In Search of the Multiverse by John Gribbin Wiley HK$200 How many versions of you are there - one or millions? Some physicists believe that an infinite number of universes exist parallel to the one we live in. These universes might only differ slightly from ours: for instance, one might contain a version of you with blue hair instead of black, or three thumbs instead of two. Or they could be entirely different universes with laws of physics completely at odds with our own. In In Search of the Multiverse, John Gribbin sets out the arguments for the existence of such entities. This may sound like a case of scientific imagination running riot, but it has been taken seriously in university physics departments since the turn of the millennium. That's because it's now the only theory than can explain why particles at a quantum level - that is, the atomic level and smaller - behave the way they do. Gribbin's book is written for the layman reader, and he - rather amazingly - manages to explain a subject which is generally expressed in the language of mathematics without using a single equation. In Search of the Multiverse starts with a basic introduction to quantum physics and then moves on to analyse the two main multiverse hypotheses: the 'many worlds' hypothesis and the 'cosmic landscape' hypothesis. The existence of the multiverse has not been proved, but quantum computing - a subject which is being investigated in science departments (and, because of its possibilities for code-breaking, defence departments) - opens up a path to proof. Gribbin spends time explaining this, too. It's a well-written book that achieves its purpose of educating the reader. But it does treat a highly controversial idea as proven, and fails to mention counter arguments to the existence of the multiverse. So what's it all about? The story starts with quantum physics, which is the science of how particles behave at the size of atoms and smaller. Non-scientific readers may be surprised to discover that particles act differently at the atomic level to the common-sense way that things behave in our daily lives. If I see you walking along the street, I can see where you are and what direction you are going in. But it's impossible to see both where an electron is and where it is going. This concept is impossible to explain in words - it doesn't seem to make any sense. But it's true in the language of mathematics, and it has been observed by experiments. Electrons simply don't act in a common-sense way. A famous experiment called the Two Slit Experiment illustrates this. A scientist fires one electron at two parallel slits in a sheet of cardboard. It can be proved that the single electron goes through both holes at once - an impossibility in our Newtonian common-sense world. Only one electron is observed hitting the target screen the other side of the cardboard. This experiment led physicists to theorise that as soon as the electron is fired it goes into a 'superposition of states' which collapses back to 'normal' when the electron arrives at the screen. Grossly simplified, this means that when the electron moves through two holes at once, it exists in two different realities and when it hits the screen, it collapses back into one reality. From this observation, which was famously interpreted by Erwin Schroedinger in an explanation known as 'Schroedinger's Cat', springs the idea of the multiverse. Gribbin moves forward from here to explain why he thinks the multiverse exists. The earliest hypothesis, the many worlds hypothesis, says that a reality splits into a new universe every time you make a decision. For example, if you go out to the cinema, an alternative reality of you staying home and watching TV also comes into being. Leonard Susskinn's cosmic landscape hypothesis, which is part of the string theory, takes a different tack. It says that an infinite number of different universes are continually coming into existence in one massive megaverse. What's the proof for all this? Gribbin, like multiverse pioneer David Deutsch, uses the idea of quantum computing as a pointer to proof. This is based around the idea that some mathematical computations are so big that they can only take place by making use of additional computers in parallel universes. Quantum computing is not as bonkers as it sounds, but it's something that itself has not been proved. In fact, the theory of the multiverse exists by default - it's the only theory that scientists have come up with that can explain what happens to those electrons in the quantum world. In spite of this excellent book's explanations, there is no proof yet that the multiverse does exist. So perhaps there's just one of you after all.