In the July, 2000 issue of Scientific American, the article "Where are They?", by Ian Crawford [Cra00] contained a sidebar by Andrew J. LePage, "Where They Could Hide" [LeP00]. These articles pointed to a chart, "Results of SETI Programs" that indicating that searches for KardashevType II Civilizations[FN1], radiating > 1025 W had been "Thoroughly Searched" for within the local group of galaxies (a distance of 5×107 light years). Unfortunately, as I shall show, this analysis is seriously flawed.
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A careful investigation of the criteria used for these studies will reveal they can be used to justify only very limited(!) conclusions regarding Type II Civilizations. The methods used Jugaku and Nishimura are to identify stars that have a very slight slight excess of IR radiation relative to visible radiation. This strategy is based on a conclusion by Michael D. Papagiannis, one of the founders of IAU Commission 51, in [Pap85] which stated:
"From the above (calculations) it follows that the construction of a spherical shell around a star from the material present in its planetary system is an impossible task. 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 analysis is based on a highly anthropocentric view, that Dyson Shells must consist of O'Neill style habitats [ONe74] designed for liquid water based life forms. This perspective may have been suggested by Dyson himself, who stated [Dys60]:
"One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star."Whether Dyson's statement is contradicted by Papagiannis depends on ones interpretation of the term "completely surrounds". Dyson continues:
"The most likely habitat for such beings would be a dark object, having a size comparable with the Earth's orbit, and a surface temperature of 200 deg. to 300 deg. K. Such a dark object would be radiating as copiously as the star which is hidden inside it, but the radiation would be in the far infrared, around 10 microns wavelength."The problem is this assumes that the temperatures of a "biosphere" must be those required for "liquid water". Yet, 11 years later, during the joint Soviet-American conference on CETI, the flaws in Dyson's perspective were pointed out by the AI pioneer Marvin Minsky [Min73]:
"Since radiation at any temperature above 3oK is wasteful and a squandering of natural resources, the higher the civilization, the lower the infrared radiation. We should look for extended sources of 4oK radiation. There should be very few natural such sources."Papagiannis performed his calculations and reached his conclusions 12 years after Minsky's comments were published. We may reasonably assume that Papagiannis must have read them but that he must have been enamoured with the space colonization ideas favoured by Gerard O'Neill [ONe74]. Or he might have limited his vision to that suggested by Dyson's response to criticisms about his idea [Dys60c]:
"A solid shell or ring surrounding a star is mechanically impossible. The form of "biosphere" which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star. The size and shape of the individual objects would be chosen to suit the inhabitants. I did not indulge in speculations concerning the constructional details of the biosphere, since the expected emission of infrared radiation is independent of such details."[FN2]Dyson may be forgiven because in 1960 when he wrote the article, computers were a very new, and ideas such as solar power satellites and such fields as artificial intelligence, artificial life and genome engineering would have been difficult to predict. But Papagiannis who had the benefit of the extensive CETI discussions in 1971 should have considered the possibility that a civilization might envelop its star in lightweight solar power collectors if only to power a long-distance CETI transmitter! Thus his emphasis on the burdensome mass requirements of primitive space habitats is unclear. He further failed to consider the development of robots [Mor88] or ubiquitous computing [UCI00] that would enable Dyson Shell architectures quite different from his vision.
Perhaps unknown to Papagiannis, was a very detailed earlier work by Dr. K. G. Suffern of the Department of Applied Mathematics at the University of Sydney analyzing various characteristics for Dyson shells [Suf77]. While the general tone of the anaylsis is to draw conclusions similar to those of Papagiannis, Suffern concluded:
"Of course this is not an objection in principle to optically thick Dyson spheres because in principle the radiation from any black body with a temperature T > 2.7 K is thermodynamically useful. It just points out a practical difficulty that thermodynamics imposes upon optically thick biospheres."Suffern's calculations showed that O'Neill style colonies would not effectively hide a star, but his perspective is somewhat more open minded in that he at least allows that there may be architectures that could hide the star.
It is safe to say that Dyson's use of the term "dark object" and the subsequent popularization of "hallow balls" by Science News Letters [Dys60b], rather than his intended "shells" [Dys60c], continues to generate confusion, because even in 1999, talented engineers such as Robert Zubrin make such statements as "Dyson Spheres are impractical" and "the implausibility of the Dyson Sphere is so extreme that it has even been criticized by science fiction writers." [Zub99, p. 241]. Dyson Shells are implausible or impractical only if you envision them as true spheres or primitive human habitats. And some science fiction writers, by stretching their imagination to include engineering methods such as force transfer using high speed circulating fluid streams, have described plausible Dyson spheres [Poh75]!
The Papagiannis and Suffern calculations are based on the assumption that we are limited to the available "construction material" present in our solar system. Papagiannis uses a figure of 10 Earth masses, while Suffern uses a figure of 30 Earth masses [from Bra76]. The author estimates that the solar system contains minimum of 3-10 times the amount of construction material considered by Suffern and Papagiannis (~100 Earth masses; 5.9×1026 kg). Why is this?
During the last 25 years, it has become clear that the view of traditional construction materials (iron, aluminium, glass) is somewhat limited, and that structures will ultimately be constructed out of materials with more robust physical properties. These include diamond and saphire as well as ceramics including carbides, oxides and nitrides. So substances such as methane, ammonia, carbon dioxide and carbon monoxide, which are abundant in the bodies with orbits beyond Saturn are more useful for construction purposes than was previously thought. Water ice is also present in significant quantities in these bodies and may be considered a structural material if it is kept significantly below its freezing point. In our solar system, the excess of hydrogen and oxygen over carbon (and other metals) dictates that the most efficient materials usage must use water in lieu of other materials as much as possible. In addition, the most abundant elements, hydrogen and helium may be used as radiation shields, as highly pressurized gases or possibly in liquid or solid form, leaving other materials available for more important uses.
The classic O'Neill habitat contains 4% aluminium, 2% glass, 10% water, and 82% soil, rock and construction materials [ONe74]. How can we substitute advanced materials into these habitats? The greatest mass utilization would result from constructing all structural elements out of carbon (as diamond or buckytubes), saphire (Al2O3), silicon carbide (SiC) and hematite (Fe2O3). Metals would only be used in situtations requiring great ductility or electrical conductivity. Diamond has an elasticity 5× that of high carbon steel and 13× that of aluminum alloys. Replacing the aluminum and steel in the original designs with diamond and saphire (which are also more abundant) reduces the structural material mass requirements by 10 times or more.
For example, advanced colonies could contain diamond walls of sufficient strength to hold an atmosphere, surrouded by silica aerogel insulation to keep heat in and cold out, surrounded by ice containing bubbles filled with high pressure hydrogen gas as a radiation and meteor shield, coated with ultrathin high efficiency solar cells that efficiently harvest incoming visible and infrared photons and covert them into electricty.
The traditional agriculture methods envisioned by O'Neill are not those that would be used by an advanced civilization. The soil requirements are presumably those required for growing food. Yet we know that traditional foods may be grown quite effectively hydroponically [Hyd00]. That would eliminate the soil requirement completely and replace it with a water requirement. Water as previously noted is one of the most abundant materials in the solar system. If the population adapts to non-traditional foods, or food processing can manipulate basic ingredients into traditional forms, one may envision very thin orbiting pancakes (agrosats) with glass surfaces facing the sun and ceramic radiators facing away from the sun filled with water containing algae like organisms that are bioengineered to manufacture and store a completely balanced set of nutrients. These would be very thin structures, perhaps only a few cm thick. These agrosats could periodically dock with habitat colonies where the nutrients would be harvested and the agrosats refertilized with carbon dioxide, ammonia and trace elements.
Traditional agriculture is very inefficient from an energy standpoint.
The biochemical predictions of photosynthetic energy harvesting efficiency
are quite good, theoretically 35% [Mat90,
pg 664]. But if U.S. agricultural land is ~9.5 million acres [TWA99]
Advanced societies would attempt to make the most efficient use of the
available energy as possible. Intially, this could be based on multi-layer
solar cells (~30-40% efficiencies). Ultimately it may use solar concentrators
with high temperature heat engines that radiate at the background
radiation temperature. The temperature differential
Thus if we divide the multi-purpose O'Neill style habitats into powersats and agrosats that harvest most of the available sunlight and habitats that house populations at New York or Tokyo density levels then the resources of a solar system may be utilized much more efficiently.
So, only primitive civilizations such as ours (circa 1974) would limit themselves to collecting less than 1% of the solar output of their star. Using advanced materials and technologies, advanced Type-II civilizations would harvest all of the power produced by their stars as Dyson has suggested. As they evolve their civilization, making it increasingly efficient, they would radiate heat at ever lower temperatures, potentially approaching the background temperature as Minsky and Suffern have pointed out.
As NASA scientists are beginning to engineer artificial life [Poh99], and life supporting chemistries that do not require liquid water have been postulated [Fre79, Lew98], we can envision either engineering or discovering life forms that can exist outside temperature ranges predicted by Dyson. Ultimately if nanotechnology is feasible, we may envision civilizations that operate over temperatures ranging from 3oK to 2900oK or higher.
The time spans for civilizations of intelligent beings to evolve from very primitive states (e.g. Homo erectus) to the limits imposed by classical laws of physics are very short (~105-106 years) compared with the galactic time scales (~1010 years). Work by the author [Bra99] has shown, that a Type II civilization could rapidly form an highly integrated meta-mind with a computational capacity ~1034 times that of a human brain. This meta-mind would occupy the computers of star-enveloping solar power satellites that do effectively hide the star. So exploration for and conclusions regarding extraterrestrial intelligence, Dyson Shells, and the Fermi Paradox are unlikely to be valid if one assumes an anthropocentric starting point as most individuals unconsciously do.
Conclusions should only be made if one assumes capabilities and capacities at the limits of known (experimentally validated) laws of physics. Unfortunately, astronomers and SETI researchers continue to look for signs of life assuming human perspectives (i.e. visible stars) rather than perspectives dictated by the limits of the laws of physics. Only whole sky surveys, with long exposure times in the mid-far IR ranges are likely to detect low temperature Dyson Shells. Unfortunately, even expensive telescopes, like SIRTF, that will not be available until 2002, the detectors for these wavelengths, typically, Si:As, Ge:Ga and stressed Ge:Ga are still very primitive[FN3]. It is doubtful that even very advanced telescopes, still in the planning stages, would have the capabilities required to provide definitive evidence for or against these objects.
As the LePage & Crawford articles fail to examine these points in detail they should be viewed as editorial commentary on SETI rather than definitive statements.
They went so far as to claim "ZIP, ZILCH, NADA has come out of any aliens with whom we share the galaxy. Searches for extraterrestrial intelligence have at least partially scanned for Earth-level radio transmitters out to 4,000 light-years away from our planet (yellow circle) and for so-called type I advanced civilizations out to 40,000 light-years (red circle). The lack of signals is starting to worry many scientists". It is not clear what observations and assumptions are used to derive these conclusions, but they are clearly incorrect. Current SETI search strategies almost all implicitly assume that civilizations remain around our level of development for a very long time (hundreds of years)[FN4]. If instead technological civilizations make a rapid transition from pre-Type I to Type II as appears likely to be the case [Vin93], then these efforts are highly misguided. So the only "worry" required is why scientists are so shortsighted.
LePage has devised a system [LeP96] for rating stars as potential SETI targets which ignores the basic problem that almost all advanced technological civilizations should not have a "visible stars"! Brown dwarfs [Bas00], are estimated to be twice as abundant as stars [Kir00]. Because they are smaller and cooler than stars, they are more easily disassembled. If we had the technology to conduct extensive surveys of brown dwarfs[FN5] we might find that many of them are being disassembled to supply fuel and construction materials for meta-minds. The unresolved problems of the missing baryonic dark matter and the gravitational microlensing observations, suggesting 200 billion "masses" orbiting our galaxy [Alc00]. Thus we may conclude, there is still plenty of room for progress in SETI.