From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Fri May 17 2002 - 06:39:12 MDT
I agree with the comments about dumping the waste heat back
into the star. That's one of the problems with a system
heavily populated by Asteroid Brains. A fundamental feature
of the architecture of MBrains is that they always radiate
the heat away from the star -- not true for Asteroid Brains
(unless you design them with some dumb-ass radiator on one
side, then they don't look very "natural").
But MBrains are already pretty stealthy. Depending on whether
the outer shell layers are cooled by lN2 or lNe, they are
radiating as blackbodies at ~63K and ~24K respectively.
*IF* you have enough material to cool with lH2 you can get
down to ~14K. *BUT* even at lN2 temperatures, using MgO
for the radiators and some guesstimate of the radiator
thickness (this *really* needs an accurate design, but
I stopped refining the calculations once I understood
what was going on), you still need 2x the total planetary
mass in our system for just a single layer of radiators.
For lH2 radiators, you need 113x the mass of the sun!
I'd hate to think of what the mass would be for lHe.
[Actually, I got off my sorry butt and did the calculation,
its ~500 billion solar masses -- a *very* interesting number].
Now, at 1 deg above the CMBR background temperature, you have
to be ~2.4 l.y. across to radiate the solar power equivalent.
(The T^4 efficiency for blackbody radiation is *really*
showing its ugly face here.)
So it looks to me like the modertately close-to-stealthy
MBrains are going to be leaving snail trails through
massive gas clouds to get the material they need for
radiator construction.
In thinking about this a bit further, I'm not sure that
MBrains have to go to too much more trouble than what
they would probably do anyway to stealth themselves.
Even at LN2 temperatures, you are still ~3 light-hours
across. Even a modestly prepared MBrain with high
intensity lasers, particle beam weapons or tactical
nukes is going to have plenty of time to respond once
it detects nanotech munching on its outer edge.
To mount a successful attack on an MBrain I think you
would have to come in with an overwhelming amount of
mass and its going to be hard to hide the gravity
signature that will likely have.
Now, presumably one star lifts to lengthen the lifetime
of your star, so the radiator mass requirements will be
reduced because you have less somewhat energy to deal
with. According to Criswell, it takes ~300 million years
to star lift a star like the sun using 10% of its power.
Presumably you want to use that material to store memories
produced over that period. So I'd imagine the outer layer
could get quite large and cold. Accessing those memories
is going to take days to months probably.
I agree with Amara that one could possibly detect the
gravity signature (or occultations) produced by nearby
"visitors". But it seems problematic to be presenting
oneself (particularly if that large) as a "clear" image
to all viewing angles. So I'm not sure how much it buys
you.
I don't however see any easy way to hide the MBrain's gravity
signature from others. You could choose to completely
disassemble the star and continue on as dispersed JBrain
"cloud". That would allow you to be "faster" since you
don't have to redirect the star when you want to change
course. This seems likely however to incur a penalty
of having to utilize valuable metals for your thermonuclear
reactors. I suppose you could get around this *if*
you could harness relatively small black holes (remember
you are trying to get your gravity signature below that
of a dwarf star) to do the power generation at the core
of the JBrains. I suppose this would be a BHBrain
(there's a new term for your dictionary I think Anders).
Now, whether a BHBrain could structure itself such that
it inverted the heat flow (radiating inwards rather than
outwards) isn't clear to me. Its interesting to consider
that since solar system and dust cloud temperatures are
significantly above the CMBR temperature, that if one
continued the process of creating ever smaller black holes
at the core of the BHBrains, one would end up with BHBrain
"dust" drifting through the coldest regions of intergalactic
space. There seem to remain problems of small Black Hole
evaporation, reflecting the Hawking Radiation back into
the Black Hole to maintain its mass and still come up
with additional material from which you can extract
useful energy by dumping it into the Black Hole.
Of course that process would appear to be counterproductive
because its going to increase the mass of the Black Hole
and therefore increase your gravity signature.
There may be a way out of this however -- you can obviously
maintain the mass of your black hole by simply balancing
radiation out with matter in (those spare crumbs of
intergalactic H2 you happen to come across). Now if
you can do photonic bandgap engineering [1] for temperatures
a few millikelvin above the CMBR temperature, you might
just be able to radiate all of your energy at that temperature.
This avoids the problem that using blackbody radiators you are
still producing detectable levels of non-CMBR radiation.
So it may be possible to be really cold and have a really small
gravity signature. Of course I think you might also not
have enough computronium left to be very intelligent.
"And so, finally, after the singularity, after all the stars
had been dismantled the Universe came to be dominated by
only two species of objects. The large black holes that had
once been at the centers of galaxies, now silenced because
of the scarcity of raw materials to feed them, and an unimaginable
number of Black Hole Brains, drifting through what had once been
the intergalactic voids, feeding on the remaining crumbs of
hydrogen and helium in the universe. Thus it came to pass
that in the end the Universe became really really dumb."
Robert
1. J. G. Fleming, et al., "All-metallic three-dimensional
photonic crystals with a large infrared bandgap",
Nature 417:52-55 (2 May 2002).
[Caveat -- the bandgap engineering they are doing is in
the visible & near IR region. It may not be possible
to engineer bandgaps a few millikelvin above the CMBR.]
This archive was generated by hypermail 2.1.5 : Sat Nov 02 2002 - 09:14:09 MST