'Look on my works, ye Mighty, and despair!' says Ozymandias's ruined statue in the desert of Shelley's imagination. Shelley's sonnet is often interpreted as a sober warning that human works are fleeting, but when I read it as a young boy it kindled a sense of adventure; it suggested a wonderfully mysterious past beneath my familiar suburban surroundings.
We use the word 'archaeology' to describe this effort, because looking into deep space takes us deep into the past. Interstellar archaeologists are looking for evidence of engineering on scales that dwarf our own. They assume that civilisations eventually build technologies capable of exploiting the energy resources of entire stars. They are building on the early work of the Soviet astronomer Nikolai Kardashev, who, in 1964, set about categorising these futuristic civilisations. His scheme, called the Kardashev Scale, has three types, and so far humanity does not even rate as a Type I — a civilisation that can master the energy resources of its entire planet. A Type II culture can tap all the resources of its local star, and a Type III can harness the energy of an entire galaxy. We do not, of course, know if any civilisation other than our own exists, but Kardashev's scale offers us a way of approaching the problem of detection: it gets us thinking about what kind of traces these advanced civilisations might leave behind.
Imagining the engineering of ancient extraterrestrials is difficult work, foolhardy even. The earliest attempts to do it tended to focus on the largest conceivable structures. The former Fermilab scientist Richard Carrigan, one of interstellar archaeology's pioneers, has long been a vocal proponent of the hunt for Dyson spheres, a technology proposed by Freeman Dyson in 1960. Dyson predicted that energy-seeking civilisations would surround their home stars in a technological shell, or a swarm of spacecraft, in order to capture its energy. A sphere with the radius of Earth's orbit would have an interior surface area 100 million times as large as the surface area of our planet. In 1966, Carl Sagan suggested that such spheres might be detectable, but he cautioned that they would be hard to distinguish from natural objects that gave off a similar infrared signature. Decades later, Carrigan would tell New Scientist that he wanted to try anyway, that he 'wanted to get into the mode of the British Museum, to go and look for artefacts'.
True to his word, Carrigan has conducted a series of searches for Dyson spheres, following earlier work by the Russian astronomers Vyacheslav Ivanovich Slysh and MY Timofeev. Carrigan combed IRAS, the infrared sky survey that dates back to the 1980s, looking for the distinct infrared signatures calculated for this purely theoretical technology. More recently, Berkeley's well known exoplanet hunter Geoff Marcy began studying 1,000 Milky Way star systems for evidence of large structures, looking for visible disturbances in light levels around the parent star as the techno-structures transit between their star and the Earth.
The field's deeper thinkers are starting to wonder if there might be other ways to search. Milan Ćirković, from the Astronomical Observatory of Belgrade, has suggested we go after large artificial objects in transiting orbits. He says we ought to look for something like the huge space colonies once championed by Gerard O'Neill, structures that could be involved in large-scale industrial operations, which might be furnaces for antimatter. If so, their existence could be confirmed by the detection of unusual gamma ray signatures. Alien engineers might even manipulate their own central star. In 1957, Fritz Zwicky suggested that civilisations could fire fuel pellets into their local stars, to move their solar systems to new locations, especially when interstellar dangers loomed. 40 years later, the physicist Leonid Shkadov suggested that huge spherical mirrors could be built to accomplish the same thing, by creating a feedback effect from the star's radiation, that would let its creators control the star's trajectory through the galaxy.
And what of stars that are anomalous such as the 'blue straggler' stars that seem much younger than the stars around them? Astronomers are puzzled by them because globular clusters — ancient cities of stars that sit in a spherical halo around the Milky Way — are where blue stragglers were first identified, and these are thought to contain stars that formed at the same time. Now we're finding blue stragglers in the galactic bulge itself, another unusual place for younger stars since most star formation there has stopped. The giant blue stars we see shining there should have exploded into supernovae billions of years ago.
All of these searches ask us to put ourselves in the minds of beings about whom we know absolutely nothing. The physicist David Deutsch has flagged this as a problem for prediction of all kinds, not just those involving SETI. According to Deutsch, we can distinguish between 'prophecy' and 'prediction', with prophecy being the discussion of things that are not knowable, while prediction deals with conclusions that are based on good explanations of the universe. As prognosticators from Thomas Malthus to the Club of Rome have demonstrated, we may be able to identify problematic trends in the present that can be extended into the future, but we cannot know what knowledge we will acquire in the future to manage those problems. This is why no scientific era has succeeded in imagining its successor. The scientists of the late 19th century discovered this firsthand, when confronted with the emergence of quantum theory and relativity early in the early 20th. Both theories raised questions earlier theorists couldn't have even formulated.
n the context of interstellar archaeology, the problem is that we have no analogues in our experience for what advanced cultures might create. Interstellar archaeologists are tasked with sifting through gigabytes of data, not layers of soil, but the principle is the same. In a recent paper with Robert Bradbury and George Dvorsky, Milan Ćirković offered a paradigm for a new SETI, one that would include not only searches like these but a wide range of 'future studies' that would encompass how a post-biological intelligence might emerge and make itself known — intentionally or unintentionally.
Finding the monuments of civilisations more advanced than our own would challenge us to place ourselves in a totally unfamiliar context, as cosmic newcomers who can suddenly aspire to long lifetimes. If we found a lost city in the sky, it might fire our imaginations. It might give us reason to think we'll outlast existential threats like nuclear weapons and biological terrorism. An interstellar Hisarlik would tell us that some civilisations do survive these dangers and learn to harness immense energies to grow. Rather than despair, we may see their mighty works and rejoice at what we can become.
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