We’ve never made contact with other intelligent life forms on other planets within other solar systems that possess sufficient technologies to communicate over long distances. Many ID advocates argue that’s because conditions on Earth are unique in the universe and no such other life forms exist. With an estimated 100,000,000,000,000,000,000,000 other planets in the universe such “uniqueness” seems very unlikely. Other factors are undoubtedly the immense distances between stars, the possibility that an alien species may have different technologies and communicate in ways that we don’t understand yet, etc.
Of course in order to communicate with another form of intelligent life both sides of the conversation need to have adequate technologies. If some intelligent life sent us messages as recently as two centuries ago no one would have had a sufficient technology to even receive them, much less interpret them.
Another factor that isn’t discussed much is the possibility that any civilizations with sufficient technologies to communicate over interstellar distances are inherently unstable. In other words, once such technologies are understood and implemented, something inherently happens to that society so that it destroys itself or loses that technology within a few centuries. Note that I am not suggesting that “intelligent life” itself is necessarily unstable. Instead I am suggesting that the use of advanced technologies may be unstable. It’s possible that an organism such as a dolphin could evolve significant intelligence but be unable to use new technologies simply because they lack opposable thumbs.
The most obvious possibility for instability is a nuclear conflict. If you have sufficient technologies to send radio signals, it is quite possible that you are close to understanding how to create and use nuclear weapons. It could be argued that we were very lucky here on Earth. It is unlikely that Adolf Hitler would have had any qualms about using nuclear weapons if he would have had them and he missed having that technology by just a few years.
But there are many other possibilities. Maybe strong technologies result in destruction of the environment of a planet within a short time (i.e. a few centuries). Life could then diminish to the point that mere existence becomes more important than trying to communicate with life on other planets. In his book “Collapse”, Jared Diamond demonstrates that environmental damage has been a significant factor in all societies on Earth that have collapsed such as on Easter Island, Mayans, etc. Of course those specific collapses didn’t involve the use of modern technologies. But it is very likely that such technologies could increase the rate of environmental damage.
Another alternative I’ve heard of is that eventually technologies become so fascinating in and of themselves that interactions between the living things become secondary in importance and the intelligent species stop reproducing. I think that there are signs that such things could happen.
I have a nephew who spends basically all of his free time playing video games. (Yes, I have mentioned my observation to his parents. Fortunately I can report that he now has a girlfriend and spends less time on these games.) A while ago I saw on a local news channel a video of two teenagers sitting a couple of feet away from each other on a park bench texting on their cell phones. There isn’t anything remarkable about that except for the fact that they were texting each other! Many people spend large parts of their free time on the Internet now. Others consistently find something on their 200+ cable channels to watch.
Such people are still a relatively small percentage of the overall population, but all of those technologies are also relatively recent. Video games have been with us for just a few decades. How addictive will they be a century from now? What will you be able to do on the Internet in 50 years? It is likely that in just a couple of decades you will be able to watch any movie ever made “on-demand” in high definition, 3-D on very large television screens. How compelling will that be for many people?
In fact, what new technologies will be available in the 22nd century that we can’t even imagine in 2011? There may even be some that offer sexual satisfaction in ways that we can’t imagine now.
If humans stop interacting with other humans, they don’t reproduce. Eventually the society ceases to exist.
Then, of course, there are other possible technologies that are unstable which humans haven’t experienced yet because our technology is still a bit limited. Some have suggested that it may be possible to create a significantly large black hole on Earth. Such a thing would destroy life here. We don’t know how to do that but maybe it is indeed possible.
With all of those risks (and maybe more that we are not aware of), it is quite possible that it is rare to find an intelligent civilization that lasts for more than, say, 3-4 centuries. So if intelligent life exists on any other planet we will need to be looking at that planet during the relatively very small window of time that radio signals coming from that planet may contain signs of intelligence.