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The Schwarzschild radius is the radius at which
the escape velocity equals the speed of light.
The surface of
the sphere of radius equal to the Schwarzschild radius is
called the event horizon. We on the outside of the
black hole cannot learn anything about any event taking place
within the event horizon, and thus we can think of it as
the "surface" of the black hole.
At the center of the black hole lies a
singularity, that is, a region where the current laws
of physics break down because the circumstances are so
extreme.
Note that the Schwarschild radius scales with the mass of the black hole.
The Schwarzschild radius of a 1 solar mass black hole is 3 x 105 cm.
According to Roger Penrose's Principle of Cosmic Censorship,
the fact that the laws of physics break down inside the event horizon
has no impact on the physics outside the black hole (so we needn't
worry about the failure of our current physics too much!?).
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