Braneworlds

Carolin Crawford

  • The Fabric of the Cosmos: Space, Time and the Texture of Reality by Brian Greene
    Penguin, 569 pp, £7.99, February 2005, ISBN 0 14 101111 4

Particle physics and cosmology are mysterious subjects, embracing such strange concepts as ‘dark energy’, ‘braneworlds’ and ‘wormholes’ – terms people may have heard of or perhaps read about, but still don’t really understand. Brian Greene, a leading expert in string theory, has now followed up his earlier, very successful book, The Elegant Universe, to give lucid and accessible explanations of a wider range of cosmological abstractions. In particular, he demonstrates the intimate relationship between the physics of the infinitely small and of the infinitely large, worlds that one might have supposed to be completely unconnected.

Greene starts by examining Newton’s notion of space as an immutable medium, the background against which all motion is measured. ‘Space’ is a slippery concept, and the search for its definition, from 17th-century Newtonian mechanics to the present-day notion of a space-time knitted out of vibrating strings, underpins the whole book. Even in Newton’s day, opinions differed as to whether space should be regarded as something absolute or something relative. While Newton asserted that space would exist as a frame of reference even in a completely empty Universe, Leibniz portrayed it as a mathematical concept, useful only for describing the relationship between objects and their motion, an abstraction that would be meaningless without its constituent objects. Later, Mach suggested that motion is relative not to absolute space, but to the average distribution of matter in the Universe.

The success both of Maxwell’s equations in the mid-19th century, which described the motion of light as an electromagnetic wave, and of Michelson and Morley’s demonstration of the constancy of the speed of light, paved the way for Einstein’s theory of special relativity. Einstein’s innovation was to show that two observers moving relative to each other will perceive different outcomes when conducting the same experiment – even in something as basic as measuring distances or durations. The crux of the theory is that all motion should be considered as occurring not only through space, but also partly through time. We are unaware of relativity in everyday life, since its consequences become apparent only when a sufficient proportion of motion through time is diverted into motion through space – as in the familiar example of an astronaut who has spent a short time moving close to the speed of light only to return to Earth and find that many millennia have passed. A decade later, Einstein extended these ideas by proposing an equivalence between gravity and accelerated motion: space was now no longer Newton’s fixed framework, but a flexible medium that warps and bends in response to large masses and large energies. The question of whether it should be regarded as a physical entity or a theoretical abstraction remained unanswered.

Einstein’s equations accurately describe the behaviour of objects that are very massive or travelling at very high speeds. Analysis of microscopic extremes, however, requires the physics of quantum mechanics. Quantum mechanics is concerned with the behaviour of single particles, especially with such simple attributes as their location and velocity. At this level, it asserts, there can be no certainty as to the result of any experiment; all possible outcomes are valid, though with varying degrees of probability.

So far so good, but if these two highly successful theories are combined in an attempt to address the physics of something that is both extremely small and extremely massive – a black hole, say, or the very early Universe – the results are nonsensical. This isn’t a major hurdle to most physicists, who confidently use these theories to predict the behaviour of most objects, but it is still a cause for concern, since the inability of either theory to extend across all possible physical circumstances shows that the underlying physics is not completely understood.

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