ok first off, we aren't even able to detect earth-like planets and we won't be until sometime in the next decade (new telescope going up). Second, without an example of non carbon based life, we have no idea what chemical signals to look for to be certain of life. This problem has already arisen, as the chemicals predicted by certain theories have been found to be produced through non living mechanisms. Likewise, even if we did find something we thought was evidence of life, it is quite possible we would not be able to recognize it. A friend of mine studied martian rocks that had what appeared to be fossilized bacteria in them. This hypothesis was discounted because there was no way to prove what they were looking at was not created through geological activity.
So basically my point is, when it comes to detection of life, certain things need to happen for it to matter. First, it has to be very very close to home. If it is outside of our solar system it is virtually meaningless given current technology. Second, it has to be in such abundance that life is undenyably present. Third, it has to be similar to known life, otherwise people will debate whether or not it is really alive (viruses are one example, advanced AI and robotics are another). Finally, it has to be extremely primitive compared to us.
The last point is based on an argument presented by Ray Kurzweil. No doubt you have head somewhere about the "technological singularity." Kurzweil summarized this as a double exponential growth curve reflected in almost all areas of advancement. He argues that, given current advancement rates, humanity is due to achieve "singularity" somewhere around 2033.. that is, the amount of knowledge gained per period of time will approach infinity. This is based on advancements in computer technology and human computer interaction. We already augment ourselves with computers.. we do research, communicate, buy things, and are generally more efficient with computers than we would be otherwise. We can "remember" more than ever simply by looking it up online. The problem is we are limited to the clunky interface provided by our fingers, ears, and eyes. Based on advancing brain scan techniques, living tissue to silicon interfaces, lab grown cells, and so on, Kurzweil believes that the way in which people interact with computers will advance to the point where we are able to interact directly with the machine. In any case, developments that allow us to more efficiently use computers and software developments that make computers easier to use converge to offer true augmentation. People may have memory chips embedded in their skulls, or perhaps wireless transmitters.
This matters because it will allow researchers to augment themselves to become better researchers. This is where the true double exponential becomes most apparent. Scientists working to create faster and better computer interfaces and faster and better software programs will benefit from their own developments, meaning that they have the potential to become much "smarter" and thus much more able to make breakthrough advances. This cycle will continue until some kind of physical limitation prevents further development. Until that point, however, advancement in all fields will occur at an amazing, breakneck speed.
This will be the new race of humanity, the self augmented human. This race will be to evolved, social humans as evolved humans are to non-social animals. The advances introduced by the augmented humans will come so quickly and will be so influential that most people will be left behind, just as our social and cultural development has completely left behind the animal kingdom. Modern humans will be as able to adapt and respond to augmented humans as an elephant can adapt and change to modern humans. That is, basically not at all. In the time it would take modern humans to reach a decision and take action on it, augmented humans will have made the equivalent of hundreds or thousands of years of advancement.
This is the singularity. Once humans achieve true augmentation, we will very rapidly have access to all knowledge that can be gained from our vantage point in the universe. It is absolutely impossible to know what this knowledge will be, or how much it will change us. Kurzweil predicted this to happen by 2033, but even if you are not so optimistic, and it happened within 100, 200, 500.. even 1000 years, these periods of time are small beans on the universal scale. In essence, life on earth will have evolved very slowly over millions and millions of years, only to suddenly change completely within an extremely small period of time.
This discussion applies to the current argument because of the following: Any sufficiently advanced alien society will rapidly approach a state of singularity. Once such a state is reached, light speed or faster travel will or will not have been developed. Likewise with time travel. If such things are not developed, than interstellar travel remains a very long and difficult undertaking. As modern humans, this would mean that even if we did detect such a society, we would be almost entirely unable to interact with them, and likewise they could hardly interact with us. More likely, we would reach singularity ourselves before they could send a message across space to us. The second option is that time travel and faster than light travel are discovered. In such circumstances, an post singularity society would rapidly gain access to and use resources throughout its galaxy. If a post singularity society existed anywhere in our galaxy or even possibly neighboring galaxies, we would know it. Unless they didn't want us to know. In that case, there is probably nothing we could do to detect the society, thus meaning the universe would look empty. Because of the extremely short timespan between achieving society and achieving singularity (10,000 to 15,000 years from primitive man to knowing all there is to know), it is quite unlikely that we'll find another race anywhere within this brief window. Life will either be undeveloped bacteria like organisms or masters of the universe.. If we're lucky we might find large animals..
So, possible realities:
-we are the first race to achieve this level of advancement, and may be the first to achieve singularity.
-we are not the most advanced race, therefore someone else has already achieved singularity. Since we have not detected them, either they do not want to be detected, are too far away from us, or were not able to develop faster than light or time travel.
In the first scenario we have a unique advantage, truly unique and possibly worthy of religion. The universe may well have been made for us. In the second, we are more or less screwed. Either there is already a massive post singularity society around us that doesn't want to be seen, or there is no easy way out of this solar system. Our only hope would be that post singularity societies live in harmony.