The anthropic principle is the idea that the existence of life, and in particular, our presence as intelligent observers, constrains the nature of the Universe. It was first discussed, in 1957, by Princeton physicist Robert Dicke,1 who argued that for advanced carbon-based life to exist, the Universe had to be roughly the age that we find it to be. Much younger, and there would not have been time for sufficient interstellar levels of carbon to build up by nucleosynthesis; much older, and the golden age of main sequence stars and stable planetary systems would have drawn to a close. Although not everyone was convinced, in 1973 the concept was given its now-familiar name and raised to greater prominence through the efforts of Cambridge physicist Brandon Carter,2 encouraged by the eminent pioneer of quantum mechanics, John Wheeler.
Carter distinguished between two degrees of the hypothesis: the weak anthropic principle, which was essentially a generalization of Dicke's idea, and the strong anthropic principle, which went much further and claimed that "the Universe ... must be such as to admit the creation of observers within it at some stage." It was not long before Wheeler found a crucial role for those observers in his participatory anthropic principle. Somehow, suggested Wheeler, we play a part in actualizing the world in which we find ourselves. Our observations today, at a quantum level, constrain the Universe so that it had to have evolved in precisely the way that would eventually give rise to us.
As for the implications of the anthropic principle for extraterrestrial life, they are controversial. According to one view, if the nature of the Universe is such that it must inevitably give rise to life as we know it, then its biogenic powers have presumably been exercised elsewhere and many times over. Curiously, however, two of the most ardent proponents of the anthropic principle, English astronomer John Barrow and American mathematical physicist Frank Tipler3 have argued that the principle favors the conclusion that we are unique.
1. Dicke, R. H. "Principle of Equivalence and Weak Interactions," Rev.
Mod. Phys., 29, 355 (1957).
2. Carter, B. "Large Number Coincidences and the Anthropic Principle in Cosmology." In M. S. Longair, ed., Confrontation of Cosmological Theories with Observational Data. Dordrecht: D. Reidel (1974).
3. Barrow, John D., and Tipler, Frank. The Anthropic Cosmological Principle. New York: Oxford University Press (1986).