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From: jlg@lanl.ARPA
Newsgroups: net.physics
Subject: Re: Re: "big bang" a big bust?
Message-ID: <17266@lanl.ARPA>
Date: Mon, 3-Dec-84 20:51:16 EST
Article-I.D.: lanl.17266
Posted: Mon Dec  3 20:51:16 1984
Date-Received: Thu, 6-Dec-84 05:44:14 EST
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Organization: Los Alamos National Laboratory
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> Light can't escape from within a black hole, but information can.
> 
> Two pieces of information escape from a black hole - its mass and angular
> momentum.  The mass can be found from the strength of its gravitational field.
> As I remember (from a physics lecture) the angular momentum can be found by
> sending a satellite around the black hole.  The satellite will return rotated
> as I recall, though I don't really understand why.  It probably has something
> to do with the curvature of space around the black hole.
> 
> It would seem that information transmission is not necessarily bound by
> the properties of light.  Any comments?
>

Information does NOT escape from a black hole.  The mass and angular momentum
of the object are discerned by observing the properties of the space-time
manifold in the neighborhood of the event horizon.  The charge on the black 
hole is also discernable in a similar manner (observing the electric field in 
neighborhood of the event horizon).  In each case the observation is of local 
phenomena that occur OUTSIDE of the black hole.  

Note also that light DOES escape from a black hole.  A black hole radiates
like a blackbody whose temperature is proportional to the inverse of the
circumference of the event horizon (or is it the inverse of the area of
the event horizon - I can never remember).  This is a quantum mechanical
effect.  The implication is that all black holes shrink as time passes
due to the loss of mass-energy.  Of course, this effect is countered in
the real universe since matter is always falling into the thing too.
However, small black holes have a high blackbody temperature and a low
accretion rate and will self-destruct over time.  The blackbody temperature
of a black hole can also be used to determine its mass, but no information
about the internal structure of the black hole (if any) is carried by this
radiation.