the energy will be redistributed until the ensemble of photons achieves a Planck distribution. The time taken for thermalization is much faster with condensed A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is called black-body radiation. The name "black body" is given because it absorbs all colors of light. In contrast, a white body is one with a "rough surface that The integration of Planck's law over all frequencies provides the total energy per unit of time per unit of surface area radiated by a black body maintained at a temperature T, and is known as the Stefan–Boltzmann law:

Boltzmann built on two known properties of cavity radiation. (1) The energy density, $u$, defined as $u = U/V$, depends only on temperature, $T$. (2) The radiation pressure, $p$ is given by $p= u/3$. Radiation pressure was given a firm basis c1862 by Maxwell. The factor of 1/3 arises because of the three-dimensionality of the cavity, in which radiation is propagating in all possible directions. [It's easy for us now to derive this equation by considering the cavity as containing a photon gas.] Boltzmann (1884) used a thought-experiment in which a cavity is fitted with a piston, and we take the radiation inside it through a Carnot cycle. On a $p–V$ diagram the isothermals are just horizontal lines, because $u$ is constant so $p$ is constant. The heat input along the top (temperature $T$) isothermal is $\Delta U + p \Delta V$. This works out to be $4 p \Delta V$. If the lower temperature i

"... Das Lebenswerk Boltzmanns war die Neuaufstellung der Thermodynamik. Dabei begründete er mit James Clerk Maxwell die Statistische Mechanik (Boltzmann-Statistik) und deutete die Entropie als eine mikroskopische Größe (siehe unten). Boltzmann war ein Verfechter der atomistischen Vorstellung. Er hatte dabei zahlreiche Fachgenossen seiner Zeit als Gegner, unter ihnen Wilhelm Ostwald und Max Planck. Dieser griff jedoch bei seinem Planckschen Strahlungsgesetz auf die Vorstellungen Boltzmanns zurück. Das Stefan-Boltzmann-Gesetz (die ausgestrahlte Leistung ist der 4. Potenz der absoluten Temperatu