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Why must the event horizon be a null surface?

  1. Dec 11, 2014 #1
    The event horizon is a null surface in spacetime separating events that are causally connected to an observer from those that are not. Does anyone have a good, simple argument on why it must be a null surface?
     
  2. jcsd
  3. Dec 11, 2014 #2

    PeterDonis

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    You gave the key fact in your OP: the event horizon is a causal boundary. Any causal boundary must be a null surface, because causal influences can travel at any speed up to the speed of light, but can't travel faster than light.
     
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