Extraterrestrial Civilizations - Isaac Asimov [89]
*This is one early and dramatic theory that is not generally accepted now.
* The English astronomer Fred Hoyle (1915- ) is sufficiently impressed by this to suggest that in comets (which in some ways have the composition of interstellar clouds) compounds form that are complex enough to possess the properties of life; that the equivalent of viruses are formed; and that comets may therefore be the cause of the occasional pandemics that afflict the Earth by sending new viruses into the atmosphere. It is an interesting suggestion, but it is hard to see how it can be taken seriously.
* Of course our physical shapes will surely change as time passes, thanks to evolution, or to the deliberate genetic engineering introduced by human beings, but that does not affect the line of argument.
CHAPTER 10
Civilizations Elsewhere
OUR GIANT SATELLITE
In a way, our speculations concerning extraterrestrial intelligence have ended on a triumphant note. Doing our best to make reasonable and conservative estimates and assumptions, we end with a Universe that may be incredibly rich in intelligence. Along with our own, 390 million sets of companions in the great adventure of learning and speculating have entered into civilization right here in our own Galaxy.
If those 390 million civilizations are spread evenly about the Population I outskirts of the Galaxy, then the distance between two neighboring civilizations would be, on the average, about 40 light-years. That is not very great as cosmic distances go.
But then there is a question that, in a way, spoils everything.
Where is everybody?
If there were indeed hundreds of millions of advanced civilizations in our own Galaxy, we should think that they might well have ventured beyond their own worlds; they might have formed alliances; they might have formed a Galactic Federation of Civilizations with emissaries sent to other galactic federations beyond the intergalactic spaces. And, in particular, they should have visited us. Why haven’t they?
Where is everybody?
There are a number of possible explanations for this puzzle. It may be, for instance, that the analysis presented in this book is wrong in some key point after all, and there are, therefore, no habitable worlds except our own Earth.
Almost every stage in the analysis might hide an error arising out of our incomplete knowledge. Perhaps binaries are much more common than we think and much more influential in distorting planetary formation. In that case, there might be very few single Sunlike stars and very few planetary systems like our own Solar system.
It might be that the ecosphere is very shallow, as some calculations indicate, and it might be that almost no planets manage to be located in just the thin shell of space around a star that would make habitability possible.
It might be that, for some reason we as yet do not understand, planets with the mass of Earth form only rarely, and that in planetary system after planetary system, there are planets that are too large and others that are too small and virtually nowhere are there planets that are just right.
It might be an incredible cosmic accident that liquid water has collected on our world in appropriate amounts, or that other things are just so, and that we are, therefore, the only habitable planet in the Galaxy, or even in the Universe.
We have, however, no reason to think these things just yet. Evidence that will justify such thoughts may arrive at any time-tomorrow, for all we know. Until then, we have no choice but to stay with our line of reasoning and see if we can find an explanation for the absence of positive evidence of other civilizations elsewhere.
Perhaps it is not some error that arises out of our ignorance. Perhaps there is an error that arises out of something that is perfectly obvious but that we have been ignoring. For instance, is there something so unusual about the Sun, or its planetary system, or Earth itself, that we cannot truly make use of the principle of mediocrity?
As far as the Sun and the planetary system