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Black holes and light

  1. Sep 30, 2015 #1
    It heard it stated that light cannot escape from a black hole, yet light continues to propagate at the speed of light even in a black hole. Can someone explain to me how it can be that light can't escape yet does not slow down?
  2. jcsd
  3. Sep 30, 2015 #2


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    Staff: Mentor

    Have a look at the "Similar Discussions" links at the bottom of the page -- do those discussions help? :smile:
  4. Sep 30, 2015 #3
    Some, but not exactly what I'm looking for.
  5. Sep 30, 2015 #4


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    Gold Member

    Things in spacetime travel on what are known as geodesics, which is the space-time geometry equivalent of straight lines. Using Euclidean Geometry as a reference frame, we say that spacetime "bends" light under the force of gravity (Google "Einstein Rings").

    Because light is affected by gravity, it "slows down" when coming out of a gravity well. I put "slows down" in quoted because it does NOT slow down locally, it just looks that way to a remote observer. The "slowing down" shows up as red-shifting. That is, the light is still traveling at c but its frequency shifts towards the red end of the spectrum.

    The gravity in a black hole is so strong that the geodesic points back towards the singularity at the center of the BH and the light, while traveling at c, locally, away from the singularity is in fact moving towards it along the geodesic.
  6. Sep 30, 2015 #5


    Staff: Mentor

    "Not slowing down" is a local concept: light always moves at ##c## when measured by observers in the same local patch of spacetime as the light. This is true in any spacetime, regardless of its global geometry.

    "Escape" is a global concept: light can't escape from inside the event horizon because the global geometry of a black hole spacetime won't let it. Geometrically, there is simply no path the light can follow that will allow it to escape. Observers who observe the light, locally, moving at ##c## can't escape either; in fact they will fall into the singularity sooner than light that is moving radially outward, relative to them, at ##c#.
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