Scientific perceptions of advanced extraterrestrial life are based upon a trio of ideas that first appeared in the religious and philosophical thought of antiquity and the Middle Ages. The first idea is that the universe is very large, if not infinite in extent. The second, that we are not alone in the universe, there are other inhabited worlds somewhere in the vastness of space. The third, that there is an essential difference between the superior beings of the celestial world and the inferior ones who live on Earth. These three ideas are relevant to the work of scientists today. Modern cosmologists have determined that the universe is expanding at an increasing rate and is unlikely to slow down and collapse on itself in a final Big Crunch. Within our immense universe, astronomers have recently identified more than 100 extrasolar planets. An extrasolar planet is one that orbits a star located far beyond our solar system. Some scientists believe that extrasolar planets are inhabited by creatures with a level of intelligence and civilization that surpasses the intellect and civilized life of humans. Astronomers, however, have just begun their investigations and have found no evidence of extraterrestrial civilizations. Any examination of extraterrestrial civilizations must begin with the debt modern science owes to the trio of ideas that shaped our ways of thinking about the universe and its inhabitants. These key assumptions, which appear so often in the modern search for extraterrestrial intelligence, arose in much earlier times and within different contexts.
The Infinitization of the Universe
The ancient Greek atomists were among the first to introduce the idea of an infinite universe. In the fifth century b.c., they claimed that tiny bits of matter (atoms) moved randomly in infinite empty space. Because an infinite number of atoms collide an infinite number of times in an infinite void, an infinite number of universes exist. Each universe has its own sun, planets, stars, and life forms. A century later the influential philosopher Aristotle (384– 322 b.c.) rejected the atomists’ infinite void and their many universes. In its place, he put a single finite universe with the Earth located at its exact center. The planets, Sun, and stars all circle the motionless Earth. The stars marked the outer limits of Aristotle’s geocentric (Earth-centered) universe, and he refused to consider the existence of space beyond the stellar boundaries. There are no voids, vacuums, or empty spaces in Aristotle’s universe because the region between the Moon and the stars is filled with a solid, transparent, crystalline material. Aristotle’s view of the universe explained all known astronomical phenomena and satisfied the ordinary observer’s feeling that the Earth was at the center of things. It lasted for nearly two thousand years and inspired some of the greatest scientific, philosophical, theological, and literary minds in Western civilization. By the fourteenth century, however, critics argued that Aristotle was wrong to place limits on God by confining Him to a finite universe. God extends Himself, they said, filling infinite space with His immensity. Influenced by their conception of an infinite God, philosophers and theologians in the Middle Ages accepted a universe that was infinite in extent. The identification of God with infinite space, sometimes called the divinization of space, lasted well into the seventeenth century. Some Renaissance astronomers and philosophers were not satisfied with the medieval understanding of the cosmos. They more than a theological construct. By interpreting astronomical observations mathematically, they argued, it was possible to obtain a true picture of physical reality. The crucial figure in this intellectual revolution was the Polish astronomer and Church administrator Nicolaus Copernicus (1473– 1543). He proposed a heliocentric (Sun-centered) model of the universe. It featured a stationary Sun at the center of a system of orbiting planets that included the Earth. The Copernican universe remained finite, but it was substantially larger than the old geocentric model made popular by Aristotle. The infinitization of the universe grew out of the fundamental changes Copernicus made in the arrangement of the Sun, Earth, and planets and in the motions of the Earth. By the second half of the sixteenth century, followers of Copernicus claimed that the universe extended to infinity. The first printed illustration of an infinite universe dates to 1576, just thirty-three years after Copernicus published his theory of a heliocentric universe. Most astronomers, if not the general public, soon accepted the infinite nature of the universe. In the seventeenth century, Sir Isaac Newton made a static infinite universe an integral part of his new physics of moving bodies on Earth and in the heavens. Despite the work of generations of physicists and astronomers who succeeded Newton, the precise nature of the universe remains an unresolved problem for modern science.