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
>>> - Nucleons can become nuclei when the universe expands and cools
>>> - 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.
> 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."
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