The Goldilocks Enigma, by Paul Davies

You might think that the world is a pretty miserable place, what with global warming, binge drinking and the decline of grammar. But by cosmic standards things could easily have been a whole lot worse. If the force binding neutrons to protons had been just a little bit bigger or smaller, the stars wouldn't have been able to manufacture carbon, and the whole universe would have been a lifeless desert.

In fact, once we start looking, many physical quantities turn out to have been "fine-tuned" to conveniently life-friendly values. The smallest variation in the constant of universal gravitation, or the mass of the neutron, or the charge of the electron, would have messed everything up. No chemistry, no planets, no life. This is "The Goldilocks Enigma" of Paul Davies's new book. Isn't it curious that the constants of nature are not too big, not too small, but just right? In the words of Fred Hoyle, the bluff Yorkshire physicist who first figured out what a fluke it is that the stars can make carbon, the whole thing smacks of a "put-up job".

Most scientists are unmoved. The constants of nature have to have some value, they say. Why should there be an explanation for their life-friendliness? Just be thankful. The trouble with this response is that there is an obvious explanation to hand: God arranged things. We may no longer appeal to God to explain the evolution of species, at least not outside Kansas. But a benevolent God seems to be just the ticket to explain why the universe should be friendly to life.

Many experts are suspicious of this argument, but there's nothing much wrong with the logic. Of course, no committed atheist is going to be moved, but agnostics will regard the fine-tuning of the universe as a powerful argument for God. Here's an analogy. If you're absolutely sure that the club raffle is decided by chance, you'll think it an amusing coincidence if the organiser's mistress wins. But if you harbour any suspicion that the raffle might be fixed, then the same result will surely increase your belief that it's a put-up job.

Still, as Davies explains, the scientific facts aren't all on the side of the faith community. A number of scientists have started taking the fine-tuning argument seriously, and have come up with an alternative to the God hypothesis. What if our universe is just one among many real universes, each with their own constants chosen at random? This "multiverse" theory would explain the fine-tuning as well as God does. Most of the universes won't be friendly to life, but a few will be just right. Given this, it is scarcely surprising that we should find ourselves in one of the friendly places.

Positing a multiverse may seem a desperate move, motivated only by a desire to keep God at bay. But some physicists can offer independent arguments for the multiverse. One way of understanding the Big Bang is to suppose that the universe we can observe is just one among many "pocket universes" spawned by quantum fluctuations in an eternally inflating uberspace. This might sound crazy, but it is a serious theory, quite apart from solving the fine-tuning problem.

Fine-tuning and pocket universes are just some of the subjects that Davies covers. He is a practising physicist as well as a popular writer, who gives us a whistle-stop tour of every hot topic in foundational physics. His breezy style may sacrifice depth, but it allows him to touch base on everything from quarks to quantum cosmology.

Davies isn't happy with the multiverse hypothesis. He feels that, even if it makes life unsurprising, it still leaves unfinished business. Why do we have this multiverse, rather than a reality unfriendly to life all the way though? Why is there anything at all? By the end, Davies is flirting with the idea that consciousness somehow creates the conditions for its own existence, like the Escher illustration of two hands drawing themselves.

By this stage I thought that Davies had moved a long way from his original puzzle. He started with a well-defined conundrum, about the lucky values of so many physical quantities. The multiverse theory purports to explain this, but Davies complains that it isn't good enough, and we still need to know the meaning of it all. This is changing the rules of the game. Ordinary physics already gives us enough reason to embrace crazy theories, without our trying to answer mystical questions that aren't there.

David Papineau's 'Thinking about Consciousness' is published by Oxford