On Sat, 5 Feb 2005 08:40:15 +0900, Martin DeMello
<martindemello / yahoo.com> wrote:
> Josef 'Jupp' Schugt <jupp / gmx.de> wrote:
> >
> > In principle the condition that the Schwarzschild radius is larger than
> > the physical size of the object can be met for *any* mass. Now enters
> > empirics. It is an unsolved question if black holes with small masses do
> > exist. Theoretical physics cannot decide, only nature knows. Up to now
> > no way of asking nature has been found.
> 
> Hawking proved they'd be unstable, at any rate.

More precisely, all black holes would, by their emission of Hawking
radiation,  be fundamentally unstable, and the smaller they are the
more radiation they would emit. However, for a typical black hole
generated by stellar collapse the amount of Hawking radiation emitted
is so low that the predicted possible lifetime of such a black hole
would be of the order of 1e63 years, much, much, much longer than the
age of the universe. A small primordial black hole with a mass of
several billion tons OTOH would have a lifetime of about 1e10 years,
roughly the age of the universe, so if Hawking's theory is correct
most of them would have evaporated by now, but the larger ones might
still be around. Such "black holes" would actually be emitting vast
quantities of Hawking radiation, so a sensitive gamma ray detector
might be able to detect their presence, and their final demise could
possibly account for some of the large gamma ray bursts that have been
observed.

If Hawking radiation is true, then that would be one way of detecting
low mass black holes.