Abstract
GENERAL relativity and quantum theory are among the greatest intellectual achievements of the 20th century. Each of them has profoundly altered the conceptual fabric that underlies our understanding of the physical world. Furthermore, each has been successful in describing the physical phenomena in its own domain to an astonishing degree of accuracy. And yet, they offer us strikingly different pictures of physical reality. Indeed, at first one is surprised that physics could keep progressing blissfully in the face of so deep a conflict. The reason is that phenomena for which both theories are essential occur at the Planck scale and the values of fundamental constants in our universe conspire to make the Planck length Pl = G®/c ~ 10 cm truly minute and Planck energy EPl = ®c/G ~ 10 19 Gev absolutely enormous compared to laboratory scales. Thanks to this coincidence, we can happily maintain a schizophrenic attitude and use the precise, geometric picture of reality offered by general relativity while dealing with cosmological and astrophysical phenomena, and the quantum-mechanical world of chance and intrinsic uncertainties while dealing with atomic and subatomic particles. Clearly, this strategy is quite appropriate as a practical stand. But it is highly unsatisfactory from a conceptual viewpoint. Everything in our past experience in physics tells us that the two pictures we currently use must be approximations, special cases that arise as appropriate limits of a grander theory. That theory must therefore represent a synthesis of general relativity and quantum mechanics. This would be the quantum theory of gravity. The burden on this theory is huge: Not only should it correctly describe all the known gravitational processes, but it should also adequately handle the Planck regime. This is the theory that we invoke when faced with phenomena, such as the big bang and the final state of black holes, where the Planck scale is reached and worlds of general relativity and quantum mechanics unavoidably meet. It may come as a surprise that the necessity of a quantum theory of gravity was pointed out by Einstein already in 1916 – barely a year after the discovery of general relativity. In a paper in the Preussische Akademie Sitzungsberichte, he wrote:
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