The anthropic principle, depending on whom you believe, is either the most profound idea in cosmology or a load of old hokum. Essentially the principle says that the universe has to be the way that it is otherwise we would not be here to observe it in the first place. This may at first glance appear to stating the obvious – but this idea has very deep implications about our universe.
In particular, some cosmologists have appealed to the principle to help explain what is know as the fine tuning problem. The constants of nature such as Planck’s constant, the speed of light, the universal gravitational constant, etc. have very precise values, in fact if some of these constants were even slightly different (i.e. to the nth decimal place) then the physical nature of our universe would be radically different to what it is. Stars, galaxies and in fact life itself would not be able to exist in such conditions. The problem is, of course, why these constants should have values that have made our universe so hospitable to life.
This is a question that science authors have written about before, most notably Martin Rees’ two superb books: Just Six Numbers, and Our Cosmic Habitat. These two books put forward the hypothesis that we live in a multiverse – where the constants of nature have all possible values – leading to some universes being hospitable to life like ours, and others being completely unfriendly to life.
Davies’ book is far more ambitious and sweeping in providing an overview of the anthropic principle than Rees’ work. He considers several explanations including the possibility that we live in a computer simulation, an infinite sea of ‘habitable regions’ in a single universe as predicted by inflation theory, and the possibility of the universe obeying a self organizing principle. The most remarkable suggestion though is Davies own idea that revolves around the delayed choice experiment.
A thought experiment first proposed by John Wheeler, this is a variation of the famous double-slit experiment, except that the detector screen can be removed at the last moment, directing light into two remote telescopes, each focused on one of the slits. In a conventional double-slit experiment, the light acts as waves, causing interference patterns, if no check is made as to which slit a photon goes through, but acts as individual, non-interacting photons if the photons’ paths are checked. In Wheeler’s version there is a ‘delayed choice’ for the observer, not making the decision on which way to observe until after the light has passed the slits. Physicists are currently working on a real version of this experiment, and in fact Wheeler himself suggested an astronomical equivalent of his delayed choice set up – which may give observational weight to his idea.
Davies suggests that our universe may well operate in a similar ‘delayed choice’ manner and that the observations we make now have retroactively fixed into place the constants of nature which existed in a ‘fluid’ state shortly after the big bang.
Unfortunately the material that gets to the heart of the book’s subject matter only takes up its second half. The first half is a very readable account of the current state of big bang cosmology and how it leads to the anthropic principle, but readers who are familiar with these ideas may find the first half of the book a little long winded. Nevertheless this is an excellent read, though some might find Davies’ own ideas at the end of the book a bit impenetrable and improbable.