Did you hear the one about the two astrophysicists on the bus? It was January 2010, during a workshop in Abu Dhabi, when they took off for a tour of Dubai—a city so bright it could be seen from outer space, the guide boasted. Later that day the astrophysicists, Avi Loeb of Harvard and Edwin Turner of Princeton, tried figuring out how far from Earth they’d have to go before the lights of Dubai became invisible. Then they turned their perspective around and wondered what it would take to seek alien civilizations by looking for distant analogues of Dubai, or Las Vegas, or Times Square.
Up until now, the search for extraterrestrial intelligence, or SETI, has primarily involved listening for radio signals deliberately or inadvertently sent by alien cultures into space. But away from the academic grind, Loeb and Turner saw city-spotting as an alternative way to hunt for ET. One virtue of the scheme was that it made few assumptions about the aliens. They didn’t have to be beaming messages at us; they merely had to share our fondness for artificial light, something common to every modern society on Earth. But the astronomers’ initial back-of-the-envelope calculations were not encouraging. Detecting light pollution from planets orbiting other stars is far beyond the capabilities of today’s instruments, they realized.
Loeb and Turner shelved the notion, but then they talked to Freeman Dyson, the physicist renowned for his seminal contributions to quantum field theory and equally famous for his provocative speculations. In the 1960s he conceived of Dyson spheres—structures manufactured by advanced civilizations that would completely surround stars, capturing most of their energy. More recently he suggested that parabola-shaped plants (which he called “sunflowers”) could survive on the solar system’s cold fringes by concentrating the weak sunlight available there. When Loeb mentioned the concept of searching for the lights from extraterrestrial cities, Dyson urged him to write it up.
