Almost 50 years ago, Philip Morrison and Giuseppe Cocconi famously suggested that the most likely electromagnetic wavelength at which an advanced alien civilization would try to signal its existence is the ubiquitous 21-cm microwave emission line of neutral atomic hydrogen. And until recently, the search for extraterrestrial intelligence (SETI) has been conducted almost exclusively by radio astronomers sifting through 21-cm radiation from space in search of anything that might be a message.
Half a century ago, the 21-cm line was the only known interstellar microwave emission line, and lasers had not yet been developed. Since then, radio astronomers have found many shorter-wavelength microwave lines that would serve better because they suffer less interstellar dispersion. Furthermore, today’s laser technology makes it possible for petawatt (1015 W) lasers to emit highly collimated nanosecond optical pulses that briefly outshine the Sun by a factor of 10 000. Because no known astrophysical...
