Hi all,
Been a bit busy the last few days - lost track of this thread. On looking it over a few things occur to me.
MBroderick brought up the idea of a container with perfectly reflection walls. This idea was original thought of by Adolfo Bartoli in 1876. He was arguing, from Maxwell's relatively new theory of electromagnetism, that light should exert a pressure. He imagined light trapped in a cylinder, with a piston at one end, all with perfectly reflecting walls. He used the second law of thermodynamics to deduce that the trapped light would and in fact must make a pressure. This pressure is not very big and it was not until about 1905 when Nichols and Hull and independently by Lebedev actually measured it.
The Bartoli Cylinder was again used by Boltzmann to derive Stefan's radiation law. This is the law that Pasti is quoting. It says that the total radiated power of a body at temperature T is gigen by the Stefan-Boltzmann law
P = sigma T^4
where P is power, sigma is called - what else - the Stefan-Boltzman constant and T is the absolute temperature.
The Bartoli cylinder was transformed into a sphere by Wilhelm Wien. He was studying the adiabatic compression of radiation with an eye toward finding a good radiation law. He was using a mostly thermodynamic arguement.
Now all these ideas were mere thought experiments. Richard Ulbricht (1849-1923) actually made what is called a spherical integrator that is a rigid version of a Wien sphere. This is normally used as a kind of radiation calorimeter or to provide a homogeneous isotropic light for test purposes.
By the way light does have inertia. In fact this is equivalent to E = mc^2. To see the derivation look at Max Born's book entitled "Atomic Physics". It is a Dover book so so it is easy to get. I would quote it here - because the derivation is easy and visilizable - but I have to runn off right at this moment.
Dr. R.
P.S. Made some small mods here.
