Why must the event horizon be a null surface?

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SUMMARY

The event horizon is definitively a null surface in spacetime, acting as a causal boundary that separates events connected to an observer from those that are not. This characteristic arises because causal influences can propagate at any speed up to the speed of light, but cannot exceed it. Therefore, the nature of the event horizon as a null surface is essential for maintaining the principles of causality in physics.

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  • Understanding of spacetime concepts
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  • Knowledge of causal boundaries in physics
  • Basic grasp of light speed limitations
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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?
 
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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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