RE: Can the arrow of time reverse?

From: Jesse Mazer <>
Date: Mon, 06 Jun 2005 17:09:37 -0400

Norman Samish:
>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
>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
>does this mean that "the 'arrow of time' would reverse during the
>contraction"? Wouldn't this violate causality?

The current understanding is that the arrow of time we see is explained in
terms of increasing entropy--that the 2nd law is why we see eggs shatter
when dropped but don't see pieces of eggs jump together to form intact
eggs--so as long as entropy continues to increase the arrow of time will go
in the same direction, and if the entropy *did* decrease in the contracting
phase, then the contracting phase would be like a reversed movie of the
expanding phase, with broken eggs re-forming and so forth. Thinking about
boxes of gas is a bit misleading, because we normally assume an isolated box
is at the maximum entropy possible given the size of the box and the energy
of all the molecules, while the universe is not at the maximum entropy
possible for its size and energy at any given moment (if it were, there'd be
no thermodynamic time asymmetries like with breaking eggs, and life would
not be possible in such a universe).

Received on Mon Jun 06 2005 - 17:14:25 PDT

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