In a recent commentary, published here on the pages of Scientific American, Jason Wright, professor of astronomy at The Pennsylvania State University, outlined a clear and persuasive argument for NASA to resume funding SETI - the search for extraterrestrial intelligence.
One of the most insightful points Jason makes is that there is no a priori reason to suppose that technosignatures (from intelligent life elsewhere in the universe) should be any less detectable than biosignatures (from any kind of life elsewhere in the universe). Indeed, I think indications of this fact go back quite a way. Take for example the famous, and famously elegant, experiment performed in December 1990 when the Galileo spacecraft made a gravity assist flyby of the Earth.
Carl Sagan and his colleagues had planned ahead for exploiting this maneuver. Galileo's real purpose for the flyby was to gain a velocity tweak as part of an elaborate orbital ballet to get out to Jupiter. But at a closest distance to Earth of some 960 kilometers it was a golden opportunity to do a baseline astrobiology experiment. As the spacecraft sped by the planet it switched on its instruments - telescopes, spectrometers, radiometers - and surveyed this strange blue dot for signs of life.
The striking thing about the results, published in the journal Nature in 1993, is that while there were strong hints of biology seen in the planet's atmospheric gases and surface colors, the real kicker came from our human transmissions from the surface. Or, as the researchers put it: "…the presence of narrow-band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence."
To me it's pretty remarkable that during an extreme close-up of a well-inhabited planet, using a space-probe bristling with scientific instruments, the most incontrovertible sign of life was a pure technosignature.
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