Re: SETI/ASTRO: MBrains/JBrains & Globular Clusters

Date: Mon Jul 16 2001 - 23:43:43 MDT

Robert Bradbury wrote:
> Mike Lorrey wrote:
> > Or old black holes. What is the Hawking estimates on the lifetime of
> > black holes in the range of .1-1.0 jupiters?
> I'm pretty sure any black holes that size evaporate over times
> much greater than the current age of the universe.
has some interesting comments on the topic:

   However, during the Big Bang, the Universe underwent severe density
   fluctuations, which might have been capable of creating "mini" black
   holes with masses many orders of magnitude less than the black holes
   that are created today. Those with masses much less than 10^12 kg
   would have exploded by now, and those on the order of 10^12 kg would
   have lifetimes comparable with the current age of the Universe (~15
   billion years).

   One can get a feel for what such a primordial black hole would "look"
   like by examining a black hole with an initial mass of ~ 10^12
   kg. The only assumptions needed are that (1) the Universe is ~15
   billion years old, (2) the black hole is non-rotating and uncharged,
   and (3) the black hole has incurred no increase in mass. The Cosmic
   Microwave Background Radiation in which the black hole has been bathed
   for the extent of its lifetime can also be neglected. The evaporation
   time for such a black hole is around 20 billion years. This would
   give it a current mass of ~5 * 10^11 kg (about half of its initial
   mass). Using eq. (9), we obtain a surface temperature of about 2 *
   10^11 K! This black hole is anything but black, as by eq. (12), it
   is emitting ~1500 megawatts of power. Considering the incredible
   surface temperature, this may even seem a little small. However,
   the Schwarzschild radius for such a black hole is less than a fermi...

Jupiter's mass is about 2 * 10^27 kg, far above this value, hence a black
hole of the size Mike is asking about (0.1 to 1.0 Jupiters) would have
a lifetime essentially infinite compared to the age of the universe.
The energy output is inversely proportional to the mass squared,
so a Jupiter sized black hole would be outputting 10^31 times less,
which would be an unmeasurably tiny amount of power, at a temperature of
almost absolute zero. Hawking radiation is insignificant for planetary
sized objects.

The 10^12 kg threshold for Hawking radiation/evaporation to be significant
would correspond to roughly a kilometer-sized chunk of rock, something
for which a gravitational signature would normally be undetectable.
So if you can see effects of its gravity, it could always be a black
hole in a shell, and Hawking evaporation will not be an issue.


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