Crooked Timber is engaging in some wild speculation regarding space travel today and I thought the topic sounded fun.
What I particularly wanted to engage with was the notion that since we haven't detected any extra-terrestrial signals advanced alien civilizations must be rare. While this may very well be the case, my speculation is no better than that of anyone else on this matter, I think this premise is too weak to draw this conclusion from.
Most notably, it assumes that any alien civilization will be using technology that gives off electromagnetic radiation similar to ours. Personally, I find this implausible for a few reasons.
First of all, this kind of communication is deeply impractical for an interstellar civilization. There are lag times between Earth and the space station, go out further, even to Mars, and this form of communication becomes increasingly impractical. Even a multi-planet civilization in one solar system will have powerful incentives to develop something else. Assuming that physical laws permit it, I think it is a safe assumption that the use of radio waves and other forms of energy detectable to us is a technological phase that lasts only briefly in any civilization.
A second problem is that we tend to assume that an alien civilization would be broadly similar to us technologically. But there is no good reason to assume this. The universe is billions of years old, extant alien civilizations may be millions or billions of years old as well.* While we're awfully impressed with our industrial revolution and the development of radio they may not consider us much more advanced than savages wielding stone tools, they may have no interest in contacting a civilization still fumbling around with a primitive toy like radio waves. Perhaps our planet is awash in signals we have no way of detecting and they won't bother with us until we can detect them and respond in kind.
Even if we assume these advanced species have something akin to academic departments that may be listening for antiquated signals we still have the problem of a lag time. We have only been broadcasting detectable signals for a bit more than a century, any alien academic would have to be within about 100 light years to detect us and have to be about half that to respond in kind with technology detectable to us. This vastly narrows the range of plausibly detectable life.
Another problem is an assumption of technological teleology. There is no reason to believe that technology has to, or even often will, develop in the order it did here. There may be many broadly similar civilizations that developed different ways of communicating that never embraced the widespread use of the electromagnetic spectrum. Even if not more advanced than us overall these civilizations would be undetectable as well.
A closely related problem is the notion that if there were alien civilizations they would have already taken over the galaxy or would be hostile to us. As to the first notion, this hasn't happened so all it tells us is that any aliens, if any, aren't like this. It doesn't really add any meaningful data.
The second notion deserves a little more attention since it is so common in science fiction. I have serious doubts that analogies from our own history are applicable. This is for the sole reason that I have no idea what an alien civilization would want from us in a material sense. Their biochemistry would be different from ours; I find it wildly implausible they would have any use for biological materials that evolved based on a different chemical basis except for as curios. They wouldn't have the enzymes to eat our food, our atmospheric chemistry would likely be different enough to be toxic (or at least incapable of supporting their kind of life),** and it would have to be more difficult to acquire resources from a toxic, biologically active world than a non-biological one. Our enslavement also seems unlikely, we're already good at using machines for labor, surely any advanced civilization will find machines more efficient than trying to deal with a bunch of toxic aliens with different mindsets who are difficult to communicate with.
Given that the universe hasn't been denuded of resources by a species billions of years old already we can also probably safely assume that constant, breakneck expansion isn't a quality that is common in alien species. Eventually, needs are satisfied and the expansion of population and material goods slows. This is hopeful view for the destiny of our own species, but I find it implausible that a desire for ever larger houses and more goods is really a characteristic of any species. At some point, material replication will be a quality of machines with living things focusing on other goods (like good food and cat videos).
* Of course, given the need for many heavier elements to form the complex molecules used by advanced life it may be the case that advanced life has only been possible for a relatively short period in the universe's history.
** This makes some science-fiction plots look less plausible. I wouldn't expect an earth virus or bacteria to be able to interact with a different cellular chemistry. Even among earth bound organisms there are limits to how much interaction is possible across organisms, it's the swine flu or bird flu, not the fish flu. While particular parasites can jump across larger species boundaries these tend to be rather particular jumps. It would be highly unlikely for a parasitical organism to be able to make the jump to an extra-terrestrial cellular chemistry (with enough time it might happen, but this would be a very, very long time).