Thread: "The Great Silence"

Originally Posted by Syme

Well, it's a common supposition that the evidence by which we could notice extraterrestial civilizations (or they could notice us) would be the radio emissions created by normal wireless communications activity on the planet in question, but that's not really accurate. It's not normal megawatt-range radio and TV transmissions that will be probably provide the evidence, but much more powerful and probably more tightly focused transmissions. As mentioned above, this is because of the inverse-square law to which radio transmissions (and other forms of EM radiation) are subject. For instance, there's a SETI FAQ which says that even using an instrument as sensitive as Arecibo, normal TV/radio transmissions from Earth would only be detectable out to a range of 0.3 light years--less than a tenth of the way to Alpha Centauri, our nearest neighboring star. This means that at ten times that distance, 3 light years (still not all the way to Alpha Centauri), the signals would be a hundred times weaker still, and thus would require an instrument 100 times more sensitive than Arecibo to detect. At 30 light years, they would require an instrument ten thousand times more sensitive than Arecibo to detect. At 300 light years, the instrument would have to be a million times more sensitive than Arecibo. And so forth.

Put simply, TV and radio signals are probably much too weak to be detected in other star systems even before satellite telecommunications starts reducing the strength of the transmissions. Unless aliens are building really ridiculously huge radio telescopes, our TV and radio transmissions can't be detected at a range of even five light years, much less fifty. Any signal which can be detected in another star system would have to be much more powerful and directional. Terawatt-range transmission powers (1 million times stronger than a megawatt-range TV/radio signal) are often mentioned in discussions of interstellar radio communications.

Alien use of satellite technology is therefore not a compelling explanation for the Great Silence: Because even if they didn't use satellites, we would nevertheless be unable to detect the transmissions that their civilizations use for ordinary communications on and around their planet(s). And they wouldn't be able to detect ours unless they are building radio telescopes with 500-mile-wide dishes (which isn't strictly not out of the question, we must admit).

Well Syme, you certainly seem to have thought this through. Your explanation makes a lot of sense, but I have a question, or four. How sensitive is the equipment we're using to detect radio transmissions? Can we detect the waves if they enter our solar system for sure? What about our galaxy? If not, then aliens would have to deliberately target us, correct? Depending on the sensitivity of our instruments, it could be that aliens have no interest in contacting us. That would certainly explain everything. Sorry if you have already discussed this, I don't really have time yet to read everything.