Photon in a massless mirrored box

  • Context: Graduate 
  • Thread starter Thread starter Rap
  • Start date Start date
  • Tags Tags
    Box Massless Photon
Click For Summary

Discussion Overview

The discussion revolves around the concept of mass associated with a photon contained within a massless mirrored box, exploring whether such a system possesses inertial and/or gravitational mass. Participants consider the implications of a photon gas in a black body cavity at a temperature above absolute zero and the relationship between the mass of the cavity and the mass attributed to the photon gas.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether the system of a photon in a massless mirrored box has inertial and/or gravitational mass, suggesting that the mass might be the sum of the mass of the cavity and the mass associated with the photon gas.
  • Another participant agrees with the initial proposition but does not elaborate further.
  • A request for references regarding the inertial/gravitational mass of a photon is made, indicating a search for supporting literature.
  • One participant asserts that the photon gas system has zero total momentum in a specific frame of reference, leading to the conclusion that the energy of the photon gas can be equated to a non-zero mass using the equation E = mc².
  • A later reply reiterates the idea that while a photon has no rest mass, a system containing a photon in a massless mirrored box will have a rest mass greater than zero, seeking references to support this claim.

Areas of Agreement / Disagreement

There is no clear consensus among participants; while some agree on the mass implications of the photon gas, others seek further clarification and references, indicating ongoing debate and exploration of the topic.

Contextual Notes

Participants express uncertainty regarding the definitions of mass in this context and the implications of different frames of reference. The discussion does not resolve the complexities surrounding the mass of a photon or the system as a whole.

Rap
Messages
829
Reaction score
11
Suppose we have a photon bouncing around in a massless mirrored box - does this system have inertial and/or gravitational mass? To be more realistic, if we have a massive black body cavity at some temperature above absolute zero, will the inertial and/or gravitational mass be equal to the mass of the heated cavity enclosure plus a mass associated with the enclosed photon gas, which I guess would be the sum of [tex]h\nu/c^2[/tex] over every photon enclosed?
 
Science news on Phys.org
Yes.
 
Do you know of any references that deal with the inertial/gravitational mass of a photon?
 
Rap said:
Do you know of any references that deal with the inertial/gravitational mass of a photon?
Anyway it's not difficult to prove it: the photon gas system has zero total momentum p, in the frame of reference in which the box or the cavity is still, so, from the equation:

E2 = (mc2)2 + (cp)2

you deduce that E = mc2. That is: the photon gas has a non zero mass m equal to its total energy divided by c2.
Of course it's invariant mass (not "relativistic" mass or other).
 
This is what I thought. In other words, although a photon has no rest mass, a system consisting of a photon trapped in a massless mirrored box will have a rest mass > 0. I am looking for a reference for this idea.
 

Similar threads

  • · Replies 26 ·
Replies
26
Views
2K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 1 ·
Replies
1
Views
607
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 17 ·
Replies
17
Views
2K
  • · Replies 1 ·
Replies
1
Views
1K