So, I got over the caloric thing some time ago and converted to
believeing that heat was the kinetic (mechanical) energy of molecules
and their atoms banging around pretty much at random, vis Boltzmann.
To heat something up you stick it next to something else that has
atoms that have been accelerated by the release of energy from
chemical bonds (...that's not the question, but you get extra points
if you can tell me how that happens...though it might actually BE the
answer anyway...).

Then along comes Planck and Einstein and suddenly there are these
"photons" that carry energy everywhere. But all I've seen about these
guys is that they run into electrons and kick them into "higher
orbits", whence said electrons "decay" and release more and different
photons of various energies. Nowhere have I read anything about how
photons that have no mass manage to disturb giant atoms enough to make
them act like pool-balls after the first break.

Thus the real question is how does the idea of quantum photons fit
with the idea of heat as a mechanical process, and how do they
interact? If you whack an atom hard enough with a hammer do you get a
photon, and v-v?

thanks, and I'll kick in all the beer it takes to make me think I
understand your anwser...

Visible, UV, and X-ray photons are generally absorbed by moving an
electron to a higher "orbit." Lower energy infrared photons can be
absorbed by increasing the vibration rates of bonds in molecules.

In regard to conversion of increased vibrational energy to heat, I'm
presuming that when molecules collide, they can transfer energy that
comes both from translation of the entire molecule and from increased
internal motion, that is, if one molecule hits another, the first
molecule could hit at a time when it is simultaneously moving towards
the second molecule (kinetic energy) and one of its "first contact"
bonds is stretching / bending so that this part of the first molecule
imparts additional movement when it hits the second molecule.

When we experience warmth from sunshine, we are experiencing the
absorption of infrared photons by many different molecules in our
hands, etc.

The site

http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM

has a chart showing the absorption of various frequencies of infrared
light by a pure compound (looks like an alcohol to me). For our
purposes, the main point is that stretching and bending are increased
by absorption of infrared radiation.

Technical details regarding the chart: The top scale could easily be
converted to energies of infrared photons since energy is directly
proportional to frequency. Also, for this type of graph, absorbtion is
greatest at those photon energies where there is a dip in the graph.

Hi brix24,

   I think his problem is more along the lines of- atoms interact by
way of electrical forces, which are mediated by photon exchanges. If
orbital electrons always use the energy of an absorbed photon to rise
to a higher orbital shell, then how do atoms exchange momentum?

   Hint to schip: consider two rogue electrons in an otherwise empty
universe. When they interact, what happens?

Hi qed100,

Is this more along the lines of the answer?

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

(I used to think "qed" referred to "quod erat demonstradum." Now, I'm
wondering if it refers to quantum electrodynamics!)

Doing pretty well so far...thanks guys.

To paraphrase...
It sounds like there is some mechanism by which photons (electromagnetic
particles, thanks, I forgot that minor fact) can affect the kinetic energy
of macro-particles by exchanging momentum. The "virtual photon" thing
virtually says that.

Oh so many questions then:
since the photon has energy but no mass, how do you figure it's force?;
is kinetic energy quantized (but so small that you can't resolve it)?;
are there black-bodyish photon emissions starting from ++0degK?;
are those _way_ low energy photons (like near-DC) detectable?;

I guess if I had spent more time in my kitchen it would have occured
to me that my electromagnetic M-wave oven was making water molecule
bonds jiggle just like I was asking about, so there must be some
crossover between photon momentum and what us earthlings like to think
of as physical momentum. Come to think of it, those four basic forces
they are always going on about don't include the prosaic physical
momentum thing at all, so it must be some artifact of the lower levels
to start with, right?

You don't have to answer that...

thanks
MS

ps...strangely enough I was in the same room with Murray Gell-Mann
_twice_ this afternoon and never got the nerve to ask him about
QCD...just as well I think...

pps...I'd send you the beer but I just drank it all.

ppps....brix24 is pretty sweet isn't it?

Note that by "massless" we always mean "having no rest mass". There is
a relativistic mass; this will get clearer further on, I hope.

I find it's much easier to deal with the whole "massless" thing by
starting with the relativistic energy-momentum equation:
E^2 = p^2c^2 + m_0^2 c^4
where E is energy, p momentum, c you know, and m_0 is the rest mass.
Unlike all the equations with gammas in them, this is valid for both
massless and normal particles. For massless particles, in fact, it's
particularly nice: m_0 is 0, so we get E^2 = p^2 c^2 or E = pc. The
relativistic mass is of course given by E = mc^2.

For photons we also have the energy given by E = hf where h is
Planck's constant and f is the frequency. So from the frequency we can
derive both the momentum and the (relativistic) mass of any photon
(travelling in a vacuum).

For example, take a typical photon of visible light, with wavelength
700 nm and hence a frequency of 4.3 x 10^14 Hz. The energy is
therefore about 3 x 10^-19 J or 2 eV. The momentum is thus around 9 x
10^-28 kg m /s and the mass is about 3 x 10^-36 kg, or about 1/300000
as much as an electron. Not very much!! But the fact that it's
travelling at the speed of light gives it a momentum large enough to
be effective at the atomic or molecular level.

thanks manuka...that seems to be the answer I was looking for.
But all the rest was great too...
MS