How likely intelligence?
Carl Sagan, the great scientific pagan of the mid-20th century, estimated that there must be at least 10,000 inhabited planets per galaxy. It would be ridiculous to think we had the universe to ourselves. He was very famous, and the number was repeated again and again.
Still — no sign of anyone else.
In 1965, Penzias & Wilson found the radio signal that Gamov had said would vindicate LeMaitre’s Big Bang and lock physics into a finite universe. The universe was not eternal; it had an age, and in consequence of its age, it had a finite dimension, somewhere between 1024 and 1027th meters. We had a universe size at last. Not perfectly definite, but fairly so. Not infinite. Not even as much as 30 billion light years in diameter. Probably 13 or 14 billion light years radius.
1027 * 1024 * 1021 galaxy size * 1018 * 1015 *
1012 * 109 sun size * 106 * 103 * 100 =1 people size
10-3 * 10–6 * 10-9 molecules * 10-12 * 10-15
Just at the turn of the century, Robert Jastrow of NASA wrote a book called God and the Astronomers, in which he acknowledged that this discovery had forced him to abandon a lifetime of atheism, and he invited his Catholic subordinate, John O’Keefe, to write an Afterword. O’Keefe had several interesting things to say, including a suggestion on how to approach the relative likelihood of other intelligent life. It could be very simple. If there were, say, 23 independent conditions for the development of intelligent life, and if each one had a 10% chance of turning up near a given star, then the chance of developing life in the universe would be 1/1023rd. That is about the number of stars in the universe. If the conditions were more likely, then we should continue to look around; if less, we’re probably alone.
This was much better than just by saying, “Gee, it’s awful big for just one human race.” Math is always nice; you can get somewhere.
In the 1990’s Ward and Brownlee came along with a book, Rare Earth, which listed all the known conditions for life, including very unexpected conditions such as tectonic plate motions. Many of the conditions were much less than 10% likely and they concluded that, yes, we might be the only ones.
The sense of human specialness was again on the march. It deepened as further consideration of the Big Bang – the explosion at the origin of the universe – showed that it had to have been incredibly specific in order to work – too intense and the universe would have blown to dust without forming stars, planets, or people – too slight and matter would have been recaptured by gravity before it had time to form stars, planets, or people. It was very, very special. The sort of exactitude its numbers required went to 51 decimal places, which is about what it would take to locate your nose within the solar system — the entire system, out to the furthest comets.
It began to look as if the intelligent radio signal had indeed been found, right there in the Big Bang, and it meant that our one intelligent companion was the creator.