LizR wrote:
On 8 November 2014 16:53, John Clark <[email protected] <mailto:[email protected]>> wrote:On Thu, Nov 6, 2014 at 3:56 PM, meekerdb <[email protected] <mailto:[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 ofscience. 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).
I think you should stop saying this. It is not true. You have not defined what you mean by "maximum entropy" nor have you specified how that maximum is calculated. If the maximum is defined as when all available degrees of freedom are in thermal equilibrium, then the universe has never been in such a state of maximum entropy, and it probably will not be until all matter has collapsed into black holes and these have decayed by Hawking radiation.
At any finite time, one useful concept of maximum entropy is to consider the state in which all mass energy is in the form of black holes. This has never happened either.
Neither of these maxima is in any way affected by the expansion of the universe as a whole.
So you cannot get around the need to postulate a low entropy condition at the BB.
Bruce
> The AoT has to point in the direction of entropy increaseBut 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 wouldbe 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 loweryesterday 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".
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