But lets pretend again that we have made it to the Oort cloud. The first thing you might notice is how very peaceful it is out here. Were a long way from anywhere now-so far from our own Sun that its not even the brightest star in the sky. It is a remarkable thought that that distant tiny twinkle has enough gravity to hold all these comets in orbit. Its not a very strong bond, so the comets drift in a stately manner, moving at only about 220 miles an hour. From time to time some of these lonely comets are nudged out of their normal orbit by some slight gravitational perturbation-a passing star perhaps. Sometimes they are ejected into the emptiness of space, never to be seen again, but sometimes they fall into a long orbit around the Sun. About three or four of these a year, known as long-period comets, pass through the inner solar system. Just occasionally these stray visitors smack into something solid, like Earth. Thats why weve come out here now-because the comet we have come to see has just begun a long fall toward the center of the solar system. It is headed for, of all places, Manson, Iowa. It is going to take a long time to get there-three or four million years at least-so well leave it for now, and return to it much later in the story.
So thats your solar system. And what else is out there, beyond the solar system? Well, nothing and a great deal, depending on how you look at it.
In the short term, its nothing. The most perfect vacuum ever created by humans is not as empty as the emptiness of interstellar space. And there is a great deal of this nothingness until you get to the next bit of something. Our nearest neighbor in the cosmos, Proxima Centauri, which is part of the three-star cluster known as Alpha Centauri, is 4.3 light-years away, a sissy skip in galactic terms, but that is still a hundred million times farther than a trip to the Moon. To reach it by spaceship would take at least twenty-five thousand years, and even if you made the trip you still wouldnt be anywhere except at a lonely clutch of stars in the middle of a vast nowhere. To reach the next landmark of consequence,
Sirius, would involve another 4.6 light-years of travel. And so it would go if you tried to star-hop your way across the cosmos. Just reaching the center of our own galaxy would take far longer than we have existed as beings.
Space, let me repeat, is enormous. The average distance between stars out there is 20 million million miles. Even at speeds approaching those of light, these are fantastically challenging distances for any traveling individual. Of course, it is possible that alien beings travel billions of miles to amuse themselves by planting crop circles in Wiltshire or frightening the daylights out of some poor guy in a pickup truck on a lonely road in Arizona (they must have teenagers, after all), but it does seem unlikely.
Still, statistically the probability that there are other thinking beings out there is good. Nobody knows how many stars there are in the Milky Way-estimates range from 100 billion or so to perhaps 400 billion-and the Milky Way is just one of 140 billion or so other galaxies, many of them even larger than ours. In the 1960s, a professor at Cornell named Frank Drake, excited by such whopping numbers, worked out a famous equation designed to calculate the chances of advanced life in the cosmos based on a series of diminishing probabilities.
Under Drakes equation you divide the number of stars in a selected portion of the universe by the number of stars that are likely to have planetary systems; divide that by the number of planetary systems that could theoretically support life; divide that by the number on which life, having arisen, advances to a state of intelligence; and so on. At each such division, the number shrinks colossally-yet even with the most conservative inputs the number of advanced civilizations just in the Milky Way always works out to be somewhere in the millions.
What an interesting and exciting thought. We may be only one of millions of advanced civilizations. Unfortunately, space being spacious, the average distance between any two of these civilizations is reckoned to be at least two hundred light-years, which is a great deal more than merely saying it makes it sound. It means for a start that even if these beings know we are here and are somehow able to see us in their telescopes, theyre watching light that left Earth two hundred years ago. So theyre not seeing you and me. Theyre watching the French Revolution and Thomas Jefferson and people in silk stockings and powdered wigs-people who dont know what an atom is, or a gene, and who make their electricity by rubbing a rod of amber with a piece of fur and think thats quite a trick. Any message we receive from them is likely to begin Dear Sire, and congratulate us on the handsomeness of our horses and our mastery of whale oil. Two hundred light-years is a distance so far beyond us as to be, well, just beyond us.
So even if we are not really alone, in all practical terms we are. Carl Sagan calculated the number of probable planets in the universe at large at 10 billion trillion-a number vastly beyond imagining. But what is equally beyond imagining is the amount of space through which they are lightly scattered. If we were randomly inserted into the universe, Sagan wrote, the chances that you would be on or near a planet would be less than one in a billion trillion trillion. (Thats 1033, or a one followed by thirty-three zeroes.) Worlds are precious.