So here we are at the end of the twentieth/beginning of the twenty first century with two shiny new theories. General Relativity, which explains gravity on cosmological scales, and quantum theory, which explains the microscopic world in terms of a number of particles and forces. What more could a scientist ask for? Well there is one snag. The two theories appear to be incompatible. Both are “right” within their own ambits, yet attempts to perform a marriage end in an unconsummated disaster. Attempts to make predictions give nonsensical infinite answers. The problem is that the quantum theory of the microscopic world ignores the force of gravity. For many purposes this is fine, since gravity is so weak. However, Einstein's theory of gravity appears to be inconsistent with quantum mechanics.

The goal is clear: to find a self-consistent theory that unifies all the forces, including gravity, and explains all the particles, the fundamental building blocks of matter. Some physicists think the candidate is here, the details just need to be worked out.

Faraday had a good swing at it; Einstein was working on just such a scheme. It was his great sadness that he did not manage to achieve an overarching unification. With his love of beauty he might well have appreciated the symmetry and richness of this new possibility. He even corresponded with one of its progenitors. In 1921 Einstein received a paper pointing out that it was possible to include electromagnetism with relativity by working out the mathematics in a five-dimensional world. Perhaps Einstein would have been shocked to hear physicists at the end of the century talking seriously about a ten-dimensional universe.

Superstring theory started to look interesting in 1984 when Michael Green from England and John Schwarz from North America showed their colleagues that they could get rid of the nonsensical answers that had been plaguing the subject.

The problem that point-like particles were generating infinities was solved with strings which have extension. A string has no specific identity, attributes such as mass vary according to the way in which it oscillates. Up to the advent of string theory, all elementary particles have been assumed to be point-like. A string, unlike a point, can vibrate in an infinite number of ways, giving string theory a very rich structure. Strings have tension that varies according to environmental temperature. At low temperatures (the universe today is ‘cool’, relative to its extremely hot beginning) the tension is high; the string is contracted and point-like. String theory describes the so-called elementary particles (such as electrons, photons, quarks) as different vibrational modes of a single string-like object, a bit like the harmonics of a violin string. The theory therefore has the possibility of unifying the bewildering array of observed particles. Point-like particles when incorporated with the force of gravity produce infinities or nonsensical answers in the mathematics. The extension in these stringy objects prevents this mathematical problem.

One version of string theory, known as superstring theory, may possibly unify the forces and particles and in a way that builds a consistent quantum theory of gravity. These strings are small; we're talking miniscule. A string is so small that it will never be seen, smaller than even a quark; it would take 1,000,000,000,000,000,000,000,000,000,000,000 (10 to the power of 33) to stretch one centimetre.

This business about ten dimensions is difficult to grasp. Theorists suggest that when the universe began, four dimensions unfolded from ten (three of space and one of time), leaving six compactified spatial dimensions. Most healthy people cannot conceptualise a higher-dimensional world. But one can get a sense of ‘dimension’ by comparing the feeling of walking along a beach with that of swimming underwater over exotic coral reefs. Underwater, one is not confined to the sandy surface. The difference can be expressed by extra degrees of freedom. The ten-dimensional structure of superstrings contributes to its richness.

Symmetry is an important concept in physics. The ‘super’ in superstring refers to supersymmetry. Physicists and mathematicians moan over the elegance and rich symmetry in superstrings. Sadly, a field equation has not yet made my spine tingle with its resonance and harmony, but there are aspects of symmetry that are easy to comprehend. The world exhibits symmetry that we are all familiar with and find pleasing. Sheep, cart-wheels and snowballs for example. If you draw a line down a sheep's back from nose to tail, you have two roughly symmetrical shapes. A cart-wheel can be halved along any axis and will be symmetrical. A snowball has greater symmetry than either of the others. If perfectly spherical, it is symmetrical through three axes. Symmetry could be described as aspects that remain invariant under transformation. A sphere may be spun any way and it will still look the same, unlike a sheep, whose symmetry is only preserved along the longitudinal axis.

Symmetry is an important concept in mathematics. The discovery of unifying laws hidden in the complex and diverse world is closely related to the principle of symmetry. Supersymmetry is a symmetry that operates in conventional and abstract spaces. The world we live in has pattern, but is not entirely symmetrical. The universe has cooled and evolved into a highly differentiated entity. It is thought that in earlier times, there was more symmetry. The Big Bang itself might have been the point of perfect symmetry from which the rich variety of difference issued over 15 billion years.

String theory has attracted brilliant notices. It also has its critics. We are perhaps accustomed to quick fixes of information. The massive amount of media consumption prevalent now can lead us to expect confirmation or refutation of last night's hypothesis to be on the breakfast table next to the grapefruit. This is unlikely to happen in the case of superstrings. It was 26 years before a half-way decent understanding of quantum theory was developed. A theory as rich, comprehensive and powerful as superstrings will doubtless take years to unravel.

Already it has been fruitful in bringing together the two disciplines of mathematics and physics in a mutually enriching way. New or little-known branches of mathematics have been explored with great zest. Green and Schwarz's discovery in August 1984 caused so much excitement that for a time the number of published papers dropped as physicists stopped writing and spent time familiarising themselves with the new and difficult mathematics needed to work in the theory. Superstrings have been described by some as a theory properly belonging to the twenty-first century that dropped into the twentieth by chance. Ed Witten, winner of the 1991 Fields Medal (Mathematics' Nobel equivalent), predicted that it would dominate physics for the next 50 years:

We're still at the relatively early stages of a scientific revolution comparable only to the invention of quantum mechanics. It's just a vast process, one that's going to change everything we know in theoretical physics, at the really fundamental level. It's going to take decades. Maybe none of us will live to see it really come to fruition. But for a physicist, superstrings are life.