It was a match made in the stars. In 1964, about the time the California Institute of Technology announced the discovery of a weird, super-intense radio source in the sky, Russian astrophysicist Nikolai Kardashev published his classification of extraterrestrial civilizations. He rated advanced civilizations by their available energy (possibly to be used for interstellar communication): A Type I civilization could harness all the energy available on its home planet; Type II, all the energy available from its star (trapping its output with a Dyson sphere, for instance); Type III, all the energy available in its entire galaxy.
Caltech's bright radio source CTA-102 seemed to fit either a Kardashev Type II or III civilization, and when the Soviet news agency TASS announced that CTA-102 must be a beacon from a distant civilization, it caused a worldwide sensation. That is until a few weeks later, when astronomers at the Mount Palomar Observatory showed the putative "beacon" to be a distant quasar, a natural quasi-stellar object powered by gas falling into a supermassive black hole.
All this drama happened during a particularly heady time for SETI, the search for extraterrestrial intelligence. SETI was born in 1960, when radio astronomer Frank Drake conducted the first (unsuccessful) search for anomalous radio signals from two nearby stars. He later developed what became known as the Drake Equation: By multiplying a series of parameters — the fraction of stars that have planets, the fraction of those planets that could support life, etc. — one could estimate the number of communicative extraterrestrial civilizations in the Milky Way. (Drake was trying to stimulate discussion about SETI, never claiming to make a serious argument.)
Despite the Russian gaffe in conflating a natural phenomenon with an artificial beacon, SETI has since evolved into a major scientific effort, not to mention its impact on popular culture — think of the movie Contact, based on Carl Sagan's book of the same name. Sagan, an enthusiastic proponent of SETI, teamed up with Drake in 1974 when they designed and transmitted a "We are here!" signal from the newly upgraded Arecibo radio telescope ("The Great Silence," June 3, 2010). These days, SETI programs mostly search for radio messages and anomalous spectra from putative extraterrestrials, hoping that "they" are using sufficient energy to be visible to us.
Such searches assume that the more a civilization progresses, the more energy it will use. But unchecked growth, by definition, is unsustainable. At some point, any civilization will collapse when its energy demands outstrips the available resources (unless, happy thought, it self-annihilates first). So Kardashev's assumption that "progress" can be measured by energy consumption doesn't hold water over the long haul. Drake was aware of this, of course. The final parameter of his equation, L, stands for "lifetime," how long a civilization can transmit signals into space, wittingly or unwittingly, before it founders.
The other option is for a civilization to use progressively less energy. One thing we've learned from technology is that it gets smaller, both physically and in its energy needs (think of your smartphone or LED flashlight). So instead of looking for "big" — strong radio signals from potential ETs — maybe SETI should focus on "small," assuming that any long-lived civilizations will have evolved beyond "bigger is better" and will instead be super-efficient, and therefore hard to detect.
Paul Gilster, author of the blog Centauri Dreams, writes: "... the more advanced a technological civilization becomes, the less likely we will be to distinguish it from natural phenomena. We may confront a cosmos rife with advanced civilizations whose work is so harmonized with their surroundings as to be invisible."
Barry Evans (he/him, email@example.com) is honored that Frank Drake wrote the forward to his first book, The Wrong-Way Comet.