Skip to contentWillowOne's Positions
TagsWhat’s newMediaStatistics
Search
Notes
WillowOne's Positions

Anthropic Principle

Overview

The anthropic principle is the observation that what we can expect to observe about the universe is constrained by the fact that we exist to observe it. A universe whose constants did not permit observers would contain no one to be puzzled by them; so finding ourselves in a universe that permits observers is not, by itself, surprising, and not evidence that it was arranged for us. The term is from the Greek anthropos, "human", and was coined by the physicist Brandon Carter in a 1973 paper for a symposium marking Copernicus's 500th birthday, as a corrective to the Copernican principle: we need not occupy a privileged position in the universe, but we necessarily occupy one in which observers can exist.

Carter distinguished two forms:

  • The weak anthropic principle: the observed values of physical and cosmological quantities are not equally probable; they are restricted by the requirement that there be places and times at which observers can exist. This is a selection effect, and it is close to a tautology.
  • The strong anthropic principle: the universe must have those properties that allow life to develop within it at some stage in its history. Carter's own version of this was conditional, as a way of thinking about ensembles of universes; the stronger teleological readings, in which the universe is somehow required to produce observers, were added later, notably by Barrow and Tipler (The Anthropic Cosmological Principle, 1986).

Notes

  • The weak principle is the standard reply to the Fine-Tuning Argument and the Teleological Argument. The apologist says the constants are improbably suited to life, so they were set by a designer. The reply is that we could only ever observe constants suited to life, so the observation carries no information about how they were set. It is a selection effect of the same kind as Survivorship Bias: the planes that came back are the only planes you can inspect.
  • The weak principle does not by itself explain why the constants have the values they do. It only removes the inference from "suited to us" to "designed for us". Whether there is one universe or many (the multiverse hypothesis), and whether the constants could have been otherwise, are separate questions that the principle does not answer.
  • The strong principle, in its teleological readings, is close to the design argument it was meant to replace, and most physicists who use the term mean the weak one.
  • The Boltzmann Brain Hypothesis is an anthropic argument turned against a cosmology: if a model predicts that most observers are random fluctuations, then observers like us are not what the model predicts, and that counts against the model.

Also see

Wikipedia: https://en.wikipedia.org/wiki/Anthropic_principle

Sources

  • Brandon Carter, "Large Number Coincidences and the Anthropic Principle in Cosmology", in M. S. Longair (ed.), Confrontation of Cosmological Theories with Observational Data (1974), pp. 291–298.