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.

FURTHER READING:

Barrow, J.D., *Theories University of Everything* (Oxford Press, Oxford,
1991).