"Chris L Peterson" wrote in message
...
On Sun, 26 Jul 2009 12:45:36 +1000, "Peter Webb"
wrote:

This raises a couple of questions for me ....

1. The equations for black body radiation do not involve the physical
nature
of the radiating body.

Don't they? The equations rely on the emissivity of a material, which is
surely a parameter that describes its physical nature. A true black body
has an emissivity of one. Anything less and the equations become
approximations, or otherwise need modification. So what do you have if
dark matter has an emissivity of zero?


Emissitivity doesn't affect black-body radiation. What it does affect is how
much of the EM energy that is incident upon it is absorbed as heat.

Pick a simple example - glass. Transparent, emissivity zero. Heat it up to
5000K and it still glows white hot. Similar deal for the gasses in
fluorescent tubes.

Indeed, a completely white object - reflects all light - still glows only
red hot if heated in a dark oven.


As an additional complication, can DM have a temperature above 0K? I
don't know the answer to that, but if it doesn't absorb any EM, how is
energy transferred to it? By definition, a black body absorbs 100% of
the EM that hits it. Nearly by definition, DM absorbs none.


Well, if you are talking about individual particles in space, then one
observer's temperature is just another observer's relative kinetic energy.

If DM consists of more than one particle bound together, then it can
certainly have a temperature, being the difference between the kinetic
energy of the object as a whole and the kinetic energy of the constituent
particles.


Are you claiming that a macroscopic body composed of
DM would not emit radiation as per the black body equations?

I think that describes the commonly accepted viewpoint, where DM is
assumed to consist of non-baryonic particles. There's certainly no
evidence



Why does "non-baryonic" matter? Is there something in the formulation of BB
radiation that is somehow tied to baryons?


2. On the other side ... I have always just accepted bb radiation as a
fact
of hot bodies. Now I have to think about this. I assume that the coupling
between heat and EM derived from the existence of charged particles
(electrons and atomic nuclei) in the radiating body which ultimately
accept
EM waves, turn them into electric potential (physical separation of
positive
and negative charges) which re-appears as thermal energy, and vice versa.
Note that this is all mediated by charged particles being accelerated by
the
E component of the EM wave, or in reverse the acceleration of charged
particles causing EM waves. Thinking about it, how does this work for
neutron starts, which have no charged particles? They absorb and emit bb
radiation, right? If so, what is the physical mechanism for the exchange
of
energy between an EM wave and uncharged matter?

I'm not sure of the answer here, but my first thought is that the
mechanism depends on force carriers, but is not necessarily limited to
simple charged particles.

I suspect that DM woule emit BB radiation if it was hot, the same as
everything else does. Its just that (if it exists) is consists of individual
particles - which cannot have a temperature in the traditional sense, or its
all floating around in space and very cold, or it does emit BB radiation and
we can observe it, its just that we incorrectly ascribe the radiation to
normal matter.

Hell, for all we really know, the Sun could be 10% DM, and 10% of the BB
radiation we see from the Sun could be caused by DM.

As to the nature of the physical coupling between heat energy and BB
radiation ... maybe someone else here can enlighten us.



_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com