Anthropic Principle

anthropic principle, in cosmology, any consideration of the structure of the universe, the values of the constants of nature, or the laws of nature that has a bearing upon the existence of life.

Clearly, humanity’s very existence shows that the current structure of the universe and the values taken by the constants of nature permit life to exist. Indeed, it appears that many features of the universe that are necessary for the evolution and persistence of life are the results of unusual coincidences between different values of the constants of nature—quantities such as the mass of the electron, the strength of gravity, or the lifetime of the neutron. The significance, if any, of these coincidences is not understood. What is understood is that, if these quantities were slightly altered, then no form of complexity or life could exist in the universe.

At present, it is not known why the constants of nature take their observed values. Perhaps their values are the only possible ones that they could take; in other words, there may be only one logically possible universe. If so, then it must be judged entirely fortuitous that the combination of values taken by the constants allows the existence of life—or it might be believed that life is so adaptable that it would arise in some form in many possible universes. However, it is likely that at least some of the constants of nature (and possibly all of them) had their values influenced, or even completely determined, by random fluctuations of quantum mechanical origin in the very early history of the universe. If that was the case, then the constants of nature could have taken different values from those observed, and the probabilities of obtaining different collections of values for them might be calculable. It is hoped that superstring theories may eventually be able to predict the values of these constants. The predictions may be probabilistic, and there may exist many logically self-consistent superstring theories of the forces of nature, each with different laws and constants of nature. In this case the likelihood of the universe’s possessing properties that allow life to exist could be evaluated.

The anthropic perspective is also important in evaluating the predictions of quantum cosmological models that make only probabilistic predictions about the structure of the universe. If, for instance, the models ignored the issue of whether living observers could exist, then they might erroneously conclude that one need only compare the observed universe with the types of universe that a particular theory predicts as being the most probable, instead of comparing the observed universe with a universe that would be the most probable given the condition that it permit life to exist and evolve.