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Unformatted text preview: each point that matters, but also the distribution of which way they are going,
and of what energies they have. Since the momentum vector p encapsulates
both the latter concepts (direction and energy) it is useful to think, at some
instant in time, of each photon being at a point in a 6dimensional space of
position and momentum:
(x; y; z; px; py ; pz ) :
(I can only draw 2 of the dimensions:)
dx dpx
p
x x The phase space density N is the number of photons dN per 6dimensional
unit phase space: dN = N dx dy dz dpx dpy dpz N d3x d3p :
Things get more interesting if we decide to keep track of the momentum part
of phase space in spherical, not Cartesian coordinates.
p
z d3p = p 2dpd Ω
py dΩ is a solid angle px 26 ...
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 Fall '09
 dion

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