# What is Photon gas: Definition and 20 Discussions

In physics, a photon gas is a gas-like collection of photons, which has many of the same properties of a conventional gas like hydrogen or neon – including pressure, temperature, and entropy. The most common example of a photon gas in equilibrium is the black-body radiation.
Photons are part of family of particles known as bosons, particles that follow Bose–Einstein statistics and with integer spin. A gas of bosons with only one type of particle is uniquely described by three state functions such as the temperature, volume, and the number of particles. However, for a black body, the energy distribution is established by the interaction of the photons with matter, usually the walls of the container. In this interaction, the number of photons is not conserved. As a result, the chemical potential of the black-body photon gas is zero at thermodynamic equilibrium. The number of state variables needed to describe a black-body state is thus reduced from three to two (e.g. temperature and volume).

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1. ### The Pressure-Entropy Relationship for a Photon Gas

This is from Problem 7.45 of Thermal Physics by Daniel Schroeder.
2. ### I Does a photon travel freely through the CMB photon gas?

A photon often travels billions of years (Gyr) through the CMB photon gas (410 photons per cubic centimeter) to reach us. Does it travel freely? Let’s share our thoughts about this. For discussion purpose, let’s assume the photon has a wavelength of 500 nm, close to the peak of the solar...
3. ### I Why can you not describe massive neutrinos with a temperature?

In the Wikipedia article for CvB, it mentions the following: "The above discussion is valid for massless neutrinos, which are always relativistic. For neutrinos with a non-zero rest mass, the description in terms of a temperature is no longer appropriate after they become non-relativistic; i.e...

5. ### B Photon Gas in a Box w/ Heat Conductor

If I have a box evacuated of air with 5 of the sides mirors and one side a heat conductor. will the photon gas inside have photons that get absorbed by the heat conductor and re-emitted when the photons strike the heat conductor
6. ### Isothermal Compressibility of Photon Gas

I am really stuck at this question. I tried to get the equation of volume with independent variables P and T, but the equation itself does not give a nice form, and thus I cannot get the derivative of V with respect to P. What should I do?
7. ### Specific Heat at constant pressure for photon gas

Homework Statement A blackbody photon gas is contained within an evacuated cavity (V = 0.01 m^3). Calculate C_p for the photon gas at T = 1000K Homework Equations C_p - C_v = T(\frac{\partial S} {\partial V}) (\frac{\partial V}{\partial T}) C_v = T(\frac{\partial S} {\partial T}) S =...
8. ### T-S Carnot Cycle for Photon Gas

So I have been thinking about the photon gas, and I have read several papers talking about how a Carnot cycle could be created for it. This is fantastic, and it is something I am quite comfortable with. All of the papers present the P-V diagram as the "golden" Carnot cycle for the photon gas...
9. ### Equations of state for photon gas

Equations of state for photon gas and relativistic electron gas This entry develops equations of state that are useful in calculations about cosmology and about the insides of stars. The first calculation is for a photon gas and the second is for a 'relativistic' gas of particles with mass...
10. ### The Pressure of a Photon Gas: A Derivation Using the Ideal Gas Law

Homework Statement Consider a photon gas (particle-like nature) with N photons of monochromatic light in a box that has a volume V. You can assume everything is perfectly reflecting. What is the pressure of the photon gas based on the ideal gas law derivation? Homework Equations N/A. The...
11. ### Pressure-energy relationship in photon gas

Exercise 22 on p108 of Schutz's 'A first course in General Relativity' is to prove that, for an isotropic, monochromatic, photon gas, p=ρ/3, where p is pressure and ρ is mass-energy density. When I try to do it I get p=ρ/6. I was hoping somebody could tell me where I'm going wrong. Here is...
12. ### Equation of state of photon gas

Dear all, I am using stress-energy tensor to derive equation of state of photon gas (assuming it as a perfect fluid). I completed all the steps except one: average value of [cos(θ)]^2 over unit sphere = 1/3. I have no idea how this is so. (θ is polar angle). I tried integrating over...
13. ### Understanding Energy Density of Photon Gas

Homework Statement ##u(\omega) d\omega \propto \frac{(\hbar \omega) (\omega^2)}{e^{\hbar \omega \over k_B T}-1} d \omega ##Homework Equations The Attempt at a Solution ##\hbar \omega ## is the energy of a photon ##\frac{1}{e^{\hbar \omega \over k_B T}-1} ##and this is the density of states...
14. ### Density of states of ideal gas and photon gas

Homework Statement I know how to derive the density of states for an ideal gas by using the energy equation: E_n = A*n^2, where A = (h_bar^2*pi^2)/(2mL^2) but what about for a 'photon gas'? Do I use the same energy equation as above, or the following: E_n = (h_bar*pi*c/L)*n...
15. ### Photon gas as medium for electromagnetic propagation

Hi Guys, I came across this article by Jikang Chen and it is of importance to me to know what measure of credibility this concept holds in the general physics fraternity. I do not have the background to make sense of the mathemetics or physics cited. I would appreciate your comments. The...
16. ### Estimating Redshift of Photon Gas in Universe Transition

Homework Statement The problem is translated from a different language, so I hope I am not missing anything. I need to estimate the redshift of the photon gas in the universe, at the time of transition from radiation to matter domination. Cosmological parameters: k=0 (meaning a flat universe)...
17. ### The photon gas in the curved space

In a flat space, the momentum of a photon gas distributes isotropically. Every direction is equivalent. If the space is curved,like the space outside a black hole, what will happen to the photon gas? Will the momentum distribution be not isotropic any more?
18. ### Energy of a photon gas: two ways to get it, two different answers

Homework Statement I've been asked to calculate the energy of a photon gas in terms of the temperature. Assume non-interacting. I'll spare the details, unless someone would like to see them, because the calculations can be found in most textbooks. Here's the problem: When I do it using the...
19. ### Resolving Discrepancies in Photon Gas Thermodynamics

Homework Statement I’m struggling to reconcile two results about the behaviour of a photon gas, any help would be appreciated: First of all the Gibbs free energy=0, which means that dG=0=Vdp-SdT But also p=1/3 U/V and S=4/3 U/T which means p=1/4 ST/V. Now if we call the entropy per unit...
20. ### Does the Rest Mass of Photon Gas Change with Unidirectional Motion?

Consider a gas of photons in a vessel. The total momentum of the radiation is equal to zero. Its total (rest) mass is m=E(rad)/cc. Consider an unidirectional motion of the vessel. Question: Will the (rest) mass of the radiation increase due to the fact that the total momentum is no longer equal...