Quantum gravity

The nature of the search for a quantum theory of gravity has undergone significant changes over the last few years. This is partly because the success of renormalized Yang-Mills gauge theory has stimulated interest in quantum field theory leading to a number of new ideas (for example instantons, solitons, monopoles, asymptotic freedom) which, focusing as they do on non-perturbative aspects, are potentially of considerable importance in a gravitational context. There has also been the development of supersymmetry and the associated supergravity theories for which the prognosis for quantization is brighter than normal General Relativity. Finally, a major impact was made by Hawking’s (1975) discovery of the thermal radiation produced when a quantum field propagates in a black hole background. This leads to a remarkable synthesis of thermodynamics, quantum theory and general relativity whose significance for physics has still not yet been fully explored. Traditionally, the methods for quantizing the gravitational field have been divided into ‘canonical’ and ‘covariant’ (Isham et al. 1975). A number of years ago the main attack on the canonical front was the quantization of the classical constraints