On 10 January 2014 12:58, Jesse Mazer <laserma...@gmail.com> wrote: > On Tue, Jan 7, 2014 at 7:49 PM, LizR <lizj...@gmail.com> wrote: > >> On 8 January 2014 13:14, Jesse Mazer <laserma...@gmail.com> wrote: >> >>> >>>> The expansion of the universe is the most likely explanation for the >>>> entropy gradient - there are a number of ways in which it generates >>>> "negative entropy", briefly some of these are... >>>> >>>> - Quarks can become nucleons when the universe expands and cools >>>> enough >>>> - Nucleons can become nuclei when the universe expands and cools >>>> enough >>>> - Plasma can become atoms when the universe expands and cools enough >>>> - Gas can become stars when the universe expands and cools enough >>>> >>>> ...and there are probably a few others I've missed. >>>> >>> >>> I don't think Price would agree with you there, since your argument >>> tries to show that known dynamical laws alone guarantee entropy increases >>> with expansion, and as I said he is talking about speculative ideas about >>> unknown future theories (like the Hawking "no boundary" proposal which >>> represents a speculation about quantum gravity) that might explain the >>> boundary conditions themselves. >>> >>> Sure. My other half has corresponded with Prof Price on this, so I know >> he's operating at a higher level of speculation, and ultimately one comes >> down to the Big Bang, which isn't explained by the above of course. >> > > But it doesn't just come down to the basic fact that there was a Big Bang > that started the universe expanding, it comes down to the fact that the Big > Bang started off the universe in a very "smooth" and homogenous state, > whereas we can easily imagine an alternate universe where the Big Bang > still happens, but in a far more "lumpy" state which should correspond to > higher entropy in a gravitational context (the highest-entropy Big Bang in > general relativity would probably just create a bunch of black holes, or at > least that's what Penrose argues when he discusses the arrow of time in the > Emperor's New Mind). In general relativity the prediction is that in a Big > Bang/Big Crunch universe where there was sufficient time between the two, > the collapsing Big Crunch would in fact be a lumpy collection of black > holes--the mystery of the thermodynamic arrow of time can then be thought > of as why the Big Bang didn't look like a time-reversed version of how > physicists would expect a Big Crunch to look (assuming no special future > boundary condition), dominated by black holes. >
Yes, indeed. I didn't mean to make it just the BB - a fairly smooth BB is required. Inflation is one attempt to explain this, as well as to solve the "horizon problem". > However, the above list is sufficient to show that something very like the > thermodynamic arrow can be derived from universal expansion, simply through > a series of "relaxations" of the cosmic fluid - i.e. through simple > dynamical processes that become possible successively as the universe grows > less dense. I don't think Price would object to this as far as it goes. > I just looked over his book, and it seems that he would. Price talks a > bunch about Penrose's arguments in Ch. 4 of "Time's Arrow and Archimedes' > Point" and endorses the view that the smoothness of the Big Bang is a > puzzle, and that the arrow of time can only be explained in terms of this > smoothness, not in terms of expansion alone. On p. 79 of my edition (second > page of ch. 4 in case editions differ) he talks about how "The 'natural' > state for a system dominated by gravity is thus a clumpy one, in which case > the gravitational force has caused the material in question to collect > together in lumps". Then on p. 81 he specifically addresses the idea that > the arrow of time could be wholly explained in terms of entropy, and says > the idea doesn't work: "An early suggestion was that the expansion itself > might increase the maximum possible entropy of the universe, in effect by > creating new possibilities for matter. The thermodynamic arrow might simply > be the result of the process in which the universe takes up these new > possibilities ... The idea that the arrow of thermodynamics is linked in > this directed way to the expansion of the universe turns out to be > untenable, however. The main objection to it is that the smooth early > universe turns out to have been incredibly 'special,' even by the standards > prevailing at the time. Its low entropy doesn't depend on the fact that > there were fewer possibilities available, in other words." > > I meant that I don't think he would object, GIVEN that the "smoothness problem" has been solved. One has to distinguish the problem of explaining the boundary conditions from the problem of explaining the AOT within a universe with the observed boundary conditions. A lot of people object to explaining the AOT *even on the basis of the observed boundary conditions + time symmetric laws of physics*, which is all that I have been arguing for. We don't have any explanation for the BCs, except for speculative ones like colliding branes, inflation and Edgar's universal bounce. I agree this is a problem, but using it to discount explanations of, for example, EPR based on time symmetry is akin to objecting to the theory of evolution because it doesn't explain the origin of life. However, as an aside, it *may* be "probablistically reasonable" that the BB started off smooth even if it started from something random, depending on what that something was. In some cases a smooth background is the highest entropy state available. However, a high entropy state can be converted to a low one by, for example, expanding the background space (this is what the "relaxations" I mentioned do, as the universe successively removed itself from the thermodynamic equilibrium it would otherwise have quickly tended towards without the expansion). It's possible a hot, dense, smooth, flat universe is the high-entropy result of some previous process, which manages to turn the tables and become a low-entropy state (I imagine a big bang created inside a black hole might exhibit these properties?) -- 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
