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Photons and zero chemical potential ?

  1. May 23, 2007 #1
    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?
  2. jcsd
  3. May 23, 2007 #2


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    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.

  4. May 23, 2007 #3
    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.


    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 [Broken]

    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
    Last edited by a moderator: May 2, 2017
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