Brian Greene

THE HIDDEN REALITY

Parallel Universes and the Hidden Laws of the Cosmos

Anyone who takes an interest in science will know that there is a lot of speculation these days about the possibility that our universe is not unique. Articles about this turn up quite frequently in magazines such as New Scientist and Scientific American, but these usually concentrate on one theory or group of theories. Greene, in contrast, deals with the whole field.

It is a truism that one cannot get a real understanding of modern physics without resort to mathematics, but, equally, publishers know that to include equations in a book for the general public is the surest way to lose sales. Greene's solution to this dilemma is to avoid formal equations but to describe how physicists go about using mathematics in their work and to put the gist of the equations into verbal form.

This is a difficult thing to do but Greene is adept in finding vivid metaphors and analogies to illustrate his meaning, and the text is accompanied by line diagrams and halftone illustrations that represent the ideas visually. As a result, a reader who lacks advanced knowledge of mathematics doesn't feel that he or she is being short-changed. You have to work hard at times to understand but the end result is very satisfying. Extensive end-notes provide further details and some mathematical treatment for those who still would like to have it.

Greene identifies nine different types of multiverse theory, although there seems to be some overlapping among them. First we encounter the Quilted Universe. If we live in an infinite universe, which is possible, everything, including us and our world, will be duplicated, and not just once but an infinite number of times.

Second, there is the Inflationary Multiverse. The theory of inflation leading to a huge expansion of the universe after the Big Bang allows for the existence of an unimaginably vast number of bubble universes, of which our universe would be one.

Greene's own work has been in string theory, and this generates three more kinds of theory about multiverses. In the Brane Multiverse our universe exists on a three-dimensional 'brane' (short for membrane), which floats on a higher-dimensional expanse potentially populated by other branes—other parallel universes.

In the Cyclic Multiverse, collisions between braneworlds can manifest beginnings similar to the Big Bang, yielding universes that are parallel in time. Alternatively, we can combine inflationary cosmology with string theory to provide for many different bubble universes related to the many different forms that multi-dimensional string theory can take. This is the Landscape Multiverse.

A different path to the multiverse arises from the 'many worlds' approach to quantum mechanics, which suggests that 'every possibility embodied in its probability waves is realized in one of a vast ensemble of parallel universes'. This gives us the Quantum Multiverse. Probably most people who have read popular accounts of modern physics have encountered this idea, but Greene goes into it in considerable depth and brings out aspects of the theory that are not usually emphasised.

A contemplation of the modern understanding of black holes leads, via information theory and entropy, to the Holographic Multiverse, which asserts that our universe is exactly mirrored by phenomena taking place on a distant bounding surface, a physically equivalent parallel universe. This idea is a distant echo of Plato's extended metaphor of the cave.

The Simulated Multiverse holds that our world, including us, is being simulated in a supercomputer. While this may seem absurd, there seems to be no logical reason why it should not be the case. This, of course, is the theme of the film, The Matrix, which in my view failed to do justice to the richness of the idea on which it was based.

Finally, and most speculatively of all, some physicists have suggested that mathematics is the primary reality, and that every possible universe that is mathematically describable really exists. Greene seems to be rather out of sympathy with this idea, which he calls the Ultimate Multiverse.

Stimulating although multiverse theories are for the imagination, not all physicists are persuaded that this kind of theorising is real science. They object that parallel universes, if they exist, are in principle beyond our capacity to observe and so the theories are unfalsifiable, more like metaphysics than science. But Greene insists that, with the exception of the Ultimate Multiverse, most if not all of the theories he describes are testable, and he suggests a number of predictions which make them potentially falsifiable. Some of the tests are at present beyond our ability to carry out, but others are already possible and indeed may soon emerge from work under way at the Large Hadron Collider and elsewhere.

As Greene acknowledges, most of the multiverse scenarios described here depend on theories that are themselves contentious, such as string theory and the many-worlds hypothesis, but that is no reason to exclude them from consideration. Indeed, the opposite may be true. For example, if string theory were one day to be accepted as valid by practically everyone, that would make more probable the existence of the parallel universes implied by the theory.

I don't know if this is how things will turn out. No one does. But it's only through fearless engagement that we can learn our own limits. It's only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality.

17 June 2011