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Strange expres. for the deriv. of solid angle

  1. Sep 23, 2013 #1
    please see attached snapshot from the book of Ligthmann & al. (problem book in relativity and gravitation).
    Can somebody please explain the expression for the derivative of the solid angle ?
    here it is given as dvxdvy where the v's are speeds !!
    How come this is so ?

    Attached Files:

  2. jcsd
  3. Sep 23, 2013 #2
    The solid angle is dvxdvy/v2 but the problem is about photons so v = c = 1 in natural units.
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