Re: Can the arrow of time reverse?

From: Patrick Leahy <>
Date: Mon, 6 Jun 2005 19:45:17 +0100 (BST)

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
Received on Mon Jun 06 2005 - 14:51:45 PDT

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