# RE: Can the arrow of time reverse?

```Norman Samish:
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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.

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
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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.
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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?

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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).
```
Jesse

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