On Sun, 26 Jul 2009 15:14:49 +1000, "Peter Webb"
wrote:

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.

But emissivity does affect black body radiation. It is the ratio of
energy emitted to energy absorbed, and provides that link to physical
materials you were questioning.

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.

Glass has a very high emissivity- over 0.9. If you want a material with
low emissivity, you have to look to something very reflective. Of
course, if you can manage to heat such a material up, it will still emit
radiation. But it becomes very hard to actually heat it up.

Gases don't normally show black body radiation unless they are under
very high pressure- much higher than found in fluorescent tubes. The
light you see there is coming from atomic emission lines, which are also
broadened by high temperatures.

If you had a material with an emissivity of zero, it would not convert
any absorbed radiation into black body radiation.

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

Only because there is no such thing as a material that reflects all the
radiation striking it, over the range of input wavelengths your dark
oven produces.

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

Yes, but particles are not black body radiators. For that, you need the
object size to exceed the wavelength of the emitted radiation. For cold
bodies, this can become very large. The CMB is 1.9mm wavelength, so you
won't see it emitted by dust.


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.

DM is assumed to be cold, in part because it can't absorb EM. So what
mechanism is left to heat it up? Of course, if it somehow carried heat,
what mechanism would allow it to cool down? Since it doesn't interact
with EM, it shouldn't be able to radiate, either. Maybe it's hot and we
can't tell, because it doesn't radiate.

Of course, even if it did radiate, we wouldn't see if unless it was in a
form that was large compared to the long wavelength of cold radiation-
millimeters.

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

Yes. Black body radiation is observed and described for a class of
particles. Electrons, neutrinos, and other non-baryonic particles do not
behave the same as baryonic material. Assuming (as most do) that DM is
made up of some type of non-baryonic material, there is no reason to
expect it to interact with EM the same way baryonic matter does.
_________________________________________________

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