I'm afraid I don't have time to give a detailed answer to everything you've said, but thank you for the response. I will respond to a couple of points (see below).
On Feb 24, 5:06 pm, Jesse Mazer <laserma...@gmail.com> wrote : > > When you mention "hidden variables," I assume you mean that particles > > are in a definite state at a given time, rather than "undecided until > > measured" ? If so, then I believe that is (supposedly) an outcome of > > Price's approach, assuming I've understood him correctly. I don't > > think there is a problem explaining why you can't send information > > back in time. Surely to obtain a useful back-in-time signal from the > > system would require some form of amplification that would also have > > to operate backwards in time? But Price is only suggesting that time- > > symmetry is significant within a given quantum interaction; it can't > > be ampified. > > What exactly do you mean by "amplification"? When we send information from > the past to the future, do you think this involves some notion of > amplification, and if so what does it mean in this case? I'm also not clear > what you mean by "Price is only suggesting that time-symmetry is significant > within a given quantum interaction"--what does "significant" mean here, > given that the laws of physics governing *all* processes, even large > collections of interacting particles, are completely time-symmetric? Is the > time-symmetry somehow "insignificant" for these more complex processes > involving many particles, and if so why? The point about amplification is that all normal detection events require amplification, such as photographic film, photomultipliers and so on. We never detect a quantum event directly, but rather the result of that event having caused some sort of cascade process. My point was that it would be impossible to arrange for this to occur in the time- reversed case. However, the comment about amplification was in response to a previous question, and was probably in any case a side issue. The original question was why we can't use this approach to send messages into the past (IIRC), and the fact that detections involve amplification and that equivalent processes can't be arranged in a past-directed time sense is, although true, not the fundamental reason, which is concerned with what it would *mean* to detect a message being sent into the past. (We would have to somehow read the state of a photon that is about to be emitted, for example.) Price's idea (if I undersand him correctly) is that the behaviour of a particle is constrained by both path and future boundary conditions, hence my use of "significant". In a broader sense, all quantum interactions are (according to Price - if I understand him correctly - please take that as read in future comments!) equally constrained by the past and future, but we live in a universe in which (for some reason) there is an entropy gradient. Time symmetry is pervasive within the network of interactions between the particles making up the world, but isn't obviously manifested on any other scale, where the entropy gradient dominates (except as puzzling results in quantum measurements). If you view the universe as a large Feynman diagram, then each branch of it should be equally constrained by the event at its past and future end points. However, superimposed on top of this is an entropy gradient that puts the pastward end of all the trajectories into a very special configuration that we describe as low entropy. Hence the time-symmetry is quite hard to see from out perspective, as creatures made up of processes that are all oriented along the entropy gradient. > Isn't his explanation for the arrow of time the same as most other > physicists, namely the idea that it should be explained in terms of the low > entropy of the Big Bang? Yes, but he is interpreting this in a slightly different way to how it is normally perceived. Many attempts to explain the "arrow of time" (at least many attempts reported in "New Scinetist", which seems to go in for them regularly) involve in some sense "local conditions", in particular something inherent in the laws of physics. Price is suggesting that there are no such local conditions, and that the initial low entropy is sufficient to explain everything about the arrow of time. This moves the explanatory burden, of course. Sorry, that seems rather obvious - did I miss the point of the question? 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.
