- The Elegant Universe: Superstrings, Hidden Dimensions and the Quest for Ultimate Theory by Brian Greene
Vintage, 448 pp, £7.99, February 2000, ISBN 0 09 928992 X
The great ambition of scientists is to grasp the far from obvious nature of the physical world at ever more fundamental levels, and in doing so, to unify our understanding of phenomena that had previously appeared to be disparate. We have been enormously successful in this, demonstrating that complex objects are made from simpler components, and they in turn are made of even simpler ones. Everything around us, ourselves included, is composed of molecules, which are composed of atoms – carbon, nitrogen, oxygen, phosphorus and so on – which are composed of negatively charged electrons, and a nucleus of positively charged protons and uncharged neutrons, which are composed of quarks. Thus underlying the immense complexity of life is a simplicity of microscopic composition.
The way in which these various particles combine and interact is controlled by just four fundamental forces: the electromagnetic force, which unifies electricity and magnetism and is the common basis of light, radio waves and X-rays (all now identified as electromagnetic waves); the gravitational force, which as well as causing things to fall onto the floor, causes the planets to move around the sun and galaxies round each other; the weak nuclear force, which is responsible for hard-to-detect effects such as the radioactive decay of neutrons into protons and electrons; and the strong nuclear force, which holds the protons and neutrons in the nucleus of an atom together, thus making it stable despite the electrical repulsion between the protons that tends to rip it apart.
It has taken centuries to arrive at the amazingly successful understanding of the structure of the physical world that we now have, whose basic feature is the law-like descriptions we can give of the behaviour of matter, testable in detail by experiments. These laws are formulated mathematically, enable us to make quantitative predictions, and are universal inasmuch as they are believed to apply to all matter at all times and in all places. If we discovered matter that didn’t obey them, that would signal that we needed to modify our understanding and look for more general relationships still.
Our understanding of both particles and the forces involved in their interactions has evolved in response to ever more refined data from experiments. In particular, the question of whether, at its most fundamental level, matter behaves in a ‘wave-like’ or ‘particle-like’ way has been a source of continuing puzzlement. Our present belief, based in quantum theory (the theory that describes the behaviour of matter at very small scales), is that, at sub-atomic levels, matter isn’t either wave-like or particle-like in nature but both, depending on the circumstances, and behaves unpredictably in a manner quite unlike anything we are familiar with in everyday life.
Our ideas about forces have also undergone a major evolution, from being thought of as simply constituting the direct action of one object on another, either through contact or at a distance, then as being mediated by the effects of various ‘fields’ pervading space and time – e.g. the electromagnetic field that currents and magnets generate, exerting forces on charged particles – and finally to being understood as involving the exchange of force-carrying particles, photons in the case of electromagnetism, gluons in the case of the strong nuclear force.
Forces and particles, then, are not as distinct from each other as was once thought. Additionally, it seems now that not even the four ‘fundamental’ forces are independent of each other, but that the electromagnetic and weak nuclear forces are in fact different aspects of a single, more fundamental force, which would have functioned as a single interaction in the very early days of the Universe but appears under present-day conditions to comprise separate forces. Attempts to include the strong nuclear force also in this unifying scheme are well advanced.