A gradual decline into disorder
Physicist Enrico Fermi famously asked the question “Where are they?” to express his surprise over the absence of any signs for the existence of other intelligent civilizations in the Milky Way Galaxy. […]
Observations have shown that the Milky Way contains no fewer than a billion Earth-size planets orbiting Sun-like (or smaller) stars in the “Goldilocks” region that allows for liquid water to exist on the planet’s surface (the so-called habitable zone). Furthermore, the search for extraterrestrial intelligent life has recently received a significant boost in the form of “Breakthrough Listen”—a $100-million decade-long project aimed at searching for non-natural transmissions in the electromagnetic bandwidth from 100 megahertz to 50 gigahertz.
Simple life appeared on Earth almost as soon as the planet cooled sufficiently to support water-based organisms. To be detectable from a distance, however, life has to evolve to the point where it dominates the planetary surface chemistry and has significantly changed the atmosphere, creating chemical “biosignatures” that can in principle be detected remotely. For instance, Earth itself would probably not have been detected as a life-bearing planet during the first two billion years of its existence. […]
[A]n excellent first step in the quest for signatures of simple extrasolar life in the relatively near future would be to: search for oxygen, but try to back it up with other biosignatures. […]
One would ideally like to go beyond biosignatures and seek the clearest sign of an alien technological civilization. This could be the unambiguous detection of an intelligent, non-natural signal, most notably via radio transmission, the aim of the SETI (Search for Extraterrestrial Intelligence) program. Yet there is a distinct possibility that radio communication might be considered archaic to an advanced life form. Its use might have been short-lived in most civilizations, and hence rare over large volumes of the universe. What might then be a generic signature? Energy consumption is a hallmark of an advanced civilization that appears to be virtually impossible to conceal. […]
More pessimistically, biologically-based intelligence may constitute only a very brief phase in the evolution of complexity, followed by what futurists have dubbed the “singularity”—the dominance of artificial, inorganic intelligence. If this is indeed the case, most advanced species are likely not to be found on a planet’s surface (where gravity is helpful for the emergence of biological life, but is otherwise a liability). But they probably must still be near a fuel supply, namely a star, because of energy considerations. Even if such intelligent machines were to transmit a signal, it would probably be unrecognizable and non-decodable to our relatively primitive organic brains.
