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: GR - The horizon a null surface?

  1. Apr 23, 2007 #1


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    1. The problem statement, all variables and given/known data
    In my GR final, there was a question that asked to show that the horizon r=2GM is a null surface, i.e. [itex]d\tau^2=0[/itex]. How does that work, since just plugging r=2GM and dr=0 in the Schwarzschild metric yields


    which is obviously not zero. :confused:
  2. jcsd
  3. Apr 23, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    A null surface has the property that its normal vector is null. What's a normal covector to the surface defined by r-2GM=0? Is it null?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook