>From: "Robert J. Bradbury" <bradbury@aeiveos.com>
>To: Larry Klaes <ljk4@msn.com>, <arnold@obs-hp.fr>, "H. Paul Shuch" <n6tx@setileague.org>
>CC: public@setileague.org, <bioastero@setileague.org>
>Subject: Detecting geometrical arrangemts of artificial objects
>Date: Thu, 7 Apr 2005 14:36:38 -0700 (PDT)
>
>
>As Paul points out in the New Scientist article [1] pointed
>out by Larry and the preprint by Luc Arnold [2] this
>approach is properly called SETT.
>
>But the fundamental problem with this approach is that it
>generally has the requirement that a civilization maintain
>itself at a slightly post KT-I level and not progress
>to KT-II level. At the KT-II level you *aren't* going to
>"see" the star so occultation or transit methods are unlikely
>to work. I find this requirement of maintaining a slight
>post KT-I level a *highly* dubious assumption and have
>never seen anyone, including some very bright people,
>explain *why* this would be the case. (As Dyson pointed
>out in his 1960 paper -- even we primitive Earthlings
>have only 800 years of growth at current rates before
>we will have to consume all of the power the sun produces.)
>The only reason I can think of is a solar system that is
>*extremely* short on its supply of metals and yet still somehow
>manages to develop intelligent life. This seems extremely unlikely
>as well.
>
>As I pointed out in my OSETI III paper [3]:
> "A small fraction of the material of the solar system provides
> KT-II level civilizations with billions of lunar diameter
> telescopes. Because these can be arrayed as an interferometer,
> their observational capabilities are rather staggering."
>
>Going back to my calculations (on which I based this statement),
>I have a note:
> "100 billion telescopes the diameter of the moon with ~0.3% of the
> Dyson Shell mass"
>
>One can go to larger quantities if one uses more of the available mass
>or engineers thiner layers of Al, Ag, Au, etc. for the mirrors.
>(I use 0.037 cm for my mirror thickness which is already quite
>thin but both Drexler and Dyson think that could be used by
>somewhat more advanced civilizations.) [See note 1] I've never
>calculated how much of a star's light would be obscured by this
>number of telescopes because it depends upon how far they orbit
>from the star. For best observing conditions one would have
>already constructed a star encompassing "shell" (a Dyson shell)
>to capture all of the energy the star produces and block the solar
>wind from disturbing the telescopes.
>
>Unless someone can come up with a reason why civilizations would
>halt slightly post KT-I (or at KT-I) and not go on to KT-II the
>only way I can think of to detect (not communicate with!) advanced
>ET civilizations would be to observe them in the process of making
>the transition from KT-I to KT-II. This can be *very* fast if
>they have advanced molecular nanotechnology. What should be observed
>is a "variable" star that keeps getting dimmer and dimmer until it
>disappears entirely. This is why the TASS survey [4] is so interesting
>because it keeps going over the same areas of sky every night.
>They are looking for variable stars. But if any of the stars
>they observe are going from KT-I to KT-II it should be possible
>to pull them out of the data they collect. The various
>gravitational microlensing surveys may also have data which
>might be useful since they looked at the same sky areas over
>and over again. But to the best of my knowledge that data isn't
>available to the public while the TASS data is.
>
>The thing to keep in mind is that the stated goal of the TASS
>and gravitational microlensing surveys is to uncover stars that
>follow regular patterns of getting dimmer then brighter or brighter
>then dimmer -- they were not looking for stars that were disappearing
>entirely.
>
>Robert
>
>Note 1. Obviously keeping telescopes mirrors that thin of
>that size in subwavelength alignment is tricky. I simply use
>the moon perspective to give people an idea of the scale we
>are talking about. Obviously there may need to be other
>structural material ("frames") needed to keep everything
>aligned to sub-wavelength requirements. But if you watch
>the evolution of telescope designs now (the Hobby-Eberly
>comes to mind) the frames are getting much much lighter.
>If one could envision something like special carbon nanotubes
>designed with specific "curves" due to precise molecular bonds
>lining the back of the mirrors you can kind of see that large
>telescope mirrors that are very thin are not "impossible"
>(though moon-sized may be pushing things a bit).
>
>1. http://www.newscientist.com/article.ns?id=mg18624944.800
>2. http://arxiv.org/abs/astro-ph/0503580
>3. http://www.aeiveos.com/~bradbury/MatrioshkaBrains/OSETI3/4273-32.html
> (This was directly following my paper in the SPIE proceedings about
> Dyson shells that was cited by Dr. Arnold.)
>4. http://www.tass-survey.org/
>
>