Photons and zero chemical potential ?

[smallphi]

Consider a photon gas in equilibrium with a material cavity (something like a furnace). Why exacly the chemical potential of those photons is zero?

The usual handwaving argument is 'because photons are easily created and destroyed' whatever that means. Hydrogen and Oxygen are 'easily created and destroyed' too in the chemical reaction H2 + O2 = H2O but we don't set their chemical potential to zero.

So what is your explanation?

 

[ZapperZ]

Consider a photon gas in equilibrium with a material cavity (something like a furnace). Why exacly the chemical potential of those photons is zero?

The usual handwaving argument is 'because photons are easily created and destroyed' whatever that means. Hydrogen and Oxygen are 'easily created and destroyed' too in the chemical reaction H2 + O2 = H2O but we don't set their chemical potential to zero.

So what is your explanation?

Your comparison here makes no sense. The oxygen and hydrogen atoms are NOT destroyed when they form a molecule. But the photon number is not a conserved number in such statistics.

Zz.

 

[smallphi]

Photons produced in photochemical reactions like in a light emitting diode don't have zero chemical potential. I am trying to understand why the handwaving 'argument' that photons are 'easily created and destroyed' doesn't work in that case. What is so special about the photon gas in thermal equilibrium that it is the only light with zero chemical potential.


References:

F. Herrmann, P. Wurfel, "Light with nonzero chemical potential", American Journal of Physics -- August 2005 -- Volume 73, Issue 8, pp. 717-721
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000073000008000717000001&idtype=cvips&gifs=yes


Time-dependent and steady-state statistics of photons at nonzero chemical potential, V Badescu 1991 J. Phys.: Condens. Matter 3 6509-6521 http://www.iop.org/EJ/abstract/0953-8984/3/33/025