SPACE.com columnist Gentry Lee is a noted space systems engineer, TV producer and SF author. This month, he draws upon several decades of thinking about extraterrestrial intelligence.

We spent the next 15 minutes discussing his question. Every one of the boys participated. Their interest and enthusiasm was marvelous. I should point out that this discussion took place in 1959, long before the invasion of aliens into our popular culture. It’s highly likely that most of the eight year olds had never even thought about the issue before.

When we were ready to return to the cabin, one of the campers suggested that we take a vote. In my cabin that summer we voted on everything important. It seemed completely reasonable that we should vote on whether there was life and intelligence somewhere in space. Six boys voted affirmatively. Five said no. One of the boys abstained. He refused to vote even when pressed by his cabin mates. "I just don’t know," he said in a tone that indicated the discussion was over.

If the subject of alien abductions is raised, I acknowledge that many ofthe stories are compelling, and that there is a tantalizing similarity among many of the tales that definitely warrants investigation. I then point out that unfortunately there exists no hard proof that any of the stories are true.
     

I will confess that the question of the existence of extraterrestrial life and intelligence has fascinated me since my early years. My desire to know if life exists, or has ever existed, someplace other than on the Earth was the primary reason that I chose to work in the aerospace industry when I finished my formal schooling. As soon as I was hired, I made it absolutely clear to my employer that my first, second and third choices for my personal assignment were projects associated with the exploration of space.

Being part of the Viking mission was a dream come true. For eight years I firmly believed that when the landers touched down on the surface of Mars in 1976 we would obtain clear and unambiguous evidence that life did indeed exist somewhere else. I didn’t expect to see a hedgehog or some other advanced life form staring up at the camera lens. But I did think that we would find microbes, or something similar, in the martian soil. Yes, I was ecstatic that the Viking missions turned out to be such a resounding engineering success. But like many of my colleagues on the project, I was disappointed that we did not discover any life on Mars.

During my Viking years I had many opportunities for informal discussions with some of the most talented scientists in the world. My favorite conversations were about the origin and evolution of life on the Earth, and the likelihood of a similar process having taken place somewhere else. I became as familiar with the Drake equation, a formula that estimates the number of advanced civilizations in our galaxy, as I was with Newton’s laws of motion. I accepted Frank Drake and Carl Sagan’s educated guesses for the values of the parameters in the Drake equation and their resulting conclusion that our galaxy alone might contain as many as a million advanced extraterrestrial civilizations. I was convinced, as was said in Cosmos, that the sky was "softly humming" with alien intelligence. The failure of Viking to find life on Mars did not alter my opinion.

I am often asked, after a speech or a lecture, if I believe in UFOs. The question is not properly posed, but I know what it means. What the questioner wants to know is if I believe that aliens exist, and that they sometimes visit the Earth. It often surprises people to hear my answer, since my science-fiction novels are full of extraterrestrials. I tell people that there exists no incontrovertible evidence that any alien has ever visited our planet. When someone subsequently starts to tell me about his or her personal UFO experience, or about Roswell, or about Area 51 in Nevada, or the crop circles, or any other phenomenon that the non-critical media has suggested implies an alien presence on our planet, I smile patiently and wait. I then explain that not one extraterrestrial artifact of any kind, not a thimble, or a drinking glass, or a fountain pen, or an eating utensil, nothing whatsoever, has ever been conclusively identified. Any actual alien artifact could quickly be determined to be not of earthly origin. A detailed examination of an item manufactured on another world would certainly reveal some kind of process that doesn’t exist on Earth.

If the subject of alien abductions is raised, I acknowledge that many of the stories are compelling, and that there is a tantalizing similarity among many of the tales that definitely warrants investigation. I then point out that unfortunately there exists no hard proof that any of the stories are true. I hasten to add that I am not accusing all the putative alien abductees of purposely fabricating their stories. They may be telling the truth. There is just no incontrovertible evidence.

One of my close scientific friends has accused me of a gross inconsistency with respect to my thinking about extraterrestrials. He can’t understand how, sometimes in the same speech, I can systematically debunk the popular conception of aliens roaming around our planet, more often than not with nefarious purposes, and then wax philosophical about the large number of likely advanced extraterrestrial civilizations. "Where is your incontrovertible evidence that they exist?" he asks. "And by the way," he adds, "if there are so many of them, why haven’t we had a definitive first contact?"

These are excellent questions. I remember having a superb conversation with Carl Sagan one night 20 years ago on these very issues. Mostly I listened, for Carl was absolutely spellbinding when he was talking about subjects related to extraterrestrial life and intelligence. He spoke about the parameters in the Drake equation, and how the scientific method led one to the conclusion that it was "highly likely," in a probabilistic sense, that there were many other advanced life forms in our galaxy. Carl contrasted the scientific approach that led to the formulation of the Drake equation with what he called the "appalling lack of skepticism" in the popular culture with respect to such subjects as UFOs and alien abductions.

Carl agreed that there was not yet any definitive evidence that these other advanced aliens, the ones implied by the Drake equation, did indeed exist. He felt confident, however, that after a reasonable amount of time, one or more of the scientific methods for searching for extraterrestrial intelligence would ultimately lead to an unambiguous first contact. "Remember," he said to me, "absence of evidence is not evidence of absence." I don’t know if that phrase was original with Carl, but it was certainly unforgettable.

Okay. If all those advanced ETs are out there, why haven’t they introduced themselves to us by now? This is a fun question. I have heard many wonderful explanations over the years. The most popular answer is what I call the "benign anthropologist" conjecture. The aliens have not contacted us, this scenario assumes, because they don’t want to disturb the evolution of our pristine culture. We are being studied by alien anthropologists. Therefore, the extraterrestrials have made the earth off-limits to all members of the Federation Galactica. They understand, correctly, that any significant contact by them with our species would irrevocably alter our society. So we are purposely avoided, in the interests of galactic science.

Another common explanation for the lack of contact could be called the immaturity hypothesis. A solid scientific argument, based on the extraordinary amount of time that the galaxy has been in existence, concludes that virtually any extant alien culture that we might encounter would be much, much older than we are. Whether one defines an advanced intelligent civilization as one that understands the electromagnetic spectrum, as some people have done, or one that has the ability to construct a radio telescope, a definition suggested by astrobiologist Chris McKay, our human culture has only recently reached the threshold. We are infants in the hierarchy of advanced cultures.

Perhaps, this explanation suggests, only a small fraction of the emerging intelligences that reach our current stage endure long enough to become full-fledged members of the galactic federation. Perhaps the older, more mature aliens wait a reasonable amount of time before contacting such a fledgling species. They may have remote monitoring stations watching our television signals and other electronic output. These mature cultures may also have developed algorithms capable of assessing the probability that an emerging intelligence will survive for at least ten or twenty thousand years. What would they conclude from their observations that our species is rapidly destroying the very environment that made it possible to rise to intelligence in the first place, and is also still spending an enormous percentage of its gross world product on weapons of destruction?

Ah, what fun it is to imagine why not one of those purportedly thousands of advanced aliens has ever made itself known to us in an incontrovertible way. Could it be that maybe, just maybe, there are not that many of them after all? This is the thesis advanced by a remarkable, accessible, new book entitled Rare Earth, by Peter Ward and Donald Brownlee of the University of Washington. The subtitle summarizes their most important conclusion: "Why Complex Life Is Uncommon In the Universe."

The galaxy and the universe, Ward and Brownlee conclude, is probably teeming with microbes. But based on a deeper insight into the factors that produced intelligence on Earth, they argue that the evolution of complex life requires such an extraordinary concatenation of unlikely events that the odds against its occurring elsewhere are vanishingly small. Hence the title of the book, Rare Earth.

Am I convinced? Not completely. Am I impressed? Absolutely. Ward and Brownlee have even included the proper disclaimer. Toward the end of the book they write that the "great danger to our thesis is that it is a product of our lack of imagination". They point out that they have taken the "perhaps jingoistic" stance that the "lessons from Earth are not only guides but also rules."

So where do we go from here? Are we alone in the universe? Or is there a little boy out there, or the alien equivalent anyway, looking out into the void of space and wondering if he has a counterpart on some other world? It is still one of the most compelling questions that human beings will ever ask. My guess is that many more people, after reading Rare Earth, will be like the one boy in my camp cabin who said, "I just don’t know."