The Anthropic Principle

The Anthropic principle is the idea that the existence of the Universe is intimately related to the presence of life. The term ‘anthropic’ derives from the Greek word for ‘man’. This is really a misnomer since most of the arguments pertain to life in general rather than humans in particular. The principle exists in two distinct forms, known as the weak and strong versions.

The weak anthropic principle (WAP) arises from the notion that any observations made by astronomers will be biased by selection effects that arise from their own existence. Characteristics of the Universe that appear to be quite improbable may merely arise from the fact that certain properties are necessary for life to exist. Cosmologist John D. Barrow has given this definition of the WAP: the observed values of all physical and cosmological quantities are not equally probable, but take on values restricted by the requirements that (1) there exist sites where carbon-based life can evolve and (2) the universe be old enough for it to have already done so.

The strong anthropic principle (SAP) goes further, stating that the Universe must have fundamental properties that allow life to develop within it at some stage in its history. This implies that the constants and laws of nature must be such that life can exist. A  number of quite distinct interpretations of the SAP are possible, including the suggestion that there exists only one possible Universe ‘designed’ with the goal of generating and sustaining ‘observers’ – life. The American mathematician John Archibald Wheeler has pointed out that this argument can be interpreted as implying that observers are necessary to bring the Universe into being, an idea he calls the ‘participatory  anthropic principle’ (PAP). A third possible interpretation of the SAP is that our Universe is just one of an ensemble of many different universes, and that by chance its properties are optimized for the existence of life. This idea is consistent with the ‘many worlds’ or ‘sum-over-histories’ approach of quantum cosmology which requires the existence of many possible real ‘other universes’.

To start with a very simple illustration of an anthropic argument, consider the question, why is the universe as big as it is? The mechanistic answer is that, at any particular time, the size of the observable universe is the distance traveled by light since the big bang. Since the universe’s present age is about 10¹⁰ yr, its present size is about 10¹⁰ light-years. Inherent in this unambiguous answer is the belief that there is no compelling reason the universe has the size it does; it just happens to be 10¹⁰ yr old. There is, however, another answer to this question, one given by Robert Dicke. His argument runs as follows. In order for life to exist, there must be carbon or at least some form of chemistry. Now carbon is produced by cooking inside stars and this process takes about 10¹⁰ yr. Only after this time can the star explode in a Supernova, scattering the newly baked elements throughout space, where they may eventually become part of life-evolving planets. On the other hand, the universe cannot be much older than 10¹⁰ yr; else all the material would have been processed into stellar remnants such as white dwarfs, neutron stars and black holes. Since all the forms of life we can envisage require the existence of stars, this suggests that life can only exist when the universe is aged about 10¹⁰ years.

Questions to Ponder

• What is the fundamental difference between the Strong & the Weak Anthropic Principle?
• How is the Anthropic Principle related to the Intelligent Design Theory?