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Unformatted text preview: 7. Thermodynamical equilibrium and blackbody radiation 7.1 Why is it that the things on your desk dont square away themselves? Second law of thermodynamics: nature tends to assume the state with the least complexity. Order turns into disorder, and the free energy in the systems, i.e. the exploitable potential differences, are being reduced. Three comments: This is the only law in physics with a time arrow. Besides gravity, this law is the main driver in the evolution of the universe. This is a means to characterize models of underdetermined systems, as is usually the case in astrophysics. The medieval philosopher William of Ockham noted the rule now known as Ockhams razor: Among competing models of a certain phenomenon, the simplest one is preferable as the presumably best description (= least complexity). 7.2 Statistical mechanics In astrophysics we deal with many particles, like 10 57 atoms in the sun. We cant possibly follow the microstate: position and momentum of every particle. The laws of thermodynamics indicate global arrangements and their evolution. It thus should be simpler to describe the macrostate: Number of particles in certain position and momentum intervals. This corresponds to the transition from the description of individual particles to particle dis- tribution functions f = dn/d 3 x/d 3 p (kinetic theory, Boltzmann equation), and possibly to the momenta of the distribution function = Z d 3 p mf ( x, p, t ) (7 . 1) and hydrodynamics....
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