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

From: hal@finney.org
Date: Tue Jul 17 2001 - 11:00:19 MDT

Eugene writes:
> Hal, can you look up the hole mass and power radiated for a ~10 kK

According to that web page,
temperature is inversely proportional to mass, and power radiated is inversely
proportional to the square of mass.

By 10 kK you mean 10 kilo Kelvins, i.e. 10^4 K? The example hole they gave
of about 10^12 kg had a temp of 10^11 K. You want temp to be 10^7 lower so
you need 10^7 more mass, or 10^19 kg. This would be like a 100 km asteroid,

Power goes as inverse square, so with 10^7 more mass you get 10^14 less
power. Instead of 1500 MW you'll have 150 microwatts. So it won't be
a useful source of power.

Unfortunately it appears that to get meaningful amounts of power you
need a black hole which was formed with exactly the right size to be on
the verge of evaporation right now. An order of magnitude too small and
it would have evaporated billions of years ago. An order of magnitude
too large and you'll get a few megawatts at best.

Hal

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