ay45c3-page2 - each point that matters but also the...

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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 6-dimensional 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 6-dimensional 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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