Jacket of The Hidden Reality by Brian Greene

The Hidden Reality by Brian Greene (Allen Lane, £25)

The laws of physics “appear” to be fine-tuned for us to be here. Take the force of gravity. If it were ever so slightly weaker, the material in the heart of stars would not be crushed and heated enough to ignite – there could be no Sun. On the other hand, if gravity were ever so slightly stronger, stellar interiors would get so hot that stars would burn out in less than a billion years – too short a time for the evolution of complex planetary life.

What are we to make of this apparent fine-tuning? One logical possibility – though one not supported by other observational evidence – is that a Supreme Being fine-tuned the laws of physics for life to arise. Another is that there are a large number of universes, each of which has different laws of physics, and we find ourselves in one where those laws are fine-tuned for us to exist. After all, how could we not?

Recently, a bishop in New Zealand – who shall remain anonymous – read one of my books and, on his blog, remarked on the lengths scientists will go to to avoid a God explanation. Incredibly, he said, they are even willing to countenance the more complex concept of a “multiverse”.

Putting aside the fact that, paradoxically, a multiverse may actually be simpler than a single universe, this is to completely misunderstand the process of science. Scientists have no freedom to avoid any truth, no matter how unpalatable to them. The ultimate arbiter of what is is not what they would like but the evidence provided by nature. And, currently, nature is banging us over the head and yelling at us repeatedly that this is not the only universe.

The evidence comes not just from the fine-tuning of the laws of physics but from many other directions as well. And, in The Hidden Reality, string theorist and best-selling popular science writer Brian Greene sets out to survey comprehensively all those different multiverses and their motivations.

There are multiverses in which, beyond the “horizon” bounding the universe, there exist other domains where all possible histories are played out. There are multiverses in which all such universes are merely inhabitants of a single bubble-universe among countless others in an eternally expanding sea of “inflationary” vacuum. There are multiverses in which such bubble-universes are contained like Russian dolls within even bigger bubble-universes, each with different laws of physics (this is the view of Greene’s beloved string theory). And I can go on. And on – as Greene does.

The question is not whether a multiverse is likely – I think we have now gone way beyond that point – but how all these visions of multiverses dovetail together. “No one knows”, admits Greene with refreshing honesty at the end of his clearly written and accessible exposition of the present state of fundamental physics. These are indeed exciting times. How many people are lucky enough to be alive when the conception of the universe is undergoing such cataclysmic revision?