On 8 January 2014 07:13, Jesse Mazer <laserma...@gmail.com> wrote:

>> They seem to have in common the idea that the maximum entropy can
> continually increase due to the expansion of space. But I don't think
> Layzer's account works as a full explanation for the arrow of time, since
> you imagine a universe that on a cosmological scale looks like the
> time-reverse of an expanding universe, but without needing to reverse the
> arrows of time due to local increases in entropy (for example, the
> psychological arrow of time for intelligent beings would be such that they
> measure the universe to be contracting rather than expanding).

You can imagine it, but that doesn't mean it's physically possible. Lots of
people have argued that psychological time wouldn't reverse in a Gold
universe, but the arguments always come down to hand-waving about how
things must depend on initial conditions etc - they smuggle the arrow of
time into the discussion, then point to it triumphantly.

The expansion of  the universe is the most likely explanation for the
entropy gradient - there are a number of ways in which it generates
"negative entropy", briefly some of these are...

   - Quarks can become nucleons when the universe expands and cools enough
   - Nucleons can become nuclei when the universe expands and cools enough
   - Plasma can become atoms when the universe expands and cools enough
   - Gas can become stars when the universe expands and cools enough

...and there are probably a few others I've missed.

Give time-symmetric laws of physics there is no reason to think that a
contracting universe wouldn't operate on the same principle, difficult
though it is for us to imagine, and have the arrow of time pointing out
from the big crunch towards the remote past. If so, one couldn't ever find
oneself born into a contracting universe (although this will ruin some of
the SF novels I hope to eventually write...)

So far self-sampling bears this out.  :-)

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