It’s a big galaxy out there. Even the most skeptical scientist has to accept that if a civilisation like our own exists, then there’s a good chance we’re not the only one to have ever done so. When most people think about SETI (the search for extraterrestrial intellgence), they imagine someone like Ellie Arroway searching the skies for radio transmissions. But what about looking in other ways? Perhaps a highly advanced alien civilisation might build structures large enough for us to see.
Vast structures, constructed on astronomical scales by advanced civilisations, is what the field of astroengineering is all about. This, admittedly, sounds audacious – and for the human race right now, it is. For us, astroengineering is still very much the realm of thought experiments, theoretical calculations, and science fiction. So it may be surprising to know that certain astronomers have made some quite serious attempts to look for astroengineered artifacts around other stars. With telescopes becoming ever more sensitive, and images being taken of exoplanets, the idea is starting to captivate imaginations once more.
In 1960, Freeman Dyson published a paper entitled Search for Artificial Stellar Sources of Infrared Radiation. His suggestion was that any megastructure constructed around a star should show itself by emitting more infrared light than it should. The solution was, simply, to look for any sources of infrared which appeared artificial.
Dyson put forward the ideas that any potentially advanced civilisation may need a tremendous amount of power to sustain itself. A method he proposed was to build a vast array of satellites which would enclose an entire star to harvest its energy – a concept which later came to be known as a Dyson sphere. While the concept wasn’t taken too seriously by Dyson himself, it was a powerful enough notion that it garnered a lot of attention.
Unfortunately, it isn’t as simple as looking for infrared light. Many stars, our own included, are surrounded by a disk of dust, and that dust emits plenty of infrared. To find a Dyson sphere, you need to look for a specific signature of infrared light, emitted at just the right set of wavelengths.