Quantum theories of gravity

Abstract

The working out of the implications of the quantum theory and the theory of relativity, and the study of their impact on each other, has been the major preoccupation of theoretical physics in this century. This effort has led to the development of a great number of conceptual schemes having their origins in the formal structure of 'naive' quantmn field theory, primarily within the framework of special relativity. Of much less immediate concern to the physicist at large, but perhaps of equal long range significance, have been a number of efforts in recent years to recast quantum field theory in the setting of general relativity. The latter efforts have involved only a small number of people, but the results achieved are already of some general interest. In narrowest terms the union of general relativity and the quantum theory is called 'the quantization of the gravitational field'. When this nomenclature is used, the almost total irrelevance, to the experimentalist, of the activity in this field becomes obvious. Its significance lies not in the direction of current experimental physics (at least not yet) but in the realm of new perspectives on the foundations of physics and the origins of the universe. It is now clear that no obstacles of principle exist which prevent the unification of the quantum theory and general relativity. Several formalisms providing such unification are very much alive and kicking. These formalisms invoke no new axioms of physics; rather they provide us with new insights into what we already have and what we can yet do with quantum field theory. J. A. Wheeler has attempted to capture the flavor &these unconventional developments in conventional physics by coining, as a replacement for 'quantum theory of gravity', the phrase 'quantum geometrodynamics', which deemphasizes the narrow view of general relativity as a theory of gravity, and suggests a fundamental all-pervading role for the quantized