The Anthropic Principle

First formulated by Brandon Carter in 1973, the anthropic principle observes that any law or constant we can measure is, by construction, compatible with our existence as observers. The principle has a weak form (the universe is necessarily compatible with observers in the region where observers exist) and a strong form (the universe must be such that observers exist somewhere). Each is, in different ways, a partial dissolution of the fine-tuning puzzle.

The weak form is uncontroversial. It is a sampling-bias correction: of all the values the cosmological constant or the fine-structure constant could in principle take, we measure the ones compatible with our existence because we exist to measure. The weak anthropic principle is consistent with there being a single universe whose constants are arbitrary; we happened to land in it because nothing landed elsewhere.

The strong form is more ambitious and more contested. It claims that the universe must have produced observers — that there is a structural necessity to the observer-permitting nature of physical law. Several routes to the strong form exist: (a) theistic design, (b) a multiverse in which all possible universes exist and observers necessarily inhabit the observer-permitting ones (Tegmark, Carr, Rees), (c) a self-consistent universe in which the laws are tuned to produce observers (Smolin's cosmological natural selection, in which black holes produce daughter universes with slightly varied laws, and the laws that produce more black holes are statistically favored).

The principle is often dismissed as tautological — "of course we observe a universe that permits observers" — but the tautology hides a deep puzzle. Why is the universe such that any combination of constants permits observers? The space of mathematically possible physical constants is vast; the subspace compatible with structure formation, with chemistry, with biological information processing, is a tiny island in that vast space. The anthropic principle does not explain why the island exists; it only explains why we, if we exist, are on it.

The contemporary status of the principle is that it is a necessary corollary of any complete physical theory but is rarely a sufficient explanation. The deeper question — why the multiverse, or the self-consistent constraint, exists in the first place — falls to layer 8.