Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!wuphys!ihr
From: ihr@wuphys.wustl.edu (Ian H. Redmount)
Newsgroups: sci.space
Subject: Re: Black Holes
Summary: Fade to Black (Fast)
Keywords: black holes, general relativity, gravity, collapse
Message-ID: <1990Dec14.004927.22480@wuphys.wustl.edu>
Date: 14 Dec 90 00:49:27 GMT
References: <2389@mentor.cc.purdue.edu> <1824@odin.cs.hw.ac.uk> <1990Dec12.195033.26427@watdragon.waterloo.edu>
Reply-To: ihr@wuphys.UUCP (Ian H. Redmount)
Organization: Physics Dept, Washington U. in St Louis
Lines: 46

In article <1990Dec12.195033.26427@watdragon.waterloo.edu>
jdnicoll@watyew.uwaterloo.ca (James Nicoll) writes:
>
>	Shouldn't relativistic effects prevent an outside observer
>from seeing the collapsar form? I can see observing a 'really-quite-close-
>to-being-a-collapsar object, but do we ever actually 'see' the event
>horizon 'form' or does the collapse appear to an outside observer to
>take an infinite amount of time?

This question has appeared in other forms on the net before.  It is
discussed quite thoroughly in C. W. Misner, K. S. Thorne, and
J. A. Wheeler, ``Gravitation,'' W. H. Freeman and Company (San Francisco),
1973, pp. 846-850 and pp. 872-875.

If I may be permitted to paraphrase the Masters Three and Wise (:-)):
Although the surface of a collapsing star crosses the event horizon at
infinite t, where t is the time coordinate kept by the clocks of
distant observers, this is actually an artifact of the coordinate
system.  The luminosity of the star decreases exponentially with
(distant-observer) time in the late stages of the collapse.  The
e-folding time of this decrease is proportional to the star's mass,
and is approximately 25 microseconds per solar mass.  The star ``fades
to black'' very fast:  The luminosity of a four-solar-mass star in
collapse drops by more than forty orders of magnitude in a hundredth
of a second!  Of course, since the atoms of the star emit light discretely,
and since those atoms cross the horizon in a finite amount of their own
proper time, the star does not fade forever.  In short order it is
absolutely black.*

There is a time, shortly after the start of the collapse, after which
no signal or influence from outside can reach the atoms of the star
before they cross the event horizon.  After that time the pre-collapse
object is, for all operational purposes, gone.  The ``collapse appear[ing]
to an outside observer to take an infinite amount of time'' is thus illusory.

One does not ``see the event horizon form'' because there is nothing
to see:  The event horizon itself is locally indistinguishable from
ordinary space.

I hope this clarifies the issue a bit.  For more (and better-presented)
detail, see MTW, loc. cit.

Ian H. Redmount

*Neglecting Hawking radiation, which is completely negligible for
``collapsars''---black holes---of stellar masses or larger.