Introducing the multiverse

Nearly thirty years ago I wrote an article in the journal Nature with Martin Rees [1], bringing together all of the known constraints on the physical characteristics of the Universe — including the fine-tunings of the physical constants — which seemed to be necessary for the emergence of life. Such constraints had been dubbed ‘anthropic’ by Brandon Carter [2] — after the Greek word for ‘man’ — although it is now appreciated that this is a misnomer, since there is no reason to associate the fine-tunings with mankind in particular. We considered both the ‘weak’ anthropic principle — which accepts the laws of nature and physical constants as given and claims that the existence of observers then imposes a selection effect on where and when we observe the Universe — and the ‘strong’ anthropic principle — which (in the sense we used the term) suggests that the existence of observers imposes constraints on the physical constants themselves.

Anthropic claims — at least in their strong form — were regarded with a certain amount of disdain by physicists at the time, and in some quarters they still are. Although we took the view that any sort of explanation for the observed fine-tunings was better than none, many regarded anthropic arguments as going beyond legitimate science. The fact that some people of a theological disposition interpreted the claims as evidence for a Creator — attributing teleological significance to the strong anthropic principle — perhaps enhanced that reaction.