Why does photons of a given frequency satisfy the Boltzmann distribution?

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Discussion Overview

The discussion revolves around the statistical treatment of photons in relation to the Boltzmann and Bose-Einstein distributions. Participants explore the implications of using Boltzmann statistics for photons, which are bosons, and the conditions under which this approximation may be valid. The conversation touches on theoretical aspects, interpretations of statistical mechanics, and the derivation of the Planck law.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant states that the energy of a mode of frequency ##\nu## corresponds to ##n## photons of energy ##h \nu## and presents a probability expression for finding ##n## photons, suggesting a Boltzmann distribution.
  • Another participant agrees that Bose-Einstein statistics are the correct approach but notes that in the classical limit, the differences between Boltzmann and Bose-Einstein distributions diminish, particularly at high temperatures.
  • A later reply reiterates the appropriateness of using Boltzmann statistics for large energies, where the additional term in Bose-Einstein statistics becomes negligible.
  • One participant questions the specific partition function used in the referenced PDF, suggesting that if only the number of photons is considered, Bose-Einstein statistics should apply, while the presence of multiplicities might indicate a classical partition function.
  • Another participant highlights the potential confusion between modes and photons, emphasizing that the probability expression provided relates to the state containing ##n## photons of frequency ##\nu##.
  • It is mentioned that the Bose-Einstein distribution involves averaging over all ##n##, which leads to the characteristic form of the distribution, while fixing ##n## does not differentiate between classical and quantum statistics.

Areas of Agreement / Disagreement

Participants generally agree that Bose-Einstein statistics are the appropriate framework for photons, but there is contention regarding the validity of using Boltzmann statistics under certain conditions. The discussion remains unresolved regarding the implications of the partition function and the specific statistical treatment applicable in different contexts.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the partition function and the conditions under which Boltzmann statistics may be applied to photons. The distinction between modes and photons, as well as the treatment of energy states, is also noted as a potential source of confusion.

center o bass
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A mode of frequency ##\nu## has energy ##E_n = h \nu##. In terms of photons, the interpretation that I have read several places, is that this correspond to ##n## photons of energy ##h \nu##. Furthermore, it is stated that the probability of finding ##n## photons at frequency ##\nu## is given by
$$p(n) = e^{-nh\nu}/Z,$$
where Z is the partition function (for example in: http://disciplinas.stoa.usp.br/pluginfile.php/48089/course/section/16461/qsp_chapter10-plank.pdf)

This correspond to Boltzmann statistics, and I'm a bit confused by this since photons are supposedly bosons. Should'nt this instead be Bose-Einstein statistics?
 
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Yes, Bose-Einstein statistics are the correct approach. However, in the classical limit the differences between the two distributions vanish. For example for bosonic atoms, there is not really a difference between the distributions for high temperatures and diluted gases.

For photons, using Boltzmann statistics is fine if you are dealing with large energies because the additional "-1" becomes negligible and Maxwell-Boltzmann statistics become a good approximation to Bose-Einstein statistics.
 
Cthugha said:
Yes, Bose-Einstein statistics are the correct approach. However, in the classical limit the differences between the two distributions vanish. For example for bosonic atoms, there is not really a difference between the distributions for high temperatures and diluted gases.

For photons, using Boltzmann statistics is fine if you are dealing with large energies because the additional "-1" becomes negligible and Maxwell-Boltzmann statistics become a good approximation to Bose-Einstein statistics.

I agree that this is correct in the classical limit. However in http://disciplinas.stoa.usp.br/pluginfile.php/48089/course/section/16461/qsp_chapter10-plank.pdf the complete Planck law is derived by assuming that the probability that a single mode is in a state of energy E=nhν (a state of n photons) is given by a Boltzmann distribution. Hence, the derivation does not consider any limit.
 
Sorry, I do not have access to that PDF file on my phone right now, but are you completely sure that the partition function Z they use is really the classical Maxwell-Boltzmann one?

If they just consider the number of photons in each energy state, they apply BE statistics. If they also take multiplicities into account and you see a lot of factorials, it is most likely the classical partition function.
 
Cthugha said:
Sorry, I do not have access to that PDF file on my phone right now, but are you completely sure that the partition function Z they use is really the classical Maxwell-Boltzmann one?

If they just consider the number of photons in each energy state, they apply BE statistics. If they also take multiplicities into account and you see a lot of factorials, it is most likely the classical partition function.

Might the confusion lie in the difference between modes vs photons or photons at frequency ##\nu## vs photons at any frequency?

To quote directly what is stated:

"The probability that a single mode has energy ##E_n = n h\nu## is given by
$$p(n) = \frac{e^{-E_n/kT}}{\sum_{n=0}^\infty e^{-E_n/kT}}$$
where the denominator ensures that the sum of probabilities is unity, the standard normalization procedure. In the language of photons this is the probability that the state contains ##n## photons of frequency ##\nu##. "
 
center o bass said:
A mode of frequency ##\nu## has energy ##E_n = h \nu##. In terms of photons, the interpretation that I have read several places, is that this correspond to ##n## photons of energy ##h \nu##. Furthermore, it is stated that the probability of finding ##n## photons at frequency ##\nu## is given by
$$p(n) = e^{-nh\nu}/Z,$$
where Z is the partition function (for example in: http://disciplinas.stoa.usp.br/pluginfile.php/48089/course/section/16461/qsp_chapter10-plank.pdf)

This correspond to Boltzmann statistics, and I'm a bit confused by this since photons are supposedly bosons. Should'nt this instead be Bose-Einstein statistics?
Bose-Einstein distribution involves an average over all n, giving the probability of a given frequency. It is the summation over all n that gives the characteristic Bose-Einstein form of the distribution.

When n is fixed, there is no any difference between classical (Maxwell-Boltzmann) and quantum (Bose-Einstein) statistics.
 

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