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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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 Fall '09
 dion

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