Parallel Worlds By Michio Kaku ALLEN LANE pounds 20 pounds 18 (P&P FREE) 08700 798 897
Max Planck once commented: "Science cannot solve the ultimate mystery of nature. And it is because in the last analysis we ourselves are part of the mystery we are trying to solve." Maybe this is true. After all, cosmology is based on beautiful geometry but quantum physics resembles a cosmic betting shop where anything could happen (but probably won't). They don't quite fit together in a "theory of everything", but they should.

A theory of everything would answer most of the fundamental questions we have. We live in three spatial dimensions now, but are there more? If so, why do things from our world (like smoke, which seems to go everywhere else) not disappear into, say, a fourth spatial dimension? (It would seem to me that it's because we do not make four-dimensional smoke, but perhaps I digress...) We may not be able to physically "go" to other dimensions, but we can certainly do maths in them. When we do, it turns out that things start to simplify, just as a city is easier to comprehend from the top of a hill than from ground level. Kaluza-Klein theory (a 1921 formulation of Einstein's equations in five dimensions) suggested that light could be a ripple from the fifth dimension. More recently, string theory asked if sub-atomic particles are vibrations on 10-dimensional or 26-dimensional superstrings. M-theory, which deals with membranes instead of strings, is, currently, the leading candidate for being the theory of everything.

It is probable that there are other dimensions within our universe, but could there also be other universes? We may recall the Schrodinger's Cat thought experiment, in which a cat is both alive and dead until we observe it (and the wavefunction - where all probabilities exist at once - collapses). To answer the questions posed by this experiment, Kaku tells us, "physicists have been forced to entertain two outrageous solutions; either there is a cosmic consciousness that watches over us all, or else there are an infinite number of quantum universes."

Of course, Big Bang theory suggests that our universe began life as something like a quantum particle. Is our universe then the result of its collapsed wavefunction, with God the observer? Or are all the other probabilities floating around somewhere? That the universe we live in does appear rather special - all the factors for life are so finely tuned - suggests that our creation was not accidental. Here we are, after all, with a nice planet, exactly the right distance from the sun, with water and food and no nasty black hole just around the corner. But it seems this creator wasn't actually that sure about us. In a few trillion years this "fine tuning" means that all life will be obliterated.

It is the obvious question - what the hell does humanity do then? - that forms the main focus for this book. If all the world's a stage, as Kaku suggests, then our future script will need to say something like: "Exeunt, all of humanity, stage left (carrying a time machine)". Tunnelling into another universe, or back in time, may well be the only way out. Even if we don't just end up as toast or permafrost when the sun explodes or dies, we'll have had it when the whole universe winds down. Kaku explains how, even if we became pure energy, towards the end of the lifespan of the universe we'd only be capable of one thought every trillion years. (Insert your own student joke here.)

If we can progress from a type 0.7 civilisation to a type III without blowing ourselves up, it seems we are condemned to spend eternity trying to fish wormholes out of the foamy lumps of space-time, to tie knots in gigantic cosmic strings, or working out a way of exiting via a black hole without being ripped apart. There are various recipes for this escape, but the basic ingredient in all of them appears to be negative energy, which, needless to say, you can't buy at the universe corner shop. But perhaps M-theory will provide the oven.

This book is absolutely impossible to put down. Kaku doesn't provide enough information in places (the section on supersymmetry is impossible for a non-scientist to understand) but he leaves you with the unavoidable conclusion that if and when we do find out what the universe is, and how it was created, it's going to be absolutely mind-blowing. He doesn't ask why we would actually want to survive the end of the universe (is it just me, or is there something a bit virus-like about planning to send our DNA through a wormhole to make sure humans thrive beyond the end of time?), but he asks pretty much everything else. Perhaps there really is another universe less than an atom away from your nose. And perhaps heaven is out there, too.