Bose-Einstein distribution for photons

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Discussion Overview

The discussion centers around the Bose-Einstein distribution for photons, particularly addressing the implications of the parameter A being equal to 1 for photons, the behavior of photon occupancy as energy approaches zero, and the existence of a minimum energy for photons in various confinement scenarios. The scope includes theoretical considerations and conceptual clarifications related to statistical mechanics and quantum physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why A = 1 for photons and whether this implies an increasing probability of photon presence as energy approaches zero.
  • Others propose that a Taylor expansion for low energy photons (E_p << kT) indicates mean occupancy of kT/E_p, suggesting multiple photons can occupy the same mode under certain conditions.
  • Several participants express uncertainty about the implications of low energy states for photons, particularly regarding the existence of a minimum energy or frequency for photons.
  • One participant notes that the density of states in k-space affects energy calculations, indicating that density is proportional to k^2 and approaches zero for long wavelengths.
  • A correction is made regarding the interpretation of the Bose-Einstein distribution, clarifying that it represents the thermodynamic average number of particles rather than a probability, which cannot exceed one.
  • Discussion includes a consideration of the lowest energy mode of photons in a confined space, with implications for the minimum energy depending on the dimensions of the confinement.

Areas of Agreement / Disagreement

Participants express differing views on the implications of A = 1 for photons and the nature of photon occupancy at low energies. There is no consensus on the existence of a minimum energy for photons, with various perspectives presented regarding confinement and energy states.

Contextual Notes

Limitations include unresolved assumptions about the behavior of photons at low energies and the dependence of conclusions on the specific definitions of energy states and confinement scenarios.

TheCanadian
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When computing the probability distribution of bosons, why is A = 1 for photons? Does this not imply that photons will have an increasingly high probability of being present as E approaches 0? What is the significance of such a situation?
 
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You can do a Taylor expansion for ## E_p << kT ## and it says the mean occupancy for this case (for each photon mode) is ## kT/E_p ##. This implies you will get multiple photons in the same mode if ## kT>> E_p ## where ## E_p ## is the photon energy.
 
Charles Link said:
You can do a Taylor expansion for ## E_p << kT ## and it says the mean occupancy for this case (for each photon mode) is ## kT/E_p ##. This implies you will get multiple photons in the same mode if ## kT>> E_p ## where ## E_p ## is the photon energy.

But why? Why is it more probable to find a state occupied by a photon as ##E_p## goes to 0? I suppose I am wondering if there is any minimum to a photon's energy (i.e. frequency). In this case, the mean occupancy is ##kT/E_p## but can't ##E_p## be an arbitrarily small quantum of energy?
 
TheCanadian said:
But why? Why is it more probable to find a state occupied by a photon as ##E_p## goes to 0? I suppose I am wondering if there is any minimum to a photon's energy (i.e. frequency). In this case, the mean occupancy is ##kT/E_p## but can't ##E_p## be an arbitrarily small quantum of energy?
When you compute the energy from these states, you multiply the occupancy by the energy. There is also one other factor included in this integration though, and that is the density of states in k-space, where ## E=\hbar c k ##. That density is proportional to ## k^2 ## and goes to zero as ## k=2 \pi/\lambda ## goes to zero, i.e. for long wavelengths.
 
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Some corrections. The link in the sentence
TheCanadian said:
is to a hyperphysics page. The explanation on that page requires a correction. The Bose-Einstein distribution is not the 'probability' of a particle having the energy but the thermodynamic average number of particles having the energy E.
Why is probability incorrect? well, for Bose-Einstein distribution, the value can be larger than 1 if E is sufficiently small. Probability cannot be larger than one, ever.

TheCanadian said:
is A = 1 for photons? Does this not imply that photons will have an increasingly high probability of being present as E approaches 0?
It means that as we approach zero energy, the number of photons in the given energy state approaches to infinity, i.e. infinite number of photons at zero energy.

TheCanadian said:
I am wondering if there is any minimum to a photon's energy (i.e. frequency).
This is actually a very interesting question.
Let's consider a one dimensional case, think of a cavity of length L. Then the lowest mode of electromagnetic wave (photon) will have the wavelength of 2L, That gives the energy ## E = \frac{hc}{2L}##. This is the lowest energy of a photon confined in one dimension to a cavity of length L. In 3D, if the cavity is a cube of the same length, the minimum photon energy is ##\sqrt 3## times larger.
So, as L goes to infinity, the minimum energy for a photon goes to zero.
However, if you confined photons to a finite volume, the minimum energy is inversely proportional to the dimension of the volume.
This leads to observable effect, such as Casimir effect

Henryk
 
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