The Anthropic Principle revisited

Bernard Carr

Astronomy Unit, Queen Mary, University of London

5.1 Introduction

My brief historical overview in Chapter 1 alluded to the crucial influence of the Newtonian mechanistic picture on the development of our view of the Universe. According to this, the cosmos operates likes a giant machine, oblivious to whether life or any form of consciousness is present, i.e. the laws of physics and the characteristics of the Universe are independent of whether anybody actually observes them. In the last fifty years, however, the Anthropic Principle has developed [1], and this might be regarded as a reaction to the mechanistic view. This claims that, in some respects, the Universe has to be the way that it is because otherwise it could not produce life and we would not be here speculating about it. Although the term 'anthropic' derives from the Greek word for 'man', it should be stressed that most of the arguments pertain to life in general.

As a simple example of an anthropic argument, consider the following 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 travelled by light since the Big Bang, which is about 1010 light-years. There is no compelling reason the Universe has the size it does; it just happens to be 1010 y old. There is, however, another answer to this question, which Robert Dicke [2] first gave in 1961. In order for life to exist, there must be carbon, and this is produced by cooking inside stars. The process takes about 1010 y, so only after this time can stars explode as supernovae, 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 y, else all the material would have been processed into stellar remnants. Since all the forms of life we can envisage require stars, this suggests that it can only exist when the

Universe or, ed. Bernard Carr. Published by Cambridge University Press. © Cambridge University Press 2007.

Universe is aged about 1010 y. So the very hugeness of the Universe, which seems at first to point to our insignificance, is actually a prerequisite of our existence. This is not to say that the Universe itself could not exist with a different size, only that we could not be aware of it then.

Dicke's argument is an example of what is called the 'Weak Anthropic Principle' and is no more than a logical necessity [3]. This accepts the constants of nature as given and then shows that our existence imposes a selection effect on when (and where) we observe the Universe. Finding that we live at a particular time is no more surprising than finding that we live at a particular place (e.g. on a planet near a star). Much more controversial is the 'Strong Anthropic Principle', which - in the sense that I will use the term - says that there are relationships between the coupling constants (i.e. the dimensionless numbers which characterize the strengths of the four forces) and other physical quantities which are necessary in order for observers to arise. Some of these relationships are remarkably 'fine-tuned' and do not seem to be predicted by standard physics.

Chapter 1 also referred to the paper on the subject I wrote for nature in 1979 with Martin Rees [4]. This turned out to be quite an influential article because it brought together all the anthropic arguments that were known at the time. In this chapter I will revisit some of these arguments to see how their status has changed. However, I will not give the full details since they can they found in our original paper and also in ref. [1]. I will then consider how one might interpret the anthropic relationships and discuss whether the multiverse proposal provides the best conceptual basis for understanding them. Naturally, other contributors will consider this point - since it is one of the main themes of the book - but without coming to any general consensus.

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