How do we interpret the anthropic coincidences

Even if one accepts that a multiverse exists and - contrary to Smolin's picture - gives rise to anthropic selection effects, there is still considerable ambiguity in how one interprets this. What determines the selection, or, more precisely, what qualifies as an observer? Is it just human beings, or life in general? Is some minimum threshold of intelligence required, or does the mere existence of consciousness suffice? In addressing these questions, I will necessarily veer into more philosophical domains.

As mentioned earlier, although 'anthropos' is the Greek word for 'man', the arguments have nothing to do with humans in particular. Indeed, Brandon Carter (who coined the term) admits that its introduction was unfortunate. Therefore anthropic arguments do not necessarily enhance the status of human beings or support the religious view that we have a special role in the Universe. This interpretation may still be possible if humans turn out to be the only form of life in the Universe. In this context, it is interesting that Carter has argued that we may be the only site of life within our cosmological horizon [37]. He infers this from the remarkable coincidence that the time for life to arise on Earth seems to have been comparable to the cosmological time.

Even if this were true, most cosmologists would still be reluctant to attribute great significance to humans in particular. Therefore it is more traditional to associate the anthropic constraints with life in general. In fact, Davies explicitly associates them with a 'life principle' [38]. Until recently, science would have regarded the existence of life as an incidental rather than fundamental feature of the Universe. Indeed, in the nineteenth century, the second law of thermodynamics was taken to imply that the Universe must eventually undergo a 'heat death', with life and all other forms of order inevitably deteriorating. However, recent developments in cosmology have led to a reversal of this view. According to the Big Bang theory, the history of the Universe reveals an increasing rather than decreasing degree of organization, and modern physics - without any recourse to divine intervention and without any violation of the second law of thermodynamics - is able to explain this. Heat death is avoided because local pockets of order can be purchased at the expense of a global increase in entropy, and, if the Universe continues to expand forever, intelligent beings may be able to delay their disintegration indefinitely [39].

Some of the types of organization which exist in the Universe are summarized in the so-called 'Pyramid of Complexity', introduced by Reeves [40] and reproduced in Fig. 5.1. This shows the different levels of structure as one goes from quarks to nucleons to atoms to simple molecules to biomolecules to cells and finally to living organisms. This hierarchy of structure reflects the existence of the strong force at the lower levels and the electric force at the higher ones. As one ascends the pyramid, the structures become more complex - so that the number of different patterns becomes larger - but they also become more fragile. The pyramid becomes narrower as one rises, and this reflects the fact that the fraction of matter incorporated into the objects decreases as the degree of organization increases.

The Big Bang theory explains when these structures arise because the Pyramid of Complexity only emerges as the Universe expands and cools. At early times, the Universe is mainly in the form of quarks. Neutrons and protons appear at a few microseconds, light nuclei at several minutes, atoms at a million years, and - following the formation of stars and planets -molecules and cells at ten billion years. The Big Bang theory also explains time A

Fig. 5.1. Summary of the different levels of structure which exist in the Universe and how this has arisen during the Big Bang.

why the pyramid came about. The key point is that structures arise because processes cannot occur fast enough in an expanding universe to maintain equilibrium. If it had its way, each type of force would always form the objects which are most stable from its own perspective (e.g. the strong force would turn all nuclei into iron; the electric force would turn all atoms into noble gases; and gravity would turn all matter into black holes). However, all variety would be lost if this were the case, and it is only the disequilibrium entailed by the rapid expansion of the Universe which prevents this.

For example, the reason all nucleons do not go into iron as a result of cosmological nucleosynthesis is that the Universe is expanding too fast for most nuclei to interact with each other at this time. The reason gravity does not turn all stars into black holes is because the pressure associated with nuclear energy release and eventually quantum effects support them against gravity. The forces may eventually attain their goal, but only after an enormous length of time and, even then, only for a limited period. (For example, if the Universe exists long enough, everything may eventually end up in black holes, but on a still longer timescale these black holes will evaporate into radiation.) As emphasized in Table 5.1, it is only the anthropic fine-tuning of the coupling constants that allows the ascension of the lower levels of the pyramid. Therefore, the Pyramid of Complexity can only arise in a small subset of the ensemble of universes.

Note that there is an important difference between the structures which exist at the top and the bottom of the pyramid. Those at the bottom are stable and need large amounts of energy to destroy them, while those at the top must be constantly maintained by exchanging energy with the outside world. More precisely, they must extract information from the world, and the second law of thermodynamics requires that this process is inevitably accompanied by the release of entropy. A store of information arises whenever there exists a source of entropy which has not been released by previous processes. For example, nuclear information is contained in nuclei other than iron, and this can be extacted by nuclear burning inside the Sun, the ultimate source and sustainer of all life-forms on Earth. Similarly, living organisms can feed on plants, and humans can exploit fossil fuels, because these things contain complex molecules with consumable electromagnetic information. Thus there is an inevitable link between complexity and life, and the key to this link is information.

Another crucial ingredient at the top level of the pyramid is competitiveness. This is a vital factor in evolution because, as a population grows, the competition for food leads to predation and increasingly sophisticated survival strategies. The proliferation of life-forms due to mutation plays a crucial role in this process. Different modes of perception and motor activity are also required, and this leads to the development of organisms with a central nervous system. From this perspective, brains - certainly the most complex structures on Earth - are merely data integration systems, and the main purpose of intelligence is to increase survival efficiency. Minds, of course, might be regarded as the ultimate storers and extractors of information.

Figure 5.1 suggests that the anthropic fine-tunings are more related to the emergence of complexity than life or minds; they could equally well be regarded as prerequisites for inanimate objects such as motor cars or TV sets. However, here on Earth at least, the development of minds seems to have occurred relatively quickly once the first signs of life arose, so it is conceivable that this applies more generally. Provided there are no extra 'biological' fine-tunings required for the higher levels of the pyramid to arise, the evolution of complexity may inevitably (and fairly rapidly) lead to life and consciousness. In this case, the distinction between life and complexity is not so clear-cut. The former is just a particular realization of the latter and may naturally emerge from it. Therefore the question of what constitutes an observer may be rather incidental. Complexity appears to be the key, and that encompasses everything. From this perspective, the term 'Complexity Principle' would be preferable to 'Anthropic Principle'.

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