When considering the evolution of intelligent life in the entire universe, scientists are confronted by two stupefying vastnesses: the improbability of the evolution of complex life, and the prodigious expanses of the universe. In a universe with possibly a hundred billion galaxies, each with billions of stars, even the least probable event may be expected to occur occasionally, even frequently.

Humans will probably never know whether or not the universe contains other life-forms with a human level of intelligence. Even if there were thousands of other civilizations in the universe, could they contact humans, or humans contact them? Light and other forms of photonic transmission require millions of years to travel among galaxies. Human observers would learn about these other civilizations only if they had evolved to the extent that they could have sent messages millions of years ago; and by now, they may no longer exist. Another problem is the ability to recognize a generalized transmission from another civilization. This is the idea behind SETI--the Search for Extraterrestrial Intelligence: one has to look everywhere for anything that might be a signal, perhaps an irregularity in what had been considered the lifeless throbbing of a pulsar.

Because of the enormous resources that deep space communication and travel would require, the fact that humans have neither been visited nor contacted from outer space means little. Science fiction speculates about the possibility of travel through wormholes or through other dimensions, but any travel through a wormhole or a black hole would probably result in the destructive scrambling of whatever goes through it.

The equation first formulated by astronomer Frank Drake allows a rough estimate of how many planets with advanced civilizations might exist in the Milky Way galaxy. The equation consists of a series of probabilities: the fraction of stars that have planetary systems, the number of planets in a system that have ecological conditions suitable for life, the probability that life will evolve, the probability that intelligence will evolve, and the probability that advanced technology will develop. The problem with these calculations lies with assigning values to the probabilities. Cosmologists disagree greatly on even the order of magnitude of some of them. . .

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