Anthropic Principle

The anthropic principle is an interesting, though highly controversial hypothesis in modern cosmology, which suggests that the universe evolved after the big bang in just the right way so that life, especially intelligent life, could develop. The proponents of this hypothesis contend that the fundamental physical constants of the universe (such as the speed of light [c] and the Planck constant [h]) actually support the existence of life and (eventually) the emergence of conscious intelligence—including, of course, human beings. In fact, the advocates of the anthropic principle are quick to point out, if the universe was not so suitable for life, people would simply not be around today inquiring about why things in the universe are so finely tuned and adjusted. The supporters of this hypothesis further suggest that with just a slight change in the value of any of these fundamental physical constants, the universe would have evolved very differently after the big bang.

The hypothesis that there could exist many different universes—each with different values of the physical constants—is sometimes referred to as the strong anthropic principle. Among these numerous possible universes, human beings emerged in the one particular universe—the one that contained just the right physical constants to permit carbon atoms to form and eventually to serve as the building blocks of living systems.

In contrast, the weak anthropic principle gives no operational meaning or significance to the concept of other universes; recognizes and accepts the importance of the physical constants that are found in and define the present universe; and seeks to interpret the significance of these physical constants with respect to the rise of life and consciousness. For example, soon after the big bang, only hydrogen and helium existed. Physicists who support the weak anthropic principle try to relate the creation of carbon and the other elements necessary for life within the context of a universe governed by the existing physical constants.

In exploring the implications of the anthropic principle, scientists sometimes like to play "what-if mind games or resort to gedanken (thought) experiments. For example, if the force of gravitation (as manifested by the gravitational constant G) was weaker than it is, the expansion of matter after the big bang would have been much more rapid, and the development of stars, planets, and galaxies from extremely sparse (nonaccreting) nebular materials could not have occurred. No stars, no planets, no development of carbon-based life—as scientists currently know it. If, on the other hand, the force of gravitation was stronger than it is, the expansion of primordial material would have been sluggish and retarded, encouraging a total gravitational collapse (that is, the big crunch) long before the development of stars and planets. Again, no stars, no planets, no life.

Opponents of the anthropic principle (weak or strong) suggest that the values of the fundamental physical constants are just a coincidence. Within their line of reasoning, intelligence is not regarded as an inevitable byproduct of the evolution of matter and energy in a universe governed by the current physical constants. Rather, the evolution of biological systems with intelligence is only one of a number of possible outcomes.

The anthropic principle remains a lively topic for debate and speculation within the scientific community. Does the presence of the human species on Earth today represent the



inevitable byproduct of an evolving universe defined by its physical constants in just such a way so as to bring forth intelligent human consciousness? Does the anthropic principle apply elsewhere in the universe, or are human beings the most advanced, intelligent species thus far evolved in the entire universe? Until this controversial hypothesis can actually be tested, however, it must remain—by the rules of the scientific method—outside of the mainstream of demonstrable scientific principles.

technology and then speculates on where the ever-increasing complexity of matter is leading. The emergence of conscious matter (especially intelligent creatures), the subsequent ability of a portion of the universe to reflect upon itself, and the role and destiny of these intelligent creatures are topics often associated with contemporary discussions of cosmic evolution. One interesting speculation is the anthropic principle—namely, the hypothesis that the universe was designed for life, especially the emergence of human life.

The cosmic evolution scenario is not without scientific basis. The occurrence of organic compounds in interstellar clouds, in the atmospheres of the giant planets of the outer solar system, and in comets and meteorites suggests the existence of a chain of astrophysical processes that

This artist's rendering symbolically represents complex organic molecules, known as polycyclic aromatic hydrocarbons, observed in the early universe. Scientists consider these large molecules, comprised of carbon and hydrogen, among the building blocks of life. (NASA/JPL/Caltech)

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