The paradox, which is not really a paradox, and which was stated by Enrico Fermi, the famous physicist, goes like this: There are an enormous number of stars in the Universe, and we can assume many of them have planets, which means there must be an enormous number of civilizations out there. The question is, where are they? Why haven’t we seen any sign of them? There are all sorts of answers to this, and if you think about it, the real question is, why would we expect to see them?
One answer to the “paradox” is, we have: they are called UFOs. And here we reach the first example of faulty logic that pervades this sort of discussion. A logical argument starts with a premise, and in this case it might be:
UFOs are alien space vessels,
Hold the howls of laughter, please. This is patently nonsense, but it also an extremely bad premise, and the easiest way to see this is to think of the opposite statement:
UFOs are not alien space vessels.
Can you say with absolute certainty that there has never been an alien space vessel? The short answer is, no. The fact that almost all UFO sightings are simply natural phenomena is beside the point, after all everyone has seen flying objects, and sooner or later, one will be unidentified by someone. A statement usually needs qualifiers, thus if we rearrange our second statement to
No UFO is an alien space vessel,
we see at once the problem, and we stop the discussion right there. So, when making statements, we should all make sure we qualify our terms so we say exactly what we mean, and not bend to stupid exaggeration. As an aside, if we did that, an awful lot of silly political posturing would have to cease.
The second option is that stars are too far apart, and the distance is an inhibition to interstellar travel. After all, why invest a huge amount of money to build a starship when those building it will never get any information or benefit from it (because they will all be dead long before it reaches its target.)? Further, if you cannot get to relativistic velocities, so will the crew. A further problem might be that unless you know the destination is going to be useful, who wants to go? They might have come here once before, gone home and filed the information. Why come again?
A third reason is the age difference between stars, and this also applies to SETI, and may be a reason why that hears nothing. Suppose those sending a signal wish to minimize expense? If so, to keep the signal strength to a detectable level over the very long distances they would target their signal. Now, suppose citizens of Kepler452b reached our level of civilization at about the same time, and decided to transmit and aimed at us, because we are around a G2 star. They would have to keep at it for 1.5 billion years or so before we could conceivably detect it. Of course they might now seek out stars of an equal age, but now they are starting to make things difficult because they may have to go quite enormous distances to find the right star. But even more problematical, consider the issue from military strategy. When you send, you identify yourself, but since you send because you cannot go, any alien that picks up the signal and comes to visit will be extremely more advanced technically than you are, and if they are warlike, you lose.
Finally, there is the option I opted for in my “First Contact” SF trilogy: when humans managed to find a way of contacting a highly moral alien race, they were told that while what they had done was clever, it was unfortunately premature, and humans were simply not ready for contact with such aliens. One particular problem was that they had to stop fighting amongst themselves, the argument being, if you hate each other this much, how could you be let loose on aliens? Even though the example ism fiction, why would an alien race want to have fundamentalist terrorists on the loose? Then, if they are moral, they would not wish to contact us because such contact would severely damage us. Moral aliens will not give us technology. If you want to know what happens when a more advanced race meets a lesser one, look no further that what happened to the indigenous people of America.
Finally, or those interested in seeing more of the arguments in this last paragraph, the first book in this trilogy, A Face on Cydonia will be on a discount for a few days, starting on August 20.