Radiation pressure calculation (verification)

[Bernie G]

If 1 kilogram of matter is converted into radiation and contained in 1 cubic meter, and the radiation reflects perfectly off the walls, is the pressure 9 X 10^16 or 3 X 10^16? Ie.- Is the pressure pc^2 or (pc^2)/3 (where p is the equivalent mass density) as given in http://en.wikipedia.org/wiki/Photon_gas ?

 

[Bernie G]

Or is the pressure (2pc^2)/3 ?