[norea-armozel]

>For intraplanetary communication, yes, I'd agree. But those signals shouldn't be strong enough for us to detect (otherwise, what a wasteful species).

Many of our radio signals are detectable from terrestrial sources such as radio and air TV stations. Even Ham radio signals throw out enough power for their signals to be easily detected beyond our solar system.

[cgriswald]

Eh, not really "easily." If a comparable civilization was located around Alpha Centauri, we would have to focus the equivalent of an Arecibo radio telescope at it for many months, maybe a year, to detect its presence. That's "easy" in the sense that we could do it, but it's also very expensive to do and we won't do it without having at least some idea that a civilization exists there already. The farther out we go from Earth, the harder it is, and it gets impossible (in the sense that we lack the resources to do it) very quickly with increasing distance.

For interplanetary communication, tight beams would make way more sense than broadcast. And unless one of those beams is directed at us, we aren't likely to intercept it. This is really what SETI is looking for because it's the easiest thing to look for.

[norea-armozel]

Yeah, but the fact remains that it's more likely we'll get a random stray signal before we'll get an intended broadcast from another civilization. I just don't see it as being one of those things any civilization is likely to do (what's the point of communication with another civilization at such extreme distances?).

[tired_man]

If they determine that the random signal contains any sort of information, even if we can't understand it, I think they'd simply be happy to have confirmation of another civilization.

Getting a chunk of signal with modulation or one containing repeated patterns would be a world changer.

[cgriswald]

That isn't a fact. We are more likely to detect a strong intended broadcast and will be so for the foreseeable future.

Your parenthetical is a good question.

We ourselves have sent a message to M13, a globular cluster ~20K lya. (See https://en.wikipedia.org/wiki/Arecibo_message) The only reason? To say "We are here." The thinking behind the choice of target was that the stars are close together so for one message we get more likely reception. That thinking was flawed because stars in a globular cluster are hot, young, and short-lived, so unlikely to support life, much less complex life.

We will probably begin sending similar messages when we discover more likely candidates. We are very close to being able to detect the atmospheres of Earth and super-Earth exoplanets. If we find one that contains a significant amount of free oxygen, it will be a good candidate for life, and I can certainly see us sending a signal to these planets.

The point of such a message is that for relatively small cost, we can potentially discover we are not the only intelligent life in the universe.

Additionally, the sheer knowledge that could be acquired if both species beamed constant signals to each other about their cultures, scientific discoveries, and etc., would be damn near priceless. Even if we found a civilization slightly behind ours or on the same level as ours, there would suddenly be two Earths with both similar and dissimilar problems with both similar and dissimilar solutions to those problems. Our knowledge of the universe and our ability to solve problems would grow, even if our mutual civilizations lagged behind each other by (say) 100 years due to the distance.