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Cosmology

Cosmology

Holes in a final theory?

15 Dec 1997

The Life of the Cosmos
Lee Smolin
1997 Weidenfeld & Nicolson 358pp £20.00hb

Reviewed by Bernard Carr.

There are a some ideas in science – usually those that try to extend it in unorthodox directions, or which trespass too far into the domain of metaphysics – that seem to produce very extreme reactions. Depending on their philosophical propensities, people either passionately embrace or vehemently reject them, but they cannot ignore them. One can confidently predict that the idea explored in this book – that a form of cosmological natural selection could have determined the constants of physics – will fall into this class. When I discussed the author’s proposal in a talk some years ago, a distinguished colleague found the idea so annoying that he left the room in a rage. Doubtless others will react in the same way. On the other hand, judging by the number of papers and meetings it has spawned, many people will find the idea intriguing.

The starting point is the familiar argument that certain features of the universe – in particular the values of various physical constants – seem to be necessary in order for life to arise. This is because life requires chemistry, chemistry requires stars, stars require galaxies, and all of these require special relationships between the constants, which are unlikely to arise by chance. Indeed, although it is unclear where his estimate comes from, Smolin gives the probability of a universe with life-conducive conditions as only 1 in 10229!

One reaction to this situation is to invoke the “anthropic principle”. This proposes that the presence of life explains these coincidences. This idea has been interpreted in two ways – either as evidence for a beneficent creator (God) who made the world for our convenience, or as an indication of the existence of many other universes in which the constants and laws are different. Although the second interpretation is less theological (and may even arise naturally in the context of the inflationary scenario for the early universe), both explanations are untestable and therefore equally liable to be dismissed as metaphysics.

Opponents of the anthropic principle would prefer to argue that the constants will turn out to be determined by some “final theory” that unifies all of physics. The latest developments in superstring theory suggest that we may be tantalizingly close to this holy grail, but the current theories still contain a multitude of coupling constants whose values seem to be arbitrary. Some of these constants may be explained by the final theory, but Smolin argues (and I agree with him) that it is unlikely that this will be the case for all of them. Even if they were explainable, it would be remarkable that the values predicted were precisely those required for life. Therefore there is clearly something to explain, and any world-view that does not come to grips with this must be regarded as incomplete.

Smolin, a physicist renowned for his research in quantum cosmology, offers a radically different approach to the problem. He rejects the explanations offered by the anthropic principle and the final theory, arguing instead that the physical constants (and perhaps even the laws of physics themselves) have evolved to their present form through a process akin to mutation and natural selection. (In other words, the physical constants are contingent rather than fundamental.) He harnesses various physical, biological and philosophical arguments in support of this proposal. The status of each of these three types of argument is very different, so I will discuss them in turn.

The underlying physical assumption is that whenever matter gets sufficiently compressed to undergo gravitational collapse into a black hole, it gives birth to another expanding universe in which the fundamental constants are slightly different. Since our own universe began in a state of great density (i.e. with a big bang), it may itself have been generated in this way (i.e. via gravitational collapse in some parent universe). Cosmological models with constants permitting the formation of black holes will therefore produce progeny – which may each produce further black holes since the constants are nearly the same – whereas those with the wrong constants will be infertile. Through successive generations of universes, the physical constants will then naturally evolve via small random variations to have the values for which black-hole (and hence baby-universe) production is maximized.

Although there is no direct evidence for this startling proposal (and perhaps there never can be), a theory that combines features of the big bang and Darwinian evolution certainly has some appeal. There is no need for the constants to be determined by a final theory (indeed, if they are, the proposal is wrong) and there is no need for the anthropic principle since it is the proliferation of universes and not life that is crucial. In Smolin’s view (although this part of the argument is not altogether convincing) life is just an incidental consequence of a universe having sufficient complexity to give rise to black holes. The proposal also has the attraction of being testable, since it implies that any variation of the constants from their present values should lead to a universe with fewer black holes. The effect of such variations can be calculated: many of the variations would indeed reduce the number of holes, but our knowledge of astrophysics is too incomplete to demonstrate that this would be the case for all of them.

In its own way, of course, this proposal is just as metaphysical as the anthropic one, since it is still necessary to invoke a huge number of other universes whose existence can never be ascertained directly. The idea is therefore equally likely to provoke a hostile reaction in certain quarters. It also depends on highly speculative physics associated with the early universe and black-hole collapse. To analyse the scenario properly, one needs to answer a number of questions. Why are the constants changed at the birth of each baby universe and in what way? How many universes are produced from each black-hole collapse? What happens if the entire universe recollapses, so that all of the black holes it contains merge? Until one has a proper theory of quantum gravity, there can be no consensus on these issues, so it is hard to place much confidence in any speculations based on them. Nevertheless, given that such questions may be resolved eventually, the proposal is an intriguing one. After all, as Smolin emphasizes, natural selection was accepted in biology well before the mechanism of mutation was understood.

The biological aspects of the scenario hinge on the nature of life, and are less controversial. Smolin’s description of life as a self-organizing self-reproducing non-equilibrium process maintained by energy flows from stars and manifested as a hierarchy of ecosystems on different scales is fairly standard. More unusual is his claim that this hierarchy extends all the way up to the galactic scale. This is because non-equilibrium processes in the interstellar medium (involving massive stars and phase transitions associated with molecular cloud formation) play a crucial part in the build up of complexity.

He is anxious to emphasize that this does not mean that galaxies themselves are alive but, within the terms of his own hypothesis, it is not clear why this is the case since most of them probably contain massive black holes in their nuclei (i.e. they can reproduce in the sense that they generate other universes containing more galaxies). Indeed, in view of the reproductive capacity attributed to entire universes, the puzzle is that the author does not extend the hierarchy to the cosmological scale and conclude that the entire universe is alive. The proposal is rather confusing on this point: the distinction between life and complexity is never made very clear, and one senses that something is missing here. Perhaps it is any reference to that other attribute of life – consciousness – a topic that the author studiously avoids.

Smolin’s discussion of the philosophical aspects of his proposal is the least satisfactory part of the book. His overriding philosophical standpoint is that one must reject the “absolute” view of the world espoused by Newton and replace it with the “relational” view of Liebniz. Of course, Newton’s notions of absolute space and time were demolished long ago, but the idea that there may be an absolute reality and even an absolute observer (i.e. God) who stands outside the world still prevails. In the relational view neither of these can exist. There is only a consensual reality created by many different observers, and the laws of physics, such as gauge theories, are a reflection of the fact that different viewpoints must be consistent. He argues that such a view is also implied by quantum theory and is incompatible with what he terms “atomic reductionism” – the notion that the fundamental properties of matter are fixed independent of the rest of the universe. For this reason he also rejects the idea that the fundamental constants can be explained by some final theory.

Discussing these issues certainly serves to put Smolin’s idea in a broader intellectual context, but there are always dangers when physicists stray into the field of philosophy, and I find these arguments unconvincing. One could particularly take issue with his inference that his theory disposes of the necessity for God in that it implies the universe can explain itself. As shown by the deeper discussions of some theologians, this argument is much too simplistic.

It is anyway surprising that he spends so much time arguing this point, since it does not really impinge on the viability of his proposal. Indeed, one criticism of this book is that it is unnecessarily long. Its prime message can be stated very succinctly but, to my mind, too much space is devoted to discussing philosophical and theological issues that are peripheral to the main theme. The book is also rather thin on references: several key figures in the area are uncited and it is disappointing that the closely related scenario of Andre Linde, who has discussed the anthropic principle in the context of the inflationary model, is not mentioned (see “Sizing up the universe” by Andrew Liddle Physics World November pp47-48).

Despite these criticisms, this is a provocative and intriguing book, and at least some readers will be in the “passionately embracing” category. After all, any theory that weaves together three of the most important ideas in the history of science – quantum theory, relativity theory and evolution theory – must have something going for it. Indeed, if it did turn out to be correct, it would surely qualify as one of the most important developments in the history of science. The probability of this will be judged to be small by the “vehement rejecters”, but the idea is still worth investigating and this book makes a valiant first attempt.

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