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Chapter 21 Ray D'Inverno Scalar Optics, congruence of null geodesics

  1. Mar 24, 2014 #1
    First of all this is my first thread, so I apologize for any mistake.
    Perhaps this is a stupid question, but i need some help in exercise 21.10 of D'Inverno, to write down geodesic equation for l^a, which is a vector tangent to a congruence of null geodesics and then by a rescaling of l^a:

    l^a -> A l^a how we conclude that l^a;b l^b=0 ?
     
  2. jcsd
  3. Mar 25, 2014 #2
    Hello, try to use general geodesic equation and then simplify the connection term with the definition of covariant derivative. Read about affine parameters.

    I'll try to post a solution attempt. ( Ray D'inverno is a nice book has introduction to general relativity aspects).
     
    Last edited: Mar 25, 2014
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