How will humans discover the existence of extraterrestrial civilizations?
Unless aliens decide to visit Earth, the most likely answer is by scanning the skies for "technosignatures," which are observational evidence of technological or industrial activity in the Universe.
In a recent paper published in the journal Acta Astronautica, a team of NASA-funded researchers outlined some of the most promising ways scientists and space agencies could search for technosignatures. The paper included a somewhat surprising proposition: Humanity's "first contact" with aliens is likely to be with a much more advanced civilization.
In other words, there could be many alien civilizations throughout the Universe, or even in our galaxy, but if they're similar to us in terms of technological advancement, we probably can't spot them yet. The same goes for those human-like civilizations spotting us.
That's because the "cosmic footprints" of our civilization and theirs would be relatively small, compared to highly advanced alien civilizations. The researchers call this concept "contact inequality."
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