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This chapter notes that quantum gravity places the concept of time on a new level. In the absence of experimental hints, mathematical and conceptual issues must be chosen as the guides in the search for such a theory. Just as reconceiving classical notions of time was key for Einstein, in his discovery of special relativity, so too many believe that time will again hold the clue for theoretical advancement, but this time with quantum gravity. The chapter details the challenge of reconciling quantum theory with relativity, concentrating especially on why time in particular causes trouble. It describes a result in canonical quantum gravity which is possibly of signal importance, namely, that fundamentally there is no time at all, and discusses the problem of time, quantization, semiclassical time, loop quantum gravity, and string theory.

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According to the inflationary scenario for the very early Universe, all inhomogeneities in the Universe are of genuine quantum origin. On the other hand, looking at these inhomogeneities and measuring them, clearly no specific quantum mechanical properties are observed. We show how the transition from their inherent quantum gravitational nature to classical behavior comes about -a transition whereby none of the successful quantitative predictions of the inflationary scenario for the present-day universe is changed. This is made possible by two properties. First, the quantum state for the spacetime metric perturbations produced by quantum gravitational effects in the early Universe becomes very special (highly squeezed) as a result of the expansion of the Universe (as long as the wavelength of the perturbations exceeds the Hubble radius). Second, decoherence through the environment distinguishes the field amplitude basis as being the pointer basis. This renders the perturbations presently indistinguishable from stochastic classical inhomogeneities. 455 Int. J. Mod. Phys. D 1998.07:455-462. Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 08/21/15. For personal use only.