On Feb 25, 6:41 am, Jesse Mazer <laserma...@gmail.com> wrote: > > Yes, this is the mainstream point of view, not unique to Price. It's > generally thought that reason we see an arrow of time at the macroscopic > level--including the arrow of time inherent in the fact that we can look at > records in the present and gain knowledge of past events, but we can't do > the same for future events--is ultimately explained by the low-entropy > boundary condition at the Big Bang. In a deterministic universe, information > about the future actually would be implicit in the total distribution of > matter/energy and the present, but the problem would be that the relation > between future events and present information about them would be a > one-to-many relationship--you'd basically have to know the precise position > and velocity of every single particle in the past light cone of a future > event in order to reconstruct what that future event would actually be. > Because of the entropy gradient, with past events and present records you > can have a one-to-one relationship (or at least a one-to-few relationship), > where localized collections of particles can function as records of past > events.
Yes, I agree that is the mainstream view as you say - it was a side issue that people seem to regularly try to extract a "local" reason for the arrow of time using for example causal dynamical triangulation (or whatever it's called), and in my opinion unnecessary. > The problem I was alluding to had to do with the fact that Price is arguing > for "retrocausation" not just in the broad sense of any arbitrary > time-symmetric theory, where the entire distribution of particles in the > past light cone of some future event can be said to contain information > about that future event (and thus to 'anticipate' it in a sense), but in a > more narrow one-to-one sense. He's saying that the hidden variables states > of just *two* entangled particles will depend in a lawlike way on the future > measurements performed on these particles. If these variables weren't > hidden--if you could actually know the hidden-variables states of particles > before they were measured--then you could use them to know in advance what > measurement was going to be performed in the future. And the experimenters > could base their decision on what experiment to perform on the outcome of > some complicated future event involving many particles (say, a horse race!), > so in a sense you can even have a many-to-one relationship between future > events and present "records" of these events in the form of hidden-variables > states for individual pairs of particles. This is the sticking point for me.I can't see how an experimenter can measure a future influence on a quantum system in any direct way. I mentioned amplification because normal measurements amplify the signal, and a past-directed signal would need to be similarly amplified (but presumably in a retrocausal manner), but that isn't the fundamental problem. The fundamental problem is that to detect a future influence, you need to measure the state of what is, in your time sense, the photon you are generating. (Taking photons as a simple example.) Suppose you arrange something like one of Price's polariser experiments. You will set up your apparatus to emit a photon, and at a later date arrange for it to pass through a polariser, orienting the polariser horizontally if you want to send a '1' bit to your earlier self, and vertically for a '0'. The problem is, although the polariser may affect the state of the photon before it arrives (in our time frame), the emitting device will *also* affect it. The photon's wave function will be constrained at both ends of its path. It isn't at all clear to me how we could arrange this system so that we can "read" any retrocausal influence by "measuring" the photon's earlier state. The idea doesn't seem to make sense, because we *have* to place a past boundary condition on the photon, simply because we and our apparatus are on the entropy gradient. We can't generate photons that are unaware of the generating apparatus, and hence only have a wave function with only a future constraint, and then somehow detect those photons' past states in order to read their future states. But without the ability to detect a past influence, we can't do any future prediction. Which is precisely what we find happens in practice: we get "unexpected" results in experiments "as though" the photon "knew" what measurement we'd ultimately choose to make - or as though the photon's state, while traversing the experiment, was affected by both the emitter and the detector. So it seems to me that the idea that this view fails because it should allow signalling from the future falls down at the first hurdle, namely how one could make such measurements, even in principle. (The fact that we're also dealing with quantum systems that are disturbed by "normally time-directed" measurements, never mind past-directed ones (whatever that would mean in practice) may be an additional hurdle...) Charles -- You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-l...@googlegroups.com. To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/everything-list?hl=en.
