Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Constant speed of light in vaccum?

  1. Aug 3, 2006 #1
    Just find something very interesting..

    If the speed of light is constant, at some distance from the black hole where light can neither escape the attraction of the black hole, nor succumb to its gravitation, what state is the light in? does it have a speed there?

  2. jcsd
  3. Aug 3, 2006 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As radially infalling light approaches the event horizon of a spherical black hole, the (Schwarzschild) coordinate speed of the light approaches zero. However, an observer who happens to be falling throught the event horizon will, at the event horizon, measure the physical speed of the infalling light to be the standard speed of light.

    Coordinate speeds and physical speeds are often different.
  4. Aug 3, 2006 #3
    got it

    haha thanks a lot:wink:
  5. Aug 3, 2006 #4
    The speed of light does not change.
    What happens is when light approaches the black hole the warping of space-time causes the incoming light rays to converge to a point. In other words they cannot escape anymore, there is no longer a geodesic, they all have to go to that point.
    The region were a return is no longer possible is called a trapped surface.
  6. Aug 3, 2006 #5


    User Avatar
    Gold Member

    Isn't the OP simply asking about escaping light being infinitely redshifted to zero?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook