From: hal@finney.org
Date: Sun Mar 26 2000 - 13:29:49 MST
Robert J. Bradbury, <bradbury@aeiveos.com>, writes:
> I'm not so sure, infrared photons add to the vibrational energy of the
> molecule. I only think you will get momentum transfer if the photon
> is reflected (as in the case of a light sail). Which raises an
> interesting point, if the photons are always going light speed,
> and they are transfering momentum to the light sail, then, *shouldn't*
> the photon being reflected back be of a lower frequency (to allow
> for conservation of energy)? Hmmmmm...
I think what happens is that you can consider the absorption and
reemission as two separate events. From the sail POV, it abosrbs the
photon and gets more momentum. Then it re-emits it at the same frequency.
But from a stationary POV the emission is at a lower frequency because
the sail is now moving away faster, hence it is doppler shifted downward.
> I would agree that photons, when absorbed by a gas, increase
> the energy state of the molecules, but this can be in
> vibrational or rotational modes in addition to velocity.
> Most of the time, I don't think the photons are going to be
> re-emitted but instead, the increased pressure of the gas will
> transfer momentum to the walls of the container which will then radiate
> infrared photons as heat. In the upper atmosphere, I suppose
> that you would see gas molecules functioning as a black body
> and radiating infrared photons.
Right, with gas molecules the energy will partition itself among the
various degrees of freedom, vibrational, rotational and translational.
This then corresponds to the temperature of the gas. As you say this
will heat up the walls of the container as well.
I think the basic point with respect to the original question is that
it doesn't really make sense to say that a molecule "absorbs energy".
There is no such thing as "energy" in the sense of an immaterial
"substance" that can be absorbed. Energy is the potential to do work.
Particles, including photons, can carry energy. So one way to transfer
energy to a molecule is by hitting it with a particle, whether a photon
or another molecule.
Hal
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