I'd say that expansion of the universe is almost necessary, not contingent. The AoT has
to point in the direction of entropy increase and in almost all models that's correlated
to the expansion of the universe. If it is bigger at one time than at another then the
AoT will point toward the bigger end. I say "almost" because there are some ways around
it. If the universe recontracts the AoT will probably continue to point toward the Big
Crunch, at least until the total entropy equals the Bekenstein bound. Or on the other
possibility, L.S. Schulmann has written a nice little book about his investigation of
universes in which the AoT reverses so it always points to the biggest phase of the universe.
Brent
On 11/6/2014 12:37 PM, LizR wrote:
I think we have very few measurements of the arrow of time beyond "contingent ones" like
- expansion of the universe, entropy increase, radiation etc. All we have that arises
from the laws of physics is the famous CPT violation (I think it is?) of certain mesons.
So it's problematic to even say what "reversing time" means except for reversing the
contingent arrows, which appear to arise from the expansion of the universe, which may
itself be contingent (not required to expand in a particular time direction by the laws
of physics, assuming a multiverse or string landscape or whatever). Of course eternal
inflation puts the arrow of time into the laws of physics, so it may be there, just
beyond out ken, so to speak.
On 7 November 2014 08:16, Bruno Marchal <[email protected] <mailto:[email protected]>>
wrote:
Hi Stephen,
On 15 Oct 2014, at 02:14, Stephen Paul King wrote:
Hi,
I re-read S. Mitra's paper <http://arxiv.org/pdf/0902.3825v2.pdf> again
and it
made more sense than before if I assumed that the reversible measurement
idea is to
be taken as a local reversal to the "direction of entropy flow" in an area
and not
the entire universe.
The trouble is this notion of locality. Are there any favorite definitions
of
"locality" out there? AFAIK, it does not have a fixed size in space, but
may have a
fixed size in "space-time" as location information expands at the speed of
light if
we ignore the effects of local structure that would modulate decoherence.
This
"decoherence" thing, IMHO, needs to be looked at carefully.
In deference to Bruno, I should ask a question relevant to the ongoing
discussions. Is a finite universe with locally reversible time consistent
as a 1p
world?
If the 1p world are described by the []p & p move in arithmetic, then it
seems more
plausible that they are antisymmetrical, in the finite case, and non
cyclical in the
infinite case. This match the usual phenomenology, I think. Locally, you
can't
negate the experience, when you have a pain here and now, you can't negate
the
immediate memories. Only a bad doctor would do that. It is like when you
look up if
you are in Moscow or Washington, in the WM duplication. Once you see
Washington,
that knowledge is not reversible *from* that knowledge pov". At the 3p
level, you
can reverse locally by inducing an amnesia, but that will not be part of
the 1p
experience.
Best,
Bruno
http://iridia.ulb.ac.be/~marchal/ <http://iridia.ulb.ac.be/%7Emarchal/>
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