from The Cosmic Connection, Anchor Books, 1973, pp. 200-203
We can make a similar examination, but with greater uncertainty, of the extraterrestrial hypothesis that holds that a wide range of UFOs viewed on the planet Earth are space vehicles from planets of other stars. The reported rates, at least in recent years, have been several per day, at the very least. I will not make that assumption. I will make a much more conservative assumption that one such report per year corresponds to a true interstellar visitation. Let's see what this implies.
We have to have some feeling for the number, N, of extant technical civilizations in the Galaxy -- that is, civilizations vastly in advance of our own, civilizations that are able by whatever means, to perform interstellar space flight. (While the means are difficult, they don't enter into this discussion...)
An attempt has been made to specify explicitly the factors that enter a determination of the number of such technical civilizations in the galaxy. I will not here run through what numbers have been assigned to the various quantities involved -- it's a multiplication of many probabilities, and the likelihood that we can make a good judgement decreases as we proceed down the list. N depends first on the mean rate at which stars are formed in the Galaxy, a number that is known reasonably well. It depends on the number of stars that have planets, which is less well known, but there are some date on that. It depends on the fraction of such planets that are so suitably located with respect to their star that the environment is a feasible one for the origin of life. It depends on the the fraction of such otherwise feasible planets on which the origin of life, in fact, occurs. It depends on the fraction of those planets on which the origin of life occurs in which, after life has arisen, an intelligent form comes into being. It depends on the fraction of those planets in which intelligent forms have arisen that evolve a technical civilization substantially in advance of our own. And it depends on the average lifetime of such a civilization.
It is clear that we are rapidly running out of examples as we go farther and farther along. We have many stars, but only one instance of the origin of life, and only a very limited number -- some would say only one -- of instances of the evolution of intelligent beings and technical civilizations on this planet. And we have no cases whatever to make a judgement on the mean life of a technical civilization. Nevertheless, there is an entertainment that some of us have been engaged in, making our best estimates about these numbers and coming out with a value of N. The result is that N roughly equals one tenth the average lifetime of a technical civilization in years.
If we put in a number like ten million years for the average lifetime of advanced technical civilizations, we come out with a number for such technical civilizations in the Galaxy of about a million -- that is, a million other stars with planets on which there are advanced civilizations. This is quite a difficult calculation to do accurately. The choice of ten million years for the average life of a technical civilization is rather optimistic. But let's take these optimistic numbers and see where they lead us.
Let's assume that each of these million technical civilizations launches Q interstellar space vehicles a year, so that a million Q interstellar space vehicles are launched per year. Let's assume that there's only one contact made per journey. In a steady-state situation, there are something like a million Q arrivals somewhere or other per year. Now, there surely are something like 10 to the 10th interesting places in the Galaxy to go visit (we have several times 10 to the 11th stars) and, therefore, an average of 1/10 to the fourth = 10 to the -fourth arrivals at a given interesting place (let's say a planet) per year. So if only one UFO is to visit the Earth each year, we can calculate what mean launch rate is required at each of these million worlds. The number turns out to be ten thousand launches per year per civilization, and ten billion launches in the Galaxy per year. This seems excessive. Even if we imagine a civilization much more advanced than ours, to launch ten thousand such vehicles for only one to appear here is probably asking too much. And if we were pessimistic on the lifetime of advanced civilizations, we would require a proportionately larger launch rate. But as the lifetime decreases, the probability that a civilization would develop interstellar flight very likely decreases as well.
There is a related point made by the American physicist Hong-Yee Chiu; he takes more than one UFO arriving at Earth each year, but his argument follows along the same lines as the one I have just presented. He calculates the total mass of metals involved in all of these space vehicles during the history of the Galaxy. The vehicle has to be of some size --it should be bigger than the Apollo capsule, let's say -- and we can calculate how much metal is required. It turns out that the total mass of half a million stars has to be processed and all their metals extracted. Of if we extend the argument and assume that only the outer few hundred miles or so of stars like the sun can be mined by advanced technolgies (farther in, it's too hot), we find that two billion such stars must be processed, or about 1 percent of the stars n the Galaxy. This also sounds unlikely.
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larry-zimmerman@uiowa.edu
University of Iowa Anthropology
08/18/98