Dyson Spheres and the Search for Extraterrestrial Intelligence
Imagine a future Earth where the population has grown so huge that there simply isn't enough energy to go around. Not even enough to boil a kettle. Writing in 'Science' in 1960, the physicist Freeman Dyson looked at this scenario, theorising that if the exponential growth in energy demand of the Earth were to hold true for all advanced civilisations, then eventually those civilisations would be forced to collect all of a star's energy for their use, also noting that "one by-product of their energy metabolism is likely to be the large-scale conversion of starlight into far-infrared radiation".
Dyson suggested that an array of objects a Dyson Swarm would travel in independent orbits around a star, collecting the star's energy, and providing living space for members of the booming population. The collectors would absorb, and then re-radiate, energy from the star the re-radiated energy being in the infra-red. If the proportion of the star's energy output altered by this absorption and re-radiation was significant, it could be detected at interstellar distances. Dyson therefore suggested that searching the sky for such emissions could provide evidence for advanced extraterrestrial civilisations.
Science fiction writers were quick to explore more exciting-sounding variants on Dyson's original idea. In Larry Niven's 1970 novel, 'Ringworld', four adventurers set off into space (in the year 2855) to explore a mysterious, artificial, ring-shaped structure that surrounds a star. Another idea is explored in Star Trek: The Next Generation (episode: 'Relics'), when the Enterprise is shaken by an unexpected gravity well, which ship sensors detect as originating from a nearby spherical structure, 200 million kilometres in size, and, as Picard explains to a confused Data, built by a civilization to surround their planet's entire orbit and star.
The solid sphere has been used most often in sci-fi, yet this is the most improbable of Dyson's ideas. The basic premise is that, using artificial gravity, beings are able to live on the inside of the sphere. Unfortunately, the proximity to the star would mean the inner surface of the sphere would be too hot to be habitable, besides the difficulty of setting it in a stable orbit. However, Dyson's original idea - named a Dyson Swarm to distinguish it from the Dyson Sphere of sci-fi fame - is a lot less improbable.
You may think that Dyson came from another planet himself, and that his ideas are too crazy to be seriously pursued by modern science. Yet, the California based organization, SETI (the Search for Extraterrestrial Intelligence), continuously monitors the skies for anything remotely weird, including "infrared heavy" signals indicative of Dyson Swarms. Their searches are augmented by data from the Infrared Astronomical Satellite, run by Fermilab, of the US Department of Energy. Alas, no Dyson Swarms have yet been detected, probably because it is unlikely that there are any advanced civilisations out there to have built one!
In the same year that Dyson proposed his Swarms, Frank Drake, an American astronomer and astrophysicist, and founder of SETI, devised his famous Drake Equation. This equation proposes that the number of advanced civilisations in our galaxy, that are capable of communicating with us, can be found by multiplying the rate of star formation in our galaxy; the number of these stars that have planets; the number of planets that could support life; the fraction of these that actually develop life; the fraction of these that develop intelligent life; the fraction of these that produce detectable signs of their existence; and the lifetime of such a civilisation.
When Drake wrote this equation, his estimate for the number of detectable civilisations in our galaxy was 10. But using current estimates, the chance of an advanced civilisation existing in our galaxy and building a Dyson Swarm is infinitesimally small. Undeterred, the searching continues, for as Dyson himself said, "one of the strongest reasons for conducting a search for such sources is that many new types of natural astronomical objects might be discovered".