Goober

Add the problem that most of the stars are so far away we are looking billions of years into their past, so signals may not have reached us, the problem that we don't know exactly how an intelligent race might signal their existence and we search only an extremely small percent of these stars for signals so we may be missing them, and the problem that many intelligent life forms may be uninterested in signalling their existance or incapable of actually doing so over a large enough distance to be visible to us (after all, we are not signalling in any organised way that might detecteble at all even out of this solar system, and we are intelligent life).

I'm not at all surprised that we have seen no signs of intelligent life.

contracycle

YEs, it does make sense for us to be surprised by the lack of evidence.

Think again about the sheer numbers of stars and the lengths of time we are discussing. They are not really conceivable in human terms. But also, look how firmly we have stamped our presence on our planet. It's not just deliberate communications we are looking for - but accidental emissions too. Drive systems, nav beacons, the equivalent of mine tailings. Why can't we see the any engineering, or indicatioons of engineering?

Space as large and deep as this should have multiple shells of emissions over time. Many of these should be in transit for long, long periods covering immense distances.

Goober says:
Well we have and we have not. We did send a message from the Arecibo dish, but have not mainteined the kind of continues transmission that would be needed to make a serious effort. OTOH, in certain radio frequencies, we are the brightest objects in the solar system, and are clearly visible from hundreds of light years away.

Echoes says:
They don't have to leave; presumably when they first developed space travel, they were using radio. So they should have a shell of archaic transmissions around their home system at some considerable distance. So if the societies a, b and c existed, or had existed, we should still be able to detect something of their presence.

Shadowy Man

Hundreds of light years doesn't get you very far.

markfiend

contracycle:
OTOH, in certain radio frequencies, we are the brightest objects in the solar system, and are clearly visible from hundreds of light years away.

We've only been sending any radio signals for about 100 years. So they'll not be visible "from hundreds of light years away" for a while yet.

auto-da-fe

More on the topic of looking for other planets, than what kind of evidence (if any) will be found of alient beings;

I saw a program on one of the Discovery Channels (other than the Discovery Channel, I think it is Discovery Science or something), that was talking about this very subject. What methods were being used to detect other planets. So far there are only a couple (3), and each of them are not capable of finding something the size of Earth, only Jupiter size planets. However, they are building a new telescope that will revolutionize the search.

It has 2 optic mirrors, set a distance apart (something like 30 feet), so that each mirror has a slightly different view of the star (or whatever your looking at). By combinging the 2 images so that their respective apexes (apexii?) meet at just the right angel, they can dim the star by about 10,000 times. At the same time, they have some sort of vibrating metalic pieces placed throughout, that have the ability to cancel out the "twinkle" that stars have (caused by viewing the light from the star through air of differing temps).

By doing the 2 things above, they can begin looking for planets themselves, instead of looking at the star to find planets (current methods of finding planets involve wathcing the "wobble" of a star, looking for miniscule eclipses in the star, and one other that has escaped me, but also looking at the star itself). The results could be us looking directly at a planet, earth sized, and being able to see clouds, water, mountains, and if any, non-natural formations (buildings, walls, statues, etc.).

This could prove to be the single most effective means we have for looking for signs of intelligent life forms that we have so far.

Rhaedas

The idea sounds interesting in how it may reduce the star's glare, which is a key factor in seeing anything near it. Perhaps elimination of the star's light can allow measurements of such signatures as water or other elements which could be factors for an earthlike planet.

Certainly the current techniques for planet detection are limited because we're indirectly measuring the planet's affect on the star, and only the bigger planets would be noticeable.

Hwoever, the resolution for doing what you suggest, seeing the actual planet, would be far and above anything you mention. I think I've seen mention that even a space based array of telescopes hundreds of miles across would have trouble seeing details that specific from stars close to us. Even the nearby stars are damn far away...just look at our best picture of Pluto from the Hubble. And that's practically on top of us compared to our neighbor stars.

Maybe someone with some telescopic experience could do a rough calculation of what type of resolution is needed for such a feat, but I'm very sure it's much more than a mere 30 feet.

Shadowy Man

What MegaDave is describing sounds like optical interferometry coupled with adaptive optics. The Keck telescopes are the first ones that will be able to do that, I think, with the Large Binocular Telescope (LBT, currently being built) probably being able to do it as well (I don't know too much about the LBT).

With this setup you won't be able to image anything on the planet (it will still be a dot), but you might be able to isolate it from the star it is orbiting in order to do spectroscopy on it.

Spectroscopic observations of the atmospheres of earth-like planets will provide the best way (in the near future) to look for life on planets in other stellar systems. You would look for the absorption signatures of things like molecular oxygen and ozone.

auto-da-fe

Hmmm... the program seemed to imply that we would be able to see planets enough to see clouds, etc.

Still though, this will still be a lot better. The way it was explained is the first places they are going to point this bad boy is in the direction of some known Jupiter sized planets, then from there see if it is alone, or is in a planetary system.

I wonder what kind of focus you would have to have to be able to see, say Alpha Centuari up close and personal enough to be able to see (if it has planets) the planets at a great resolution.

Anyone smart enough to figure this out? I don't know enough about optics to be able to say.

What if we were to put it into a geostationary orbit? How much would that help?

Rhaedas

From here:

So it would have to be space based definitely, for optical, preferably in a spot such as the L4/5 points (ie, low earth orbit isn't really the best place, but since the shuttle, which does maintenance on the Hubble, can't go past LEO, that's where it is). Now again, the Hubble as good as it is, can only see Pluto as a small blob. The Centauri group is magnitudes farther away, at a mere 4.5 lys, so I'd question if a space telescope with 10 times the mirror size of the Hubble could even see a Jupiter sized planet as more than a point. Arcseconds are small compared to the whole size, but I don't think such a scope would see the Centauri stars themselves as disks, much less planets.

I haven't attempted the math though, mainly because I'd probably get it wrong. But it seems the scale suggests optically, we won't be seeing planets. Shadowy Man's approach, which described it better than I did, is our best bet...it doesn't rely on the resolution factor (as much).

auto-da-fe

so what your saying is that the chances of visual confirmation of a planet earth sized is maybe being a little too hopeful?