Recall that last week we were looking at all the reasons why we may never detect alien civilizations on planets orbiting stars other than our sun.

Surely, it is argued, that with all those stars and all those planets spread out over such a wide expanse, more than a few such civilizations have developed.

Perhaps. But the vastness of time and space, and our own intellectual and physical limitations, work against our ability to ever know they exist.

5. The size of the Milky Way galaxy works against us. The sheer number of stars, around 300 billion, in our galaxy precludes a detailed study of all of them. If, for example, we simply wanted to count the stars in our Milky Way at a rate of one per second, it would take 935 years to count them.

6. The vastness of time works against us. Civilizations might feel the desire to communicate for only a short time. Our own desire to do so spans but a few decades out of the 10,000-year history of our civilization.

7. The civilizations have to match up in terms of time. The universe is over 12 billion years old. Civilizations may have risen and fallen before we were ever born as a species. And many civilizations may be born after ours. The chance that the limited period those civilizations are broadcasting and the period we are interested in listening may not match up. We may simply be looking in the wrong places at the wrong time.

8. Distance works against us. It takes a long time for a signal to get from one place to another. In 1974, astronomers at the Arecibo radio telescope sent a signal to the globular cluster M13, 17,500 light years away. It, with the power of a hoarse whisper, will reach M13 in 17,500 years. If some civilization hears it, which is itself doubtful, it will take 17,500 years for their response to get to us. 35,000 years is a long time to exchange a “Howdy, hi, how-are-ya.” Will humans even be on the planet in 35,000 years, and will they be listening?

9. It is the height of arrogance to suppose that a highly intelligent civilization will think and act like us. They may think in ways that to us are incomprehensible. We may be receiving signals right now that we simply do not recognize as signals.

10. In that regard, our own primitiveness may make it undesirable for them even to want to communicate. Would you want to have a nice sit-down with a cockroach or a bacterium? Besides, we are prone to kill each other in wars that wrack our whole planet. One look at our unfortunate tendencies toward violence and prejudice might make a more sedate species recoil in horror or disdain.

It hurts to say it, but I must. Intelligent life may not exist at all or exist in small enough quantities that the vastness of time and space and the vast differences in intellectual levels may make communication improbable at best.

If even one of the 10 reasons I have proposed in the last two weeks is true, we will most probably never receive or understand an intelligent signal from another planet. Even if the universe is teeming with life, we may never know it.

So why do we keep looking?

We are, so far as we know, the only species that is self-aware, that contemplates its place in the universe. Our spirits soar to the stars, but our bodies are trapped on a tiny ball of rock orbiting a below average star.

We are, so far as we know, the only part of the universe that has begun to understand who and where we are. We have become, in effect, the mind of the cosmos peering at itself.

Self-awareness and a consciousness of the enormity of time and space are heavy burdens to bear when you are alone in all that vastness.

We have worked ourselves to the brink of annihilation several times in the past century, partly because we cannot overcome our instinctual desire to survive by competing with other tribes for power and resources and partly because of our avaricious consumption of those limited resources. We are again at a crossroads. Will we be able to harness our primitive nature with our intelligence to survive into the 22nd century?

It would be nice to know that some civilization out there was able to think their way through its own self-destructiveness and civilization-ending crises to survive into old age.

We are, after all, a young species, babies really. We are still going through those inevitable growing pains, still trying to find ourselves. We look to the heavens and long to hear some elder voice to help us through these inevitably tough times, but the heavens remains silent.

Or to put it another way, we are lonely. And thus we will, against all odds, continue listening and hoping against hope that we hear the faint voices of others far older and far wiser than ourselves.