ay45c3-page4 - Q dz Q is area = dx dy dz is thickness of...

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Unformatted text preview: Q dz Q is area = dx dy dz is thickness of slab of photons moving at speed c Photons in volume dx dy dz will slam into wall in a time dz=c. So, (number per volume) = 1 (number per area per time); c or d3 x = c dx dy dt  c d2 dt : | {z } normal area 2 d Why do we care? Because of a very famous, subtle theorem of mechanics: Liouville's Theorem: Phase space density of particles is constant along the trajectory of any freely moving particle. 28 ...
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