On Tue, Jan 7, 2014 at 4:46 PM, LizR <[email protected]> wrote: > On 8 January 2014 07:13, Jesse Mazer <[email protected]> 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 -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
