Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549
During the 1980s, mathematical physicists, including Michael Green (of Queen Mary College, University of London), became interested in a class of theories of the fundamental interactions that departed radically from the format of gauge theories that had been so successful in unified models of the physics of elementary particles. In these theories, known as string theories, the fundamental objects are not point-like objects (particles) but one-dimensional objects called strings. These strings exist only in spaces with a particular number of dimensions (either 10 or 26).
The equations that describe the motions of these strings in the space they inhabit are very complicated, but it was realised that certain kinds of vibration of the strings could be treated as representing discrete particle states. Amazingly, a feature emerged from these calculations that had not been predicted by any other forms of grand unified theory: there were closed loops of string corresponding to massless bosons that behaved exactly like gravitons - hypothetical bosons which are believed to mediate the gravitational interaction. A particular class of string theories was found that also produced the properties of supersymmetry: these are called superstrings. Many physicists at the time became very excited about superstring theory because it suggested that a theory of everything might well be within reach.
The fact that these strings exist in spaces of much higher dimensionality than our own is not a fundamental problem. A much older class of theories, called Kaluza-Klein theories, had shown that spaces with a very high dimensionality were possible if extra dimensions, over and above the four we usually experience, are wound up (compactified) on a very small length scale. It is possible, therefore, to construct a string theory in 26 dimensions, but wrap 22 of them up into such a tight bundle (with a scale of order the Planck length) that they are impossible for us to perceive.
Unfortunately there has been relatively little progress with superstring theory, chiefly because the mathematical formalism required to treat their complicated multidimensional motions is so difficult. Nevertheless, hope still remains that string theories, or generalisations of them such as membranes or M-theory, will pave the way for an eventual theory of everything.
Barrow, J.D., Theories University of Everything (Oxford Press, Oxford, 1991).