Thermodynamic aspects of quantum gravity

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Quantum gravity is the search for the completion of the revolution in science that Einstein launched in 1905 with his simultaneous proposal of quantum theory and relativity theory. In this proposal I put forward a new strategy for completing Einstein's revolution, based on discovering new principles for quantum gravity. The aim of this proposal is to make significant progress on the problem of quantum gravity by building on a persistent theme of research the last 45 years, which is the thermodynamic nature of gravitation. ******There is ample evidence that the dynamics of general relativity, as expressed by the Einstein equations, embody phenomena that have a close analogue to thermodynamics. Such evidence comes from the classical laws of black hole mechanics, which apply to stationary black hole solutions. These suggest an identification of horizon area with entropy and surface gravity with temperature. When we take the mass of the black hole to correspond to energy, these quantities satisfy relations that have the form of the laws of thermodynamics. ******Over the last two decades, a very exciting hypothesis has been developed, according to which classical spacetime is an emergent phenomenon, arising from a fundamental quantum description, given by a discrete theory of fundamental processes. Hence, causation, energy and momentum are fundamental. Moreover, the Einstein equations of general relativity arise as the expression of the laws of thermodynamics applied to the fundamental quantum processes. ******In recent years the PI and others have developed this hypothesis, taking for the fundamental processes a model of quantum spacetime called spin foam models. There have also been inspiring insights obtained by combining entanglement entropy with holography. The present proposal builds on this recent progress in four ways.******First, we seek to discover the principles which underlie the fundamental quantum processes. These include a version of the holographic principle as well as an extension of the equivalence principle to histories of quantum processes. ******Second, we will derive general relativity directly as a consequence of the principles, combined with the laws of thermodynamics.******Third, we seek to develop new experimental predictions directly from these principles, giving us new quantum gravity phenomena by which the principles may be tested.******Fourth, we will test whether specific models of quantum spacetime satisfy the principles. These include spin foam models, proposals for background independent string theories, causal set models and dynamical triangulation models.******We believe this is a strategy to discover quantum gravity which takes best advantage of the deep links connecting gravity to thermodynamics as well as recent insights regarding holography and entanglement. It also builds on progress in various models of quantum spacetime.