On Mon, 6 Jun 2005, Norman Samish wrote:

Norman Samish wrote:
    If the universe started contracting, its entropy would get smaller,
which nature doesn't allow in large-scale systems.  This seems to me an
argument in support of perpetual expansion.

Norman Samish writes:
   Thank you for your comments.  My reasoning was that if a volume of gas
contracts, its temperature must go up because particle collisions will occur
more frequently.  Since entropy is inversely proportional to temperature,
the entropy must get smaller.
   If an entropy decrease upon contraction of our universe does not occur,
does this mean that "the 'arrow of time' would reverse during the
contraction"?  Wouldn't this violate causality?

No, it means that entropy is *not* inversely proportional to temperature.

If a volume of gas is expanded or contracted adiabatically (i.e. with no heat exchanged with it's surroundings), its entropy stays constant. If it does exchange heat irreversibly, then entropy of (gas+surroundings) increases whether the gas expands or contracts (2nd law).

Expansion of the universe (and re-collapse, if it happens) is roughly adiabatic, at least on very large scales (since there are no very large-scale temperature gradients that would drive heat transfer).

Paddy Leahy

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