Quantum field theory is obtained by combining special relativity and quantum mechanics. Until 1981 this was the primary tool for the understanding of elementary particles of matter and the nongravitational forces of matter. However, such theories were known to possess deficiencies and many calculations of observable quantities led, formally, to infinite answers. Yet, by the application of well defined rules these infinities could be removed to leave finite answers that agree with observation to as many as fourteen decimal places of precision. It was then discovered that these deficiencies could be avoided by replacing their theories of pointlike particles by string theories that treated the most fundamental entities in nature as lines or loops of energy (strings) possessing a certain symmetry (supersymmetry).

String theories avoid the infinities and paradoxes of quantum field theories and are a promising candidate for a complete theory of all elementary particles and forces of nature. The stringlike loops of energy in these theories possess a tension that increases as the temperature of the environment falls. Thus at very high temperatures, for example in the first moments of the expansion of the universe, they would have behaved in an intrinsically stringy manner. As the universe expanded and cooled, the string tensions would increase and the loops of string would behave more and more like single points of mass and energy. As a result, in the low temperature world all the predictions of the earlier quantum field theories are expected to be obtained, in agreement with experiment.

See also PHYSICS, QUANTUM; FIELD THEORIES; STRING THEORY

JOHN D. BARROW