Yay! A page that doesn't have tons of dialogue! Enjoy it while it lasts (and of course, it's followed by a massive science blurb). Only eight pages left in this chapter. The end is nigh!
So nigh.
The Fermi Paradox is the perceived contradiction between the fact that galaxy is very old and packed with billions of stars just like our own sun, and stellar systems just like our own, and yet despite that there's no evidence of any other civilizations out there, or that they've ever existed. Given the age of the Milky Way, even traveling at a small fraction of light speed, there's been more than enough time for alien probes to traverse the entire galaxy a thousand times over, so surely they should have visited us at some point, or left traces of themselves all over the place. And yet, there are no traces to be found, just an eerie silence.
One aspect of this “eerie silence” is the apparent lack of radio signals. We've been inadvertently sending radio and TV broadcasts off into space for decades, so in theory at least any nearby aliens should know we're here. And likewise, the universe should be awash in such transmissions from civilizations all over the cosmos. So shouldn't we be just drowning in extraterrestrial radio chatter?
Well, not necessarily. Radio transmissions weaken and disperse with distance. Most of our communications transmissions would only be detectable about a little less than 1 light year away with our own current technology, and as our communications technology gets better and more efficient, our signals keep getting weaker. If there are aliens hanging out in our own Oort Cloud, we might be able to eavesdrop on their television signals, but we wouldn't be able to pick up the alien equivalent of 'I Love Lucy' being broadcast from even our nearest star.
Not really, no. On their website, SETI acknowledges that we couldn't detect our own transmissions, even from the nearest star; the signals are just too weak. Proxima Centauri's SETI program would miss us entirely. There are occasional exceptions, like high powered radars and other scientific experiments that can travel very far off into space, but it would take a ton of luck to intercept such a transmission, and even if you did it would be unlikely to be repeated. SETI is not hoping to serendipitously stumble upon a random signal propagating through empty space. Rather, they're hoping to pick up a signal that's being intentionally beamed to us by someone looking to make contact, at a strength and frequency we can detect. They're operating under a number of assumptions about alien behavior – assumptions which, for what it's worth, are not true about us. With only the occasional exception, we're not broadcasting to anybody, and the emissions from our communications infrastructure is only getting weaker.
A more advanced civilization might not use radio, especially if they're communicating across interstellar distances. They'd probably use some kind of directly focused laser signal, and unless we just happen to be in the path of one of those beams (which we wouldn't, ever) then we'd never know about it.
It's worth pointing out that there are lots of other ways to detect alien civilizations if they're out there. But the idea that we should easily have picked up all kinds of obvious background noise from every direction by now if there were anyone out there… is a misguided notion. If the extraterrestrials are anything like us, it's entirely likely that we wouldn't have picked up anything from them by now, and if they're much more advanced than we are, then there may not be any signals that we can pick up.
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