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

From: Mike Lorrey (
Date: Tue Jul 17 2001 - 08:39:16 MDT wrote:
> 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:
> 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 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.

Ah, so they would be stable enough for an Mbrain to be constructed

> 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.

Yes, but what about extracting energy from purposely injecting mass into
the hole, as well as from the gravitational potential of dropping mass
into it?

> 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.

Ah, interesting, so there could be these all over the place without our
knowing. Here's an idea:

We have explicit evidence that there is no Fermi Paradox by the mere
fact that there are no such black holes in our immediate area, they've
already all been harnessed and carted away by other races.

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