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## Main Question or Discussion Point

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?