Re: And she's gone..

On 2007-10-04, Steve VanDevender (aka Bruce)
was almost, but not quite, entirely unlike tea:
"Mike Andrews" <mikea@xxxxxxxxxxxx> writes:

On Tue, 2 Oct 2007 04:12:39 +0000 (UTC),
Garrett Wollman <wollman@xxxxxxxxxxxxxx> wrote in
<fdsgfn$1jhk$1@xxxxxxxxxxxxxxxxxxxxxxx>:

In article <fdr6oc$e2$7@xxxxxxxxxxxxxxx>,
Mike Andrews <mikea@xxxxxxxxxxxx> wrote:

ISTR that the delay from the time a photon is generated by fusion to
the time it actually gets out of the photosphere of the Sun is O(800
years). I.e., that sunlight outdoors today was actually generated
about 1200 AD.

But is it really the same photon? How can you tell? (Can't label 'em!)

There's only one electron in the universe, travelling forward and
backward in time. And my name isn't Archimedes Plutonium.

That's a good question. I'd say "no", but the physicist who wrote that
bit seemed to think it was.

Simple physical argument: The Sun is observed to emit photons over a
extremely wide and nearly continuous range of wavelengths. Nuclear
fusion events would produce photons in a much more limited set of
wavelengths, based on limited set of possible fusion events and their
energies; because those photons are produced by nuclear interactions,
those energies should also be on the order of those of gamma rays, which
are not frequently observed in emissions from the Sun's surface.
Therefore we're definitely not seeing the original photons generated by
fusion.

In fact, the photons we see from the Sun are those generated by thermal
interactions between atoms in the Sun's outer atmosphere, and show
roughly the expected black-body distribution of wavelengths. The
photons generated by fusion distribute their energy into atoms in the
Sun's core, and the resulting heat may well indeed take a very long time
to diffuse to the surface of the Sun. But it's the energy of the
photons, not the photons themselves, that takes all that time to move
from the core to the surface.

No, it's the photons themselves. The (multiple) photons are
travelling just as much backward as forward after each scattering, and
thus have to perform a random walk, and thus the time taken to travel
outwards scales as the square root of the distance travelled and
inversely with respect the *very* small mean free path.

I can't remember the conditions under which radiation dominates over
conduction when it comes to stellar interiors, but given that that
argument above of the Sun is still being pushed in modern stellar
physics textbooks suggests that radiation transfer time dominates in
the sun rather than conduction or convection timescales (I think we
would have to wait for the Sun to become an Asymptotic Giant Branch
star, which it isn't going to do, before the latter would become
important).

--
TimC
"This strongly suggests to me that perl is way out of hand,
or that I need another drink, or both." -- Alan J Rosenthal
.

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