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Comment by iwr | Hacker News Reader

What does "temperature" mean with regard to photons? A photon has an energy level given by its wavelength or frequency. But temperature is a macroscopic phenomenon (equivalent to the kinetic energy of a group of particles).

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So reading more about this: http://en.wikipedia.org/wiki/Photon_gas , it looks like a scenario where a blackbody container was created, with a Boltzmann "temperature" peak equal to the energy level of the incident photons. Thus, all absorbed photons would be re-emitted at with the same distribution as the blackbody walls. And apparently, this phenomenon was tested in an ultra-low temperature scenario.

Still, I wonder. Given the low temperatures, the Boltzmann distribution suggests photons in the microwave or lower frequencies.

Actually the article says the photons were in a room-temperature scenario.

It's hard for me to say without reading the actual research article, but my guess is that temperature is referring to energy of each of the photons relative to one another. Meaning, if you were shining a laser beam down the z axis, typically the photons would also have some small velocity in the x and y directions (this would mean the light beam is diverging). So looking in a frame of reference moving along the z axis at the speed of the photons in the z direction, you would notice that most of the photons would be stationary in the z direction but are slowly moving in the x and y directions. The temperature is probably referring to the average kinetic energy of the photons in the x and y directions. So in order to get the photons all into a single state, you would want their x and y velocities to all be zero. That's my guess, I know sometimes temperature can refer to similar scenarios with a beam of atoms. Like with the LHC, they often refer to the atoms as having a very low temperature. However, the atoms are moving around in circles at very nearly the speed of light. With such a high kinetic energy, you would expect the atoms to have a high temperature. But really, the atoms are moving very slowly relative to one another, and thus temperature is referring to this kinetic energy.

Photons move at c in all frames of reference.

Photons do not move at all. Light propagates at c in a vacuum. A photon describes the interaction of the propagated light with another phenomenon(such as a detector).

...in a vacuum

Technically, photons always travel at c. Slow moving light is caused by the medium continually absorbing, delaying, then re-emitting the photon, or by other stuff which I won't pretend to understand [1]. Either way, rest assured that electromagnetic propagation always travels at c WRT any inertial frame.

[1] http://en.wikipedia.org/wiki/Slow_light

Still, I don't see how the photons would be travelling at different velocities in the same medium.

They are all moving at the same speed, yes. They are moving in different directions, meaning they have different velocities, but the components of that vector will add up to c.

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