
Arrow of time no longer double-ended - vaksel
http://arstechnica.com/science/news/2009/08/arrow-of-time-no-longer-double-ended.ars
======
Maro
There is much debate about the arrow time. (Related disciplines are
thermodynamics, statistical physics, quantum mechanics, gravity, inflation, so
it gets technical.)

Here are some good papers, look them up at <http://arxiv.org>

Hollands and Wald. An Alternative to Inflation. arXiv (2002) vol. gr-qc

Wald. The Arrow of Time and the Initial Conditions of the Universe. arXiv
(2005) vol. gr-qc

Kofman et al. Inflationary Theory and Alternative Cosmology. arXiv (2002) vol.
hep-th

Carroll. Is Our Universe Natural?. arXiv (2005) vol. hep-th

Carroll and Chen. Spontaneous Inflation and the Origin of the Arrow of Time.
arXiv (2004) vol. hep-th

Lebowitz. Boltzmann's Entropy and Time's Arrow. Physics Today (1993) pp. 1-7

Wallace. Gravity, Entropy, and Cosmology: In Search of Clarity. arXiv (2009)
vol. cond-mat.stat-mech

Maccone. A quantum solution to the arrow-of-time dilemma. arXiv (2008) vol.
quant-ph

------
jerf
That certainly explains why we never observe entropy running exactly
backwards, but there are conceivable pathways that could result in entropy
running backwards into a less entropic state. For instance, put two gasses of
equal density into a container, where one gas is on top and one is on the
bottom, separated by a sheet of glass. Remove the sheet of glass. The gasses
mix. Now, entropy could run backwards for no known reason, and re-order the
gasses, but in the other direction; the top gas now separated on the bottom,
the bottom gas on the top.

This argument, while essentially correct IMHO, doesn't seem to cover the
entire state space of what backwards-running entropy could do. But it's still
an interesting argument.

~~~
dhimes
Actually, statistical arguments explain why we don't observe (in macroscopic
systems) entropy running backwards. Yes, the molecules could, through
collisions, spontaneously re-order. But that is such an unlikely occurrence
that it's probability of being observed (for a large number of molecules) is
very near zero.

~~~
jerf
This paper, and my post in reply, is about a question deeper than that,
namely, why doesn't _time itself_ run backwards sometimes, in essence? (That
elides over some things, which unfortunately including the essence of the
question, but there isn't a snappy English formulation of the problem that I
know of.) Nothing seems to stop it, except inasmuch as it never happens so
something clearly is.

What you're talking about is a foundation to that argument, but the argument
itself is beyond that. While the physicist in question has hopefully
considered the point I make, he _certainly_ knows about the statistical nature
of entropy. However, while that is necessary knowledge, it is not sufficient
to explain the other mysteries of entropy that this paper is about.

~~~
dhimes
I understood what the article was saying, but it seems I didn't understand
what you were saying. If I understand it now, you are saying that to prove his
case he has to not only show that entropy can't directly decrease by,
essentially, undoing the process and getting back to the original state, but
he also has to show that entropy can't decrease by the system ending up in a
state different from the first, but yet which is also of lower entropy? It
seems to me that you are correct.

------
ewjordan
For anyone looking for a version of the article that you can download without
a Physical Review account: <http://arxiv.org/abs/0802.0438>

------
lisper
This actually is not a new result. See <http://www.flownet.com/ron/QM.pdf>,
section 5.4.

------
woadwarrior01
For some reason, this reminded me of OCaml's time travelling debugger.

