1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted