On Tue, Jan 7, 2014 at 4:46 PM, LizR <lizj...@gmail.com> wrote:

> 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.
>

I believe it's physically possible in the sense that, to the extent
physicists can use the equations to model local behaviors that have their
own characteristic arrows of time (like blobs of matter collapsing into
black holes) against the background of a contracting universe, there are
perfectly valid solutions where the arrow of time of all these local
behaviors have the "forward" direction pointing towards the collapse.



> 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 only plausible way in known physics to get the conclusion that the
arrows of time *would* reverse in a contracting universe is to assume that
there must be a low-entropy boundary condition on the future Big Crunch
just like there is on the Big Bang. Price is just arguing that if we assume
that time-symmetric laws can in some unknown way impose such a condition on
the Big Bang, it seems plausible to assume that they would do so on the Big
Crunch, and that arguments that try to derive a Big-Bang-only low-entropy
condition typically smuggle in time-asymmetric assumptions. But Price
doesn't argue that any *known* laws (all of which deal with dynamics, not
boundary conditions) rule out the idea of the arrow of time maintaining the
same direction during the contracting phase. In fact, he also suggests that
another way of preserving time-symmetry would be to suppose that in an
ensemble of universes generated by these laws, half would have a
low-entropy boundary condition in the past singularity but not the future
singularity, and half would have a low-entropy boundary condition in the
future singularity but not the past one.



>
> 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.
>

I don't think Price would agree with you there, since your argument tries
to show that known dynamical laws alone guarantee entropy increases with
expansion, and as I said he is talking about speculative ideas about
unknown future theories (like the Hawking "no boundary" proposal which
represents a speculation about quantum gravity) that might explain the
boundary conditions themselves.

Jesse

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