- #1

- 1

- 0

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 ?