A feasibility study in the Astrophysical Journal explains how a powerful laser on an Earth mountaintop, focused through a huge telescope, could shine a light of infrared radiation that would be detectable up to 20,000 light years away. While some scientists are concerned about alerting extraterrestrials to our presence, I agree with something said to me by Ann Druyan -- co-writer of Cosmos and many other works with her husband Carl Sagan -- when we were working on the Voyager Golden Record: 40th Anniversary Edition: It's rather cynical, she explained, to think that a extraterrestrial civilization advanced enough to notice us and make the long trip to Earth would be so emotionally stunted as to purposely destroy us upon arrival. From MIT News:
The findings suggest that if a high-powered 1- to 2-megawatt laser were focused through a massive 30- to 45-meter telescope and aimed out into space, the combination would produce a beam of infrared radiation strong enough to stand out from the sun’s energy.
Such a signal could be detectable by alien astronomers performing a cursory survey of our section of the Milky Way — especially if those astronomers live in nearby systems, such as around Proxima Centauri, the nearest star to Earth, or TRAPPIST-1, a star about 40 light-years away that hosts seven exoplanets, three of which are potentially habitable. If the signal is spotted from either of these nearby systems, the study finds, the same megawatt laser could be used to send a brief message in the form of pulses similar to Morse code.
“If we were to successfully close a handshake and start to communicate, we could flash a message, at a data rate of about a few hundred bits per second, which would get there in just a few years,” says (James) Clark, a graduate student in MIT’s Department of Aeronautics and Astronautics and author of the study...
He hopes the study will encourage the development of infrared imaging techniques, not only to spot any laser beacons that might be produced by alien astronomers, but also to identify gases in a distant planet’s atmosphere that might be indications of life.
“With current survey methods and instruments, it is unlikely that we would actually be lucky enough to image a beacon flash, assuming that extraterrestrials exist and are making them,” Clark says. “However, as the infrared spectra of exoplanets are studied for traces of gases that indicate the viability of life, and as full-sky surveys attain greater coverage and become more rapid, we can be more certain that, if E.T. is phoning, we will detect it.”
"Optical Detection of Lasers with Near-term Technology at Interstellar Distances" by James R. Clark and Kerri Cahoy (The Astrophysical Journal)