Changing approaches to SETI are getting public attention these days, as witness a new article in The Economist that makes reference to the probable cause of the interest, the publication of Paul Davies’ The Eerie Silence (Houghton Mifflin Harcourt, 2010). Sub-titled ‘Renewing Our Search for Alien Intelligence,’ Davies’ book is making accessible to the general public the kind of discussion we’ve often had in these pages, looking at the question of whether our SETI strategies at radio and optical wavelengths aren’t too limited for any chance of success. The Economist is just one sign of the new interest.
After all, technologies like spread spectrum encoding are already masking straightforward radio communications, while conventional broadcasting is giving way to such heavy use of fiber-optics that a planet like ours may go dark at radio wavelengths within a relatively short time as civilizations go, and no more than an infinitesimal flicker in cosmological terms. Thus the interest in alternatives like hunting up Dyson spheres, the search for which Dick Carrigan has so actively championed. The few searches for such spheres have been unsuccessful, but we’ve only begun to look for signs of the technologies of vastly more powerful cultures.
Dyson Spheres and Their Signature
In any case, Dyson spheres would be tough to locate, although it’s interesting to note that the Allen Telescope Array will investigate thirteen of what Carrigan calls the ‘least implausible Dyson sphere candidates’ once it becomes fully operational. Carrigan built up a list of such candidates by going through data from the Infrared Astronomy Satellite (IRAS), which spotted hundreds of thousands of infrared sources. A Dyson sphere should radiate in the infrared, but would have to be distinguished from natural sources like stars with thick dust shells, whose own infrared signal might closely mimic that of an artificial construct.
Carrigan goes into all this on his Web site, where he notes that the Calgary Group has classified all the available low-resolution spectra from IRAS into categories including some that would help identify a Dyson sphere candidate. Thus U stands for a type of object with an ‘unusual spectrum showing a flat continuum,’ while F stands for ‘featureless’ objects that may correspond to O or C stars with small amounts of dust. The C category flags possible late-type cool giant stars with circumstellar shells of carbon dust and emission in the infrared, while H is for HII regions, encompassing various kinds of nebulae.
All these could be considered Dyson sphere look-alikes, and Carrigan notes that more data are becoming available through the Spitzer Space Telescope and surveys like 2MASS. The key is to pick out an actual Dyson sphere signature from all the noise, one Carrigan describes this way:
…an advanced civilization inhabiting a solar system might break up the planets into very small planetoids or pebbles to form a loose shell that would collect all the light coming from the star. The shell of planetoids would vastly increase the available “habitable” area and absorb all of the visible light. The stellar energy would be reradiated at a much lower temperature. If the visible light was totally absorbed by the planetoids a pure Dyson Sphere signature would be an infrared object with luminosity equivalent to the hidden star and a blackbody distribution with a temperature corresponding to the radius of the planetoid swarm. For the case of the Sun with the planetoids at the radius of the Earth the temperature would be approximately 300 ºK.
Signs of a Kardashev Type II Culture
But back to The Economist, which notes that Dyson spheres are but one thing an advanced civilization might spend its time creating. A Kardashev Type II civilization is one capable of exploiting all the energy output of its star, leading to the possibility of interesting forms of stellar engineering. From the article:
Any civilisation that has built a Dyson sphere will have to have been around for a long time, of course—and in the very long run its star will start to change in unpleasant ways, ballooning to form a red giant. Another signature of advanced technology would be an attempt to slow this process down. Red giants are created when a star exhausts its supply of hydrogen at its core, with the result that the inner layer contracts and the outer layers expand, forming a redder and much larger star. If the star’s outer layers could be mixed into the core, that would slow the process of inflation down. And, presumably, a sufficiently advanced civilisation would try to do that if it could.
Let’s assume so, and fortunately for distant observers, such an engineering feat would show a particular signature, even if one that, like Dyson spheres, can be confused with natural phenomena:
Such a star would look odd, though. It would be bluer than it should be and would be of a type known to astronomers as a “blue straggler”. Although, again, there are perfectly natural reasons these might form. The universe, though, is an ancient place, so many civilisations could be very old indeed. Perhaps, then, it will be a sign like this—of a technological civilisation millions of years old—that is seen, rather than some upstart that has not even got its radio waves under control.
Leaving Assumptions Behind
New technologies will help us pursue other observational lines of research. Using space-based telescopes and, recently, instruments on the ground, we’re learning how to characterize the atmospheres of extrasolar planets. It’s a process that’s sure to pick up as we refine our techniques, but in the not so distant future we may be able to spot telltale signs of industrial activity on a distant world as well as probing its atmosphere for the chemical signs of life. Carrigan suggests chlorofluorocarbons (CFCs) as one possibility, a type of molecule that would have to correspond to a technology because no natural process produces it.
All of these approaches take us in a new direction, one that, as Dick Carrigan notes, makes no assumptions about the motivation of the originating civilization. Indeed, he likens the hunt to a form of archaeology, one closer to the search for exoplanets than the attempt to snag a radio signal. For more, see Carrigan, “Starry Messages: Searching for Signatures of Interstellar Archaeology,” FERMILAB-PUB-09-607-AD (abstract), which I’ll be discussing in more detail soon, along with cross-references to Davies’ The Eerie Silence. For as Davies shows, there are many more ways to conceive how the activities of an extraterrestrial civilization could modify the heavens.