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From: mam@charm.UUCP (Matthew Marcus)
Newsgroups: net.physics
Subject: Self-gravitating Photons
Message-ID: <681@charm.UUCP>
Date: Mon, 10-Jun-85 22:26:41 EDT
Article-I.D.: charm.681
Posted: Mon Jun 10 22:26:41 1985
Date-Received: Tue, 11-Jun-85 07:28:04 EDT
Organization: Physics Research @ AT&T Bell Labs Murray Hill NJ
Lines: 21

(E = Mc^2 +- 3dB )
	About maximum energies for photons:  Take any ol' photon you have lying
around.  Now look at it from a frame of reference going very fast in the
direction opposite to said photon.  The photon is now blue-shifted to higher
energy.  By making your new reference frame move fast enough, you can get any
energy you want, including 10^43 Hz or whatever the last-posted limit was.
Now, 'black-hole-ness' is an invariant, so an energetic photon is never a
black hole.  Another way to see this is that black holes are characterized
by three and only three parameters: charge, spin, and mass.  This mass is
an invariant, and hence must be in the nature of a rest mass.  The rest
mass of a photon, any photon is 0.  'Nuff said.
	The argument has been made that a hefty enough electromagnetic
field will gravitate itself into a black hole.  Well, Misner, Thorne &
Wheeler, aka the Phone Book, give as a show-that problem the theorem that
two light beams do not attract.  While there is energy density in each beam
(T sub 00, where T is the stress-energy tensor), there is also momentum
density (T sub 0i) which cancels out the attractive effects of light upon
light.  Therefore, you are not going to get a black hole by shining lots
of light into a small spot.

		{BTL}!charm!mam