On 8/22/2014 11:48 AM, John Clark wrote:
On Fri, Aug 22, 2014 at 1:27 PM, meekerdb <meeke...@verizon.net
<mailto:meeke...@verizon.net>> wrote:
> The second law is only approximately true for finite systems (in either
time or
space). Globally it's a tautology: the arrow-of-physical-time points in the
direction of increasing entropy, whichever way you chose coordinate time.
It's not a tautology. The second law can explain why tomorrow will almost certainly be
more disordered (have a higher entropy) than today, it's just because there are
astronomically (too weak a word but the best I could find) more disordered states than
ordered ones. However by using the exact same logic and the fact that the state of
things today evolved from the state of things yesterday and because there are
astronomically more such disordered states than ordered ones we must conclude that
things today almost certainly evolved from one of those very numerous disordered states
that existed yesterday. So entropy was almost certainly higher yesterday than today. But
that's nuts!
But entropy is relative constraints, in this case coarse graining. I you take the equation
of physics seriously, they are time-symmetric (assming MWI) and entropy never changes.
But at the coarse-grained level of description, where entropy increases, there are fewer
past states that could produce the present than there are future states into which the
present can evolve.
Everybody thinks that entropy will be higher tomorrow but nobody really thinks it's true
that entropy was also higher yesterday, and yet it is undeniable that you can not deduce
a asymmetry in time (time's arrow) from thermodynamics or from any of the known laws of
physics alone; this dichotomy is sometimes called Loschmidt's Paradox or Loschmidt's
Objection. To deduce the arrow of time and get rid of Loschmidt's Paradox the laws of
physics are not enough, you must make an additional assumption about initial conditions
called "The Past Hypothesis", it's the assumption that the universe started out in a
very very low entropy state, as low as you can go. And therefore there was only one
direction it could evolve, toward a higher entropy state.
Except that the exact same physics applies to a universe that "collapses" into a very very
low entropy states. But if we lived in such a universe, we'd live our lives and form our
memories in the direction of expansion and we'd say we live in an expanding universe (as
we do) and that's why the 2nd law globally is a tautology.
Brent
--
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 everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at http://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
