Carnot Cycle for the Photon Gas

Light is something like raindrops. Each little lump of light is called a photon and if the light is all one color, all the “raindrops” are the same.

R.P. Feynman.



The photon gas, a box filled with little light lumps, plays a very important role in physics. As a source of black body radiation — collect the photons coming from a small hole in the box and sort them by energy — it put Planck on the road to quantum mechanics, and although his hope that electromagnetic radiation was the origin of irreversibility did not work out as he intended, it is well worth studying this idea.  Here, however, I investigate the photon gas as a medium for a Carnot engine.

Read more

Entropy 1

Prospects are good for laboratory construction and testing of this solid state Maxwell demon in the near future.

D. Sheehan1 (2002).




Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don’t understand it, but by that time you are so used to it, it doesn’t bother you any more.

This quote by Arnold Sommerfeld summarizes my state of mind from the moment I had taught thermodynamics for the third or fourth time. I could do all the exercises and answer almost all student questions (for them it was the first time, so it did not matter what the answer was anyway), but now that I don’t have to do this any more my lack of  understanding does bother me. So I did the stupidest thing you can do: go back to the original literature, Reflections on the Motive Power of Fire, by S. Carnot. Read more