A little over 80 years ago, humanity first began broadcasting radio and television signals with enough power that they should leave Earth’s atmosphere and progress deep into interstellar space. If someone living in a distant star system were keeping a vigilant eye out for these signals, they would not only be able to pick them up, but immediately identify them as created by an intelligent species. In 1960, Frank Drake first proposed searching for such signals from other star systems by using large radio dishes, giving rise to SETI: the Search for Extra-Terrestrial Intelligence. Yet over the past half-century, we’ve developed far more efficient ways to communicate across the globe than with broadcast radio and TV signals. Does searching for aliens in the electromagnetic spectrum even make sense anymore ?
[...] After all, if someone from a culture that was versed only in smoke signals and drum beats found themselves deep inside the heart of a forest, they might conclude that there was no intelligent life around. Yet if you gave them a cellphone, there’s a good chance they could get reception from right where they stood! Our conclusions may be as biased as the methods we apply.
[...] But if we weren’t looking for electromagnetic signals, what would we look at? Indeed, everything in the known Universe is limited by the speed of light, and any signal created on another world would necessitate that we be able to observe it. These signals — in terms of what could reach us — fall into four categories:
Electromagnetic signals, which include any form of light of any wavelength that would indicate the presence of intelligent life.
Gravitational wave signals, which, if there is one unique to intelligent life, would be detectable with sensitive enough equipment anywhere in the Universe.
Neutrino signals, which — although incredibly low in flux at great distances — would have an unmistakeable signature dependent on the reaction that created them.
And finally, actual, macroscopic space probes, either robotic, computerized, free-floating or inhabited, which made its way towards Earth.
How remarkable that our science-fiction imaginations focus almost exclusively on the fourth possibility, which is by far the least likely !
[Also Covered By]: Three Alternate Ways Scientists Should Hunt For Aliens [inverse.com]