Suppose an alien civilisation was looking at Earth through a BIG telescope, able to analyse the atmosphere. At what point in our history could they infer that there was intelligent life on the planet. Iron Age, Industrial revolution, A-bomb tests ?
There can’t be life on Earth. The atmosphere is full of oxygen, one of the most dangerous, most reactive gases known. Any molecules of any complexity would get oxidized to CO2 or SiO2 almost as soon as they formed.
Emphasis added.
The A-Bomb and or Tests are indications of a certain lack of intelligence in the design, construction and detonation.
This takes us back to the Iron Age or earlier when man began develop tools to work with.
But of course this entire post is academic at best as a “Big telescope” to "see that far and do the impossible has never existed , as such.
It might be possible using spectroscopy. An abundance of heavy metals or radioactive isotopes might indicate the presence of a technological society, although I don’t think geological processes such as volcanism could be conclusively ruled out as an explanation for the former. I think I’m right in saying life would be the most likely explanation for abundant oxygen.
It is feasable to obtain the spectra of extra-solar planets, there are plans to do this. It is not possible to obtain a detailed image of an extra-solar planet.
But if oxygen is do reactive, why is the atmosphere still full of oxygen? I.e. why hasn’t it all reacted already to the point where it’s only found bound in compounds instead of being free in the atmosphere?
There must be something continuously breaking down oxygen-bearing compounds and producing free oxygen to keep the atmosphere so full of it. The only conceivable thing that does that is life.
Can spectroscopy detect isotopic changes at all?
I know that above-ground nuclear tests changed the carbon-14 level to the point that we can no longer do carbon dating for objects that died after 1940, and it would be hard to conceive of something else that could do that. I don’t think there’s any purely chemical change that could be conclusively attributed to civilization, though.
Looking at the site for the now cancelled Terestrial Planet Finder mission, I came across this article, which talks about how to interpret the results of the spectra from an extra-solar planet. Sounds like they were fairly confident they could detect life, but it doesn’t present any ideas about how to detect civilization.
None - there is no intelligent life on the planet - present company excluded :dubious:
So there is intelligent life? How can you tell?
There really is not anything you could see from a distance that would indicate intelligent life. Nuclear weapons tests are not powerful enough that they could be detected directly and they don’t produce enough of anything to change atmospheric composition. Even things like greenhouse gases don’t require intelligence; the Earth has seen much wider shifts in the prehistoric past.
Radio waves are probably their best bet to prove intelligence, but those are just not strong enough on an interstellar scale.
I suppose at some level of intelligence you might get some fantastical artifacts that could be detected - something like a Dyson sphere would certainly be a detectable anomaly. In fact, Dyson spheres were proposed partly as an idea for how an intelligent civilization might be detected. However, those kind of technologies are still a very long way from what we can accomplish and their construction implies an assumption about continued growth that might be unwarranted given the changes we see in population growth in industrialized countries on Earth.
First you have to define life, then you have to define intelligence, then you have to look in the right places with the right kind of telescope, and correctly interpret what you see. How long did it take us to realize how intelligent dolphins are, and they’re not light years away?
I would imagine we would look like organized chaos.
Sort of like us looking at a bunch of cockroaches swarming over a pizza box - would you want to contact those cockroaches?
If I had reason to believe that they had nuclear weapons I would.
Nonsense. Once you have oxidized everything that is oxidizable, any oxygen left over will not have anything to react with. Anyway, the only relevant elements are carbon and silicon, the only ones capable of forming complex macromolecules. Once you have oxidized them to SiO2, CO2, or to silicates and carbonates, they won’t oxidize any further. The Earth’s surface rocks are largely composed of SiO2, silicates and carbonates (together with oxides of other elements), and there is quite a bit of CO2 in its atmosphere also. Clearly everything that can be oxidized has been oxidized. Despite that, there is still a huge amount of oxygen left over, ready to burn up any self-reproducing molecule that might appear beforei t can even begin to evolve.
Earth is clearly not a planet hospitable to life, and it would not be safe to land an exploration party. Even our robotically piloted magnesium shuttle craft could not survive atmospheric entry. I suggest we continue our search in a different galactic sector.
I tend to agree. Detecting life is one thng. Differentiating that from intelligent life is another thing altogether.
It’s an engineering problem. Radio waves don’t really drop below the background like an object sinking in water. They just get really spread out, but collect enough of them… It’s really just a matter of constructing a large enough antenna. A theoretical antenna (with gain which implies directionality which means they would have to be pointing it as us) the size of a solar system would theoretically be capable of picking up an ‘I Love Lucy’ level broadcast out to 100 ly give or take. A dish the size of Arecibo? Not so much. There may also be other ways to exploit physics to pluck such faint signals (-250 dBW (:eek:)) from the background noise that we haven’t even imagined yet. But sticking to what we do know… yeah, a really, really, really big antenna or array of antennas would do it. Interestingly enough, the ‘I Love Lucy’ days of broadcasting was when we were at our noisiest excepting active SETI projects. Cable and satellite TV/radio today are much more efficient, thus requiring less power. Digital broadcasting even more so. Then there’s encryption - wouldn’t that just piss off Mr. ET who was eagerly awaiting the next installment of The Sopranos. 
There has been serious work done looking for Class II civilizations by searching galaxies for Fermi bubbles (or voids). The theory goes that such a civilization would be expanding at some fraction the speed of light, converting stellar systems into Dyson Spheres. While a Dyson Sphere itself is hard to detect as it radiates in infrared, a lot of them in a region of a galaxy would appear as an artificial absence of stars. So far, no such void has been seen.
My guess would be irrigation and dam systems. The aliens would see water moving in obviously artificial ways.
Just how large a telescope are we allowed to posit? I’ve heard theories of using sun’s gravity as a telescopic lens. Of course, IIRC prime focus was 440AU out!
Oh well, good for him, then.
Except it wasn’t a whoosh. I figured artificial lakes and canals are going to be one of the earliest technological developments that would be visible from outer space.