A very short distance. IIRC something like 20 or so light years, but I'm having trouble finding the citation. That's for radio waves (and the number is getting smaller as the technology is getting better -- EM radiation blasted into the universe is considered waste by hardware engineers).
For direct detection of life, that's still an open question. I remember a poster done by a grad student who looked at whether life would be detectable from the Moon, looking at the Earth. This was tested with data from one of the outer solar system probes which did a lunar flyby on its way out (Cassini?). The result, IIRC, was inconclusive -- you could see signs of life, but not anything that was absolutely definitive proof.
Indirect evidence might be provided by spectra of the atmosphere observed via solar transit. A biome is likely to have different spectra than would be predicted by inorganic atmospheric physics. Still, that's making some assumptions about what extraterrestrial life would be like, and only only visible along the elliptic plane.
The chemical composition of Earth's atmosphere (21% O2) is so far away from a long term chemical equilibrium, that any current astrobiologist would interpret it as a sign of planetary scale life. (And correctly so: the oxygen was produced by life.)
So life on Earth is pretty visible from the other planets in our solar system.
And spectroscopy to observe the composition of atmospheres of planets orbiting the nearest stars will probably happen in not-too-distant future.
Meh, it's a long way from saying that "we don't know how the O2 could have been produced except by life" to "the O2 was produced by life." Just look at the current debate about methane on Mars.
Am I supposed to disagree with that? High atmospheric oxygen content is suggestive of biology. So is seasonal blooms of methane on a planet like Mars. Is that suggestive that there is life on Mars? Yes! Can we definitively infer that there is presently life on Mars? Not yet.
Would be great if you could find the citation, because this seems like the best explanation so far. There are only about 54 stars which at some point had a distance of less than 17 light years (that we know of): http://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown... . Assuming that some stellar source of energy is required for life we can already remove 9 stars from that list because they only consist of brown dwarfs. Doing the math with the conservative estimates in the blog post, this puts the chance of detecting signs of life at a pretty low percentage. Assuming that 22% of these stars have at least one planet with earth like conditions and a 1% chance that some form of life develops on these planets, we arrive at the low chance of 46x0.22x0.01, ie. ~10% of finding any life in our vicinity.
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.
For direct detection of life, that's still an open question. I remember a poster done by a grad student who looked at whether life would be detectable from the Moon, looking at the Earth. This was tested with data from one of the outer solar system probes which did a lunar flyby on its way out (Cassini?). The result, IIRC, was inconclusive -- you could see signs of life, but not anything that was absolutely definitive proof.
Indirect evidence might be provided by spectra of the atmosphere observed via solar transit. A biome is likely to have different spectra than would be predicted by inorganic atmospheric physics. Still, that's making some assumptions about what extraterrestrial life would be like, and only only visible along the elliptic plane.