On 1/7/2014 1:46 PM, LizR wrote:
On 8 January 2014 07:13, Jesse Mazer <laserma...@gmail.com <mailto: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...)

If the universe reached a maximum and started to contract (which now seems unlikely) the maximum possible entropy would still be much higher than the actual entropy so entropy would continue to increase until the contracted enough to reduce the maximum to near the actual. If it were the reverse of the Big Bang it would be a very sudden contraction, aka "deflation". Of course it might actually be symmetric. Lawrence Schulman has a nice book about this "Times Arrow and Quantum Measurement". He did some computer simulations by just considering randomized initial conditions and then post-selecting the ones that satisfied the final conditions.

Brent



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

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