Aliens

First thing that I can think of is, do you know at what frequencies these messages were broadcast? I know the Sun is of type... G2V I think? This likely affects what frequencies we find appropriate for communication. The sun's spectral type is also somewhat rare, I think something like only 1/13 stars in the galaxy are of the same type. How long was the duration of the broadcasts we sent? What if the message was simply missed?

My point in stating that we've sent messages to star systems light years away had nothing to do with whether or not they actually receive the signals. It was more of a proof of concept. If we can do it in our relatively primitive state of technology, then imagine what beings who have surpassed our technological capabilities can do. If we are doing it, I assume they are doing it as well. My question is, why haven't we picked up on their signal? I am not expecting two way communication to be established any time soon.

Assuming this isn't a problem and communications have a 100% success rate, which is very unlikely, then did we match up time? Remember we live in four dimensions, not three. Considering the life of the universe, the human race has occurred in what some call 'the blink of an eye' and our civilized state of being able to send/receive electromagnetic communications is a much smaller window. Who's to say that the system we've sent messages to didn't have life or won't have life sometime in the future? If civilizations happen in the blink of an eye compared to the life of the universe, what are the odds that two civilizations so close to one another will both be capable and watching the sky trying to engage in communications at the same time?

This raises an interesting point. Is it inevitable that an intelligence will eventually extinguish itself? I think it seems that way with humans. But I'm not sure that this will be the case with all forms of intelligence. So, in regards to the argument that we may simply be temporally out of phase with other intelligences in the vicinity, one has to consider two cases: that an intelligence will inevitably extinguish itself, or it will not.

If the former is correct, then your temporal argument has merit. Perhaps our civilization arose at a time when no other civilization exists near enough to communicate with. This would explain our lack of present communication. Still, though, would it be impossible to find some vestige of their existence? Is there any evidence that our solar system has been visited any time in the past? How about some artifact left out in space that we can detect? If the nature of intelligence is to exterminate itself, this would make finding evidence of extraterrestrial life difficult, but perhaps not impossible.

If, however, intelligence doesn't necessarily destroy itself, then this blows the temporal argument out of the water. While we may have only been around for a blink of an eye, other beings may have been around a lot longer. As a corollary, they would have been transmitting for a lot longer. This should actually increase the chance of finding them, in my opinion.

Of course, in both cases there is a lot of wiggle-room. Perhaps an intelligence capable of surviving indefinitely has moved on to a more efficient medium of communication. Perhaps some dying race left a beacon on their world looking for help. Either way, whether the former or the latter is the case remains to be established. This is purely speculative on my part, but I tend to lean towards the latter case being more likely.

Lastly, what stars are you referring to? Obviously I'm not asking for a complete list, but a star's lifetime is inversely proportional to its mass at formation (Look up an H-R diagram if you're not familiar). So I would wager (this is a complete assumption on my part) that a good majority of the stars we've broadcast to are massive stars, and therefore have short lifetimes. It's been suggested that evolution of life in these star systems would not occur since there is not enough time. I think some of the most massive stars have lifetimes less than 1 million years? If you look it up correct me if I'm wrong as its been a while. I may be off by an order of magnitude, but either way they have lifetimes that are very short as far as stellar lifetimes are concerned. Assuming that some of these stars are capable of supporting life, what is the distance? Again, we find ourselves limited by the vastness of space. I did take the liberty of looking up the 50 closest stars to us. I think the 50th closest star is around 16 light years. So any communication with a civilization of that system would take 32 years (16 out, 16 back). So maybe some of these messages we've sent to different stars systems haven't even reached the star yet, let alone had time for a message to travel back. Who knows, right now an alien civilization may be reading one of our messages and deciding if it's even wise to respond .

I will provide two examples.

The Arecibo message is aimed at globular star cluster M13. This star cluster ranges across 145 light-years, and has several hundred-thousand stars. For taking a shot in the dark, this is a massive target.

The "Hello From Earth" message was aimed at the Gilese 581 star, about 25 light-years from here. Gilese 581 is a red-dwarf star, whose mass is approximately a third of the sun's mass, and it's age is estimated to be around 7-11 billion years. Red Dwarf stars constitute the majority of the stars in the universe, and have an estimated life-span longer than the estimated life-span of the universe.