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Black Hole Event Horizon

  1. Mar 10, 2005 #1
    Hey, my first post here, glad I've found somewhere on the internet to discus this crazy stuff!

    Anyway heres my question:

    Matter falling into a black hole accelerates towards it under its gravity, towards the "event horizon" (or Rs=2GM/c^2)

    The relativistic version of the law of gravity is F=GM/r^2*[1/(1-Rs/r)] so at the event horizon the force becomes infinite so then the speed of matter falling in can become the speed of light given infinite enegry, so it also has infinite mass?
    But from outside nothing ever crosses this border as we see it getting slower and slower so it never acctuly touches it. But according to the matter it passes this and then on inside the black hole.

    My question is assuming all of this, does that mean at some point there is an infinite amount of matter in the universe that we cannot see? It's all conveniently hidden behind the event horizon. So does this not violate the laws of conservation in a way, there suddenly being infinitley more enegry in the universe?
  2. jcsd
  3. Mar 10, 2005 #2


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    The "force" at the event horizon isn't infinite, it is just exactly enough to happen to equal the (gravitional pull) of the escape velocity of the speed of light. No matter and/or photons drawn into a BH have infinite energy (= infinite mass) so no, there is no BH with an infinite mass.

    In fact, quite a few BH candidates have been measured with various, finite, masses up to the largest one (I know of) in the center of the elliptical galaxy M87 at ~3 billion Ms.
  4. Mar 10, 2005 #3
    Your question one. Ther ehas been a lot of confusion over the years about what happens when something falls into a black hole. The best answer is this: The object falls in according to the equations for a falling body. The problem is with what a faraway observer sees. The gravitation of a black hole is so great that time slows down in its vicinity.

    If you imagine a flashing light on the falling object that flashes once every second, seconds pass slower and slower as the object approaches the hole. So they become farther and farther apart from the distant point of view. As the event horizon is approached, the flashes become so far apart that it seems like the object has stopped.

    I probably don't explain this very well. Search the net on black holes and you should find an explanation somewhere.
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