From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Mon Sep 13 1999 - 13:31:54 MDT
On Mon, 13 Sep 1999, Robin Hanson wrote:
> Right. Obviously the universe is very big, and we couldn't expect to
> see a single nano-alien half-way to the horizon. But we *can* say things
> about the aggregation of all aliens in a region. For example, aliens
> aren't intercepting more than 1% of the starlight from the nearest 100
> stars, at least if they re-radiate it at obvious IR temps. (For more
> examples like this, see: http://hanson.gmu.edu/filluniv.pdf)
>
I think I did read this paper quite a while ago (the date on the copy
I have saved is Sept. 1998). The problem seems to be the 1% argument
from Jugaku. I did investigate this a bit more completely and it seems
he got it from Papagiannis from "The Search for Extraterrestrial Life:
Recent Developments", 1985, pp. 263-270. Jugaku has been doing
Dyson "searches" for 15+ years based on the 1% assumption and has
looked at probably the number of stars you quote.
The problems with the Papagiannis argument are:
(a) He claims based on some simple calculations that you can't build
a solid Dyson sphere. These are correct only using his assumptions.
If you build a light enough Dyson sphere it can be supported by the
solar wind (with certain navigational problems). Pohl and Anderson
figured out how to solve the materials strength problems for a
heavier Dyson sphere in their book the "Cuckoo". While the
requirements are difficult to meet, Robert Freitas feels they
can be solved using momentum transfer technologies.
I've also never seen any calculations on variable thickness
Dyson spheres (as they finally figured out they had to do to
make Sky Hook cables work). So real "solid" Dyson spheres are
still a fairly open question.
(b) Papagiannis did "claim" without any calculations was that:
"What is possible, however is to have a large number of
independent space structures in orbit around the star, but
these would intercept only a relative fraction (~1%) of the
star's radiation. Consequently such stars, would display a
normal spectrum with only a small excess in the infrared."
This statement only appears valid if:
(1) you use the material to create planetoids that have gravity; or
(2) you create O'Neill type colonies out of all of the material.
So, you only get the 1% result if you *assume* that an ETC
must construct habitats for pitiful wet-nanotech like us.
If you assume they are only going to construct supercomputers
in space (that could care less about planetary gravity), then your
material gets spread out over a *much* larger area. Robert
Freitas has redone my rather simple calculations on the
Matrioshka Brain (layered nested shells of orbiting computer
platforms) and they do hold up. They can completely enshroud
the star. So then the questions become what is the power the
star is emitting and how much material do they have? Smaller
stars or more material make the power dissipation per m^2 by the
outermost layers lower and therefore the temperature as well.
>From a theoretical standpoint (if you live long enough and grow
big enough *or* do stellar mining to reduce the power output
of your star *or* build the MBrain around a Gas Giant and use
it to fuel thermonuclear reactors) you can make your radiation
temperature *very* close to that of the background radiation
of the universe (2.7K). This is the most efficient situation from a
thermodynamic viewpoint, and therefore a goal advanced civilizations
would presumably strive for. Anders has discussed this somewhat
in his Jupiter Brain paper about the tradeoffs between small-hot
JBrains and large-cold Jbrains. I believe the natural evolution
is from small-hot JBrains *to* large-cold MBrains. Our telescopes
are so poor, we *cannot* detect most such objects *even* in our own
solar system. I have some articles discussing IR telescopes that
say the dust in our system would hide low power or distant sources
that have a temperature below 40-60K unless we put the telescope
near or beyond the orbit of Mars. Using our *best* telescopes,
I believe the best we can currently do is see newly formed (hot)
brown dwarfs with temperatures around 800-1000C out to a few
hundred light years.
The devil is in the details in these discussions and you have to
question the assumptions of people like Jugaku or Papagiannis
in light of technologies we can see on the horizon today.
I've been intending to write to Dr. Papagiannis to request his
actual calculations, but its not near the top of my todo list yet.
Robert
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