On 8 November 2014 16:53, John Clark <[email protected]> wrote: > On Thu, Nov 6, 2014 at 3:56 PM, meekerdb <[email protected]> wrote: > > > I'd say that expansion of the universe is almost necessary, not >> contingent. >> > > I'd say that by about 1850 when people started to have a understanding of > what Entropy was physicists had all they needed to have known that the > universe must have started out in a very very low entropy state, that is to > say they could have predicted the Big Bang in the early to mid 19th > century; and they wouldn't have needed to go near a telescope to do so. But > unfortunately they didn't, it's one of the great failures of nerve or > imagination in the history of science. >
Another feature of the big bang / expanding universe is that it continually raises the entropy ceiling (maxium entropy that can exist in a given volume). > > > The AoT has to point in the direction of entropy increase >> > > But the question is WHY does time point in the direction of entropy > increase. The answer is because in the first instant of time the universe > was in a extraordinarily low entropy state, probably as low as it could > get, and because there are vastly more disordered (high entropy) states > than ordered (low entropy) states. So regardless of what the laws of > physics were by the second instant of time the chances are overwhelming > that entropy will be higher than it was at the first instant. > The universe could potentially start in a state of maximum entropy (at least in terms of the equilibrium of mass-energy) and still move to states where things can happen (if there are *any* inhomogeneities). Gravitational entropy is trickier, as it would tend to indicate the universe should start as a black hole (although that would never actually start...) But the rest of the AOT can be handled by the entropy ceiling being continually raised, almost regardless of initial conditions. > > If instead in the first instant of time the universe was in a very high > entropy state then in the second instant Entropy could have been smaller or > larger with about equal probability and there would be no second law of > thermodynamics and time would have no arrow. > > > I say "almost" because there are some ways around it. If the universe >> recontracts the AoT will probably continue to point toward the Big Crunch >> > > Even if that were true time would still have a arrow, it would just be > pointing in the opposite direction we are accustomed to. But why should > time have a preferred direction at all? The laws of physics alone can not > explain it nor is there any reason to expect that they should. Even if you > know all the laws of physics there is to know you still can't predict what > a system is going to do tomorrow unless you know what state it is in today; > you've got to know the initial conditions. The laws of physics can explain > why Entropy will be higher tomorrow than today, but it can't explain why it > was lower yesterday than today, for that you need initial conditions. > > True, although (see above) I think we can sneak around requiring any "implausibly low entropy starting conditions". -- 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 [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
