Edition: Oxford University Press, 1996
Review number: 355
It may seem that there is not very much to be said about the anthropic principle, that it is an interesting sideline in the philosophy of science which may have a minor role in explaining why the universe is the way it is. To Barrow and Tipler, it has formed the peg around which a seven hundred page book can be written, one which takes the reader on a survey of cosmology, theology, the future of the human race, and the existence of extra-terrestrial intelligence. It is a fascinating book, occasionally rather on the mathematical side for a popular science book.
The anthropic principle, as discussed by Barrow and Tipler, comes in three varieties, with a "Final" form as well as the more familiar "Weak" and "Strong" versions. The Weak Anthropic Principle is hardly contentious. It merely says that the existence of carbon based life is an observed fact, so that the universe must have properties which make such life a possibility. Barrow and Tipler make as strong a case as is possible for the explanatory power of this idea, but I still feel that it is limited. It may explain, for example, that the universe has to be large even if the Earth is the only planet containing life (to have expanded for long enough for galaxies to form and supernovae to occur to create some of the elements we require), but not why the universe happens to be this large. All the principle states is that if it weren't, we wouldn't be here to observe the fact. However, most of what can be inferred from it doesn't actually require the presence of life; the example I've referred to could be deduced just as logically from the existence of uranium. (Life is a sufficiently complex phenomenon that it requires a large collection of such pre-conditions, so the anthropic principle is a convenient summary of many similar explanations.) It also involves the deduction of causes from effects, and that is something which requires a great deal of care, to say the least.
The stronger versions of the anthropic principle are far more contentious, and more closely related to the design arguments used to "prove" the existence of God from the appearance of design in the universe. (These arguments are summarised in an excellent historical overview which forms the first chapter of the book.) The standard strong principle says that life must evolve at some point in the history of the universe, rather than that it just has evolved. As Barrow and Tipler point out, this means that life can be said to be part of the "purpose" of the universe in some way, and this doesn't make much sense without the deduction that life must at some point have a measurable effect on the whole cosmos. This point leads into a lengthy discussion of just what this effect could possibly be, which is fascinating but extremely speculative. The first point made is that it is very difficult to imagine any way in which a species confined to a single star system could affect the universe. So interstellar travel is a necessary development, and that requires intelligence. This is the motivation behind the authors' formulation of what they call the Final Anthropic Principle, which states that intelligence must at some point arise and never die out.
The discussion of how interstellar (and, indeed, intergalactic) travel could be developed is fascinating and seems convincingly feasible. Their ideas are based on the theoretical von Neumann machine, which is basically a machine which can create replicas of itself. A von Neumann machine could be made a space probe that seeks out a star likely to have the resources to enable replication (using a strategy based on analysis of the Polynesian colonisation of the Pacific islands), and then copies itself. Given sufficient processing power to be considered intelligent and a sufficient density of planetary systems - considered likely in current astronomy - this would amount to colonisation of the galaxy by intelligent systems over a period of several thousand years.
In fact, these arguments are sufficiently convincing that they are used to support the idea that there is no more advanced race of beings in the galaxy than humanity, because we should now have been contacted by probes of this sort. (Even if they did not want to directly contact other forms of life, the action of such a probe on reaching the solar system would probably be detectable.) The idea that we are alone in the galaxy, however, contradicts the equally convincing "Copernican Principle", that there should not be anything particularly special about the Earth - we are just a small planet orbiting a typical star in a typical part of the galaxy. The only way to reconcile this with the idea that a society only slightly more advanced than we are would have contacted us - and Barrow and Tipler estimate that von Neumann probes will be economically viable in a few centuries at most - is to argue that some catastrophe almost always destroys a civilisation between two and six hundred years after the Industrial Revolution (or its equivalent). This pessimism may seem justified in a society facing possible nuclear devastation, social disintegration and ecological disaster.
The authors do not dwell on this. It is really a long - and fascinating - digression. The main thread of the argument is rejoined with a discussion of the end of the universe in which some form of intelligent life has basically colonised the whole, and is trying to circumvent in some way the 'heat death' predicted by thermodynamics. This part is necessarily very speculative (cosmologists do not even agree on the broad details of how the universe will end), but certainly represents just about the only feasible way in which life could affect the whole universe.
The Anthropic Cosmological Principle is an extremely complex book, and is exactly the kind of science book I enjoy, finding a peg to discuss a large number of fascinating ideas that turn out to be connected despite appearances. The earlier chapters, about the Weak Anthropic Principle, are solid expositions of material which I've seen before (and which will probably be familiar to most people with an interest in the philosophy of science). The later writing, about the stronger principles, contains much less well-known science. I suspect that these versions of the anthropic principle are probably wishful thinking, the outcome of the desire to feel that we are significant. This doesn't invalidate much of the science contained in the book, which is an excellent one.