| Life, or intelligent life? We can infer life based on atmospheric compositions NOW, but only if that planet fits an extremely narrow margin... meaning, Earth. Earth three and a half billion years ago had life on it, but you'd be hard-pressed to tell it from orbit. If you want to move to unambiguous (intelligent) life then....... The largest radio telescope we have (the 305 meter diameter Arecibo) would need to have it's sensitivity increased by around two orders of magnitude JUST to pick up our TV/FM/AM signals from outside the solar system. So, the crap we pump out the most couldn't even be detected by Voyager 2, if we had strapped a giant dish to it. If we move into the narrowband signals then, depending on the source-strength, Arecibo could potentially pick up signals at up to a few thousand light years... if it happened to be pointed in exactly the right direction at exactly the right time. So, our most sensitive instrument is only capable of measuring a fraction of a percent of the spectrum in a fraction of a percent of the possible-directions-it-could-be-pointed if the point source happened to be sending a strong enough signal (aimed at us) at exactly the right time (which would be anywhere between 2 and a few thousand years ago). I haven't done the math, but I suspect that regardless of the frequency and amount of energy we dump into sending out a signal (within the realms of not being scifi, anyhow), it would be impossible for anyone to detect us a thousand or two lightyears out (using EM radiation) without knowing when/where to look. They also wouldn't be able to "see" that for quite a long time yet. I couldn't find a list of "when did we start sending out signals at X frequency" (and I suspect it doesn't exist), but if we take 50 years ago as a guess and we assume we pumped out noise at a frequency that could make the distance at a power level sufficient to be noticeable and that there wasn't anything to get in the way of the signal then we're only looking at something like 2000 star-systems. |