On Feb 23, 8:42 pm, Brent Meeker <meeke...@dslextreme.com> wrote:
>I think
> it's an example of the radiation arrow of time making a time-reversed
> process impossible - or maybe just vanishingly improbable.  Bruce Kellet
> has written a paper about these problems, see pp 35.
>
> http://members.optusnet.com.au/bhkellett/radasymmetry.pdf

I am reading this, and have just come across this passage:

"One possibility that is sometimes raised is that the overall
expansion of the universe provides
the local arrow for the direction of time. While cosmology,
particularly the cosmological initial
conditions, might be relevant to any final understanding of the arrow
of time, particularly the
thermodynamic arrow, it is difficult to see the expansion of the
universe as being sufficient to
explain the local asymmetry of every single independent radiation
event. The basic reason is
that the expansion of the universe is a cosmological phenomenon; the
usual understanding of the
Friedmann-Roberston-Walker solution to Einstein’s equations of General
Relativity that governs the
overall evolution of the universe is that, although the fabric of
spacetime expands on the large scale,
individual galaxies do not expand, they merely move apart. The
expansion actually takes place only
on the scale at which the universe can be seen as homogeneous and
isotropic. This is the scale of
galaxies and galactic clusters—only there is the ‘Friedmann dust’
model applicable. The model that
describes the expansion of the universe simply does not apply within
galaxies, much less within the
solar system or on the surface of the earth. So the universal
expansion is simply unable to provide
an effective arrow of time that is locally available for every
independent radiation event."

This seems to me to miss a fundamental point, namely that emission and
absorption events are only local if you ignore what happens to the
photon beforehand or afterwards. If you trace the trajectory of the
photon, you will arrive at some other event, and this event in turn is
linked to a previous / future one. Ultimately all chains of
trajectories of photons, electrons, quarks and so on connect to either
the Big Bang or the distant future (timelike infinity, say). If the
trajectories (or, presumably, waves) are constrained by whatever is at
either end of their trajectory, as time-symmetry implies, then this
stops them being local. They are part of a universe-filling web, which
is "anchored" to whatever boundary conditions obtain on the universe
as a whole.

Charles

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