From: utzoo!decvax!harpo!floyd!vax135!ariel!deimos!houxi!5941ux!kek
Newsgroups: net.space,net.physics
Title: Re: Black Holes
Article-I.D.: 5941ux.145
Posted: Tue Nov 16 05:12:32 1982
Received: Wed Nov 17 06:24:15 1982





In response to inuxa!rrm (Ron Meyer) question on black holes:

The German astronomer Karl Schwarzschild determined the field equations
describing space-time in the vicinity of a spherical lump of matter. His
solution states that if the mass is compressed within a certain radius
(called the Schwarzschild radius) space-time is so severely distorted
that nothing, even light, is able to escape.  To put it another way,
the "local gravity" cannot be overcome.  The Schwarzschild radius (R)
for a body of mass M is:

              R = 2GM/c**2

where G is a constant of proportionality known as the universal
gravitational constant, and c is the velocity of light. Thus the
Schwarzschild equations define how compressed a body would have to
become to create a black hole. (Actually, Michell and Laplace hinted
at such a possibility nearly a century before Schwarzschild put it
all together).

A body thus compressed (e.g., a collapsing star) disappears from view
since light is unable to escape from its surface (it has become a
black hole).  The boundary of the black hole is called the "event
horizon" because nothing inside can ever pass to the outside. The
event horizon is a one-way boundary: you can check in but never out!
This boundary coincides with the Schwarzschild radius. Even if the mass
inside continues to collapse into a "singularity", the event horizon
remains the same, the Schwarzschild radius. This holds true for
simple non-rotating black holes. Rotation adds some complexity.

As a couple examples, the Schwarzschild radius for our sun is just
under 3 kilometers (it's nominal radius is about 700,000 km) and
for the Earth is a little less than one centimeter!

Three good references on the subject that I have read are:

     GRAVITY, BLACK HOLES AND THE UNIVERSE
     Iain Nicolson
     John Wiley & Sons - New York 1981

     MONSTERS IN THE SKY
     Paolo Maffei
     The MIT Press - Cambridge, Mass and London, Eng (1980)

     BLACK HOLES - THE EDGE OF SPACE, THE END OF TIME
     Walter Sullivan
     Anchor Press / Doubleday - Garden City, NY (1979)


                                         Ken Kepple
                                         Bellabs - Holmdel
                                         5941ux!kek