Can you please define the term "event horizon."
I have seen it in relation to discussions of black holes,
and then of course there was a sci-fi movie of the same name that
had to do with time/space relationships and what lies beyond.
What is the real definition of this term? Thanks.
An event horizon is an imaginary surface, completely surrounding a
black hole, at which the escape velocity (from the gravitational
pull of the mass inside) is equal to the speed of light. Anything
that exists or occurs at or inside an event horizon is unobservable
by faraway outsiders like us because even light can't get from there
to here without being red-shifted to zero by the gravitational field.
Anything that starts outward from an event horizon moving slower
than the speed of light will be decelerated to zero speed by the
gravitational field and will eventually fall back.
I suppose if we were close to an event horizon, we could observe red-
shifted light even from within the event horizon, because "escape" velocity
really implies "escape to infinity", and by going ourselves down into the
gravitational well, we'd be relaxing the requirement that the light
make it all the way out.
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Update: June 2012