anthropic principle: a standalone construct

The cosmologist Brandon Carter introduced the anthropic principle in 1974, in a paper on large-number coincidences in cosmology, arguing that any statement about the universe must allow for the fact that we, the observers making it, exist within that universe and could only have arisen in a place and time that permits observers. He drew a distinction still used today between a weak version, which treats the values we measure for physical quantities as biased by the requirement that our location support observers, and a strong version, which holds that the universe itself must have properties allowing observers to develop at some stage. The book-length treatment by John Barrow and Frank Tipler in 1986, The anthropic cosmological principle, collected the surrounding physics and gave the idea its standard reference. The principle is invoked to address why the observed values of certain physical constants and cosmological parameters fall in the narrow ranges that permit stars, chemistry, and life: on this reasoning such values are partly a selection effect, meaning that observers can only find themselves in a universe, or a region of one, whose parameters already allow observers to exist. It is a principle about observational selection rather than a measurable quantity, so it constrains which explanations of apparent fine-tuning are admissible instead of predicting a specific number.[1][2]

Unit

Not applicable: a reasoning principle about observational selection, not a measured quantity

Recovery means

Not applicable

Measured by

Applied as a constraint in cosmological reasoning: a candidate value or explanation is weighed against the prior requirement that the universe, or the region an observer occupies, contain observers, for example by conditioning probabilities on the existence of observers