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Why doesnt a photon escape from a black hole?

  1. Apr 2, 2006 #1
    Explain why elight emitted at a point inside the Scwarzschild radius (r<a) cannot be seen by a distant observer. (by one at r-> infinity)
    well we know the trajectory of a photon is given by

    [tex] \dot{r}^2 = c^2 \lambda^2 - \left( 1 - \frac{a}{r}\right) \frac{J^2}{r^2} [/tex]

    where J repsents the angular momentum in units of mass
    lambda is [tex] \lambda = \dot{t} \left( 1 - \frac{a}{r} \right) [/tex]

    so when r < a does the expression for r dot become imaginary? Is that what i am aiming to prove?
  2. jcsd
  3. Apr 3, 2006 #2


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    See the argument given by Dirac in its GR book when discussing black holes.

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