Paul Davies's new book The Goldilocks Enigma wrestles with some of the deepest philosophical issues around, but concentrates on one in particular: 'why is the world the way it is?'He approaches this question through a discussion of a hot topic in theoretical physics that most scientists refer to as the 'Anthropic Principle', but which Davies chooses to label the 'Goldilocks Enigma'. This refers to the fact that the physical laws that govern the universe are 'just right' for the development of life. Relatively small changes in certain parameters would make it uninhabitable by the likes of us and we wouldn't be here. What should one make of this? Religion has a quick explanation, that God set things up so that we can exist. 'Intelligent Design' is the currently popular name for explanations of physics or biology that invoke a higher intelligence that chose to make the world the way it is. This explanation suffers from the lack of any way to ever test it.

Davies spends much of the first half of the book providing an introduction to the modern scientific view of physical laws and cosmology, working up to the latest and trendiest of these. For more than 20 years now, theoretical physics has been dominated by a very speculative idea known as 'string theory'. Very roughly, this involves replacing elementary particles with objects more like loops, and it crucially requires six extra dimensions beyond the three space and one time dimension we're familiar with.

One must do something like wrap up the six dimensions to make them unobservably small, but then the properties of particles and thus our physical laws depend on how this is done. Initially there was much optimism that there would be only a small number of consistent choices for how to handle the six dimensions, and one of these choices would agree with what we observe. Recent results in string theory appear to show that this isn't the case; instead an unimaginably large number of possibilities exist. Indications are that if one can get our observed universe this way, one can also get just about any variation of it, and legitimate scientific predictions are not possible.

Instead of abandoning string theory as a hopeless cause since it can't predict anything, some string theorists have chosen to promote the idea that our universe is just part of a 'multiverse' of all the nearly infinite possibilities allowed by string theory. One of the few things one can then predict is that we must be in a part of the multiverse that is 'just right' to allow our existence. Debate rages amongst physicists over whether or not this idea is really testable and thus scientific.

Davies provides a careful description of this currently popular multiverse scenario and its explanation for why things are the way they are, including some mind-boggling implications involving infinite numbers of copies of ourselves, and the possiblity that the universe is a simulation. He contrasts it with the common belief among many physicists that there is a simple unique mathematical structure underlying the physical laws that describe the universe. The problem he sees with this belief is that there's no reason to expect that such a mathematical structure should pick out exactly the parameters that are 'just right' for life. But then again, does it really make sense to have any expectations about this? It's not clear that a sufficient answer to the question 'Why is the universe just right for life?' isn't simply: because otherwise we wouldn't be asking the question.

The last chapter of the book moves away from points of view conventional among physicists to some much more speculative answers to the 'why is the world the way it is?' question that Davies finds appealing. These involve some version of the idea that life itself is in some way or other built into the laws of the universe, that they inherently lead to the evolution of life. He looks to information theory and quantum mechanics for hints of how this might come about. Like the multiverse, this kind of speculation tends to suffer from a lack of any known way to test it. The hallmark of the scientific method is the insistence that theories have the property that one can confront them with experiment in a way that allows one to decide whether they work or not. One's answer to the 'why is the world the way it is?' question should be a theory of this kind.

Davies concludes with the admission that, in the end, he finds all the different answers he has examined wanting. He notes that we're the evolutionary products of the pressures of a specific environment, and only recently beginning to be liberated from these. Our minds may still be far too crude and our knowledge of the universe too fragmentary to allow us to perceive the correct answers to these existential questions. In the meantime, Davies has provided an engaging and very readable account of the range of answers we have come up with so far.