From: utzoo!decvax!yale-com!leichter
Newsgroups: net.physics
Title: Re: black holes
Article-I.D.: yale-com.337
Posted: Tue Nov 16 13:45:07 1982
Received: Wed Nov 17 05:31:06 1982
References: inuxa.169

The horizon (more fully, event horizon) of a black hole is the surface in
space at which the escape velocity is the speed of light.  (This is an informal
way of putting it but is probably correct if interpreted right.)  If you are
outside the horizon, you can get away from the black hole.  Inside, you can
never escape.  Hence, no information can ever cross from inside the event
horizon to outside of it.  (If you are exactly at the event horizon, under
the right conditions, you can orbit forever, but never get away.)  In the
relatistic view of things, two events can be causally related only if infor-
mation could have propagated from one to the other at no more than lightspeed.
No event whose spacial location is inside the horizon can be the cause of
an event outside of it.

A better - certainly more accurate - way of looking at it is as follows:
General Relativity views gravity as consisting of a curve in 4-dimensional
space time.  The effect is to rotate the reference 4-d coordinate system.
As you move closer to a large mass, the rotation is such as to rotate the
time axis somewhat toward the mass.  On result is the slowing of time in
a large gravitational field.  The field of a black hole is so large that
once you cross the event horizon the direction of the time axis is toward
the black hole!  Hec
\\\Hence, to escape you'd have to have a time machine!

I believe that the Schwartzchild radius and the event horizon coincide, at
least for a non-rotating black hole.  It's been a while since I looked at
this stuff; I apologize for any errors - which I'm sure someone in net-land
will point out.
						-- Jerry
					decvax!yale-comix!leichter
						leichter@yale