Can the arrow of time reverse?

From: Norman Samish <>
Date: Mon, 6 Jun 2005 11:35:33 -0700

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.

On Mon, 6 Jun 2005, Jesse Mazer wrote:
>From what I've read, if the universe began contracting this would not
necessarily cause entropy to decrease, in fact most physicists would
consider that scenario (which would mean the 'arrow of time' would reverse
during the contraction) pretty unlikely, although since we don't know
exactly why the Big Bang started out in a low-entropy state we can't
completely rule out a low-entropy boundary condition on the Big Crunch.

Paddy Leahy wrote:
This is quite correct. The idea that there are future as well as past
boundary conditions is an extreme minority one.

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?
Received on Mon Jun 06 2005 - 14:38:05 PDT

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