if I don't quote the entire exchange, which is
now rather long. Unfortunately I only have a short time in which to
reply, as well, so excuse the brevity!

I was under the impression that Price was NOT arguing for any special
kind of retrocausation, but I may have misunderstood him. The
impression I received was that, in his opinion, it would be impossible
to send messages back in time (e.g. by performing a particular future
measurement on a photon) because there is no way to detect this
influence except indirectly. (This would be a general property of
quantum systems that operates in either time sense, and leads to us
having limited knowledge of them.) If you measure the state of a
photon, say, you introduce an extra interaction that changes the state
of the system: the state of the photon between its emission and your
first measurement is determined by the constraints introduced by these
events, and the state between your measurement and the final
measurement is determined by those two operations. This doesn't appear
to involve any special cases, hidden variables etc - as far as I know,
the wave equations involved are time-symmetric, hence it would seem to
be necessary to explain why the state of the system would NOT be
influenced by a future measurement - wouldn't it?

I don't think there is an issue with T violation; we're agreed that
this doesn't have any noticeable effect on the entropy gradient. The
type of thing I was thinking of, which as you say is always presented
in a sensational manner by NS, is more along the lines of, for
example, finding a way to use a spin-foam model of space-time to
obtain an arrow of time at a fundamental level. I can't remember the
details, but several examples of that sort of thing has appeared in NS
(and occasionally Scientific American), and seems to me to be
unnecessarily complicating the issue until the standard laws of
physics has been shown conclusively not to be up to the job.

Charles

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