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Start date: 2010Subject: search.filters.subject.Spacetime

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Now showing 1 - 6 of 6
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    Secret Life of the Spacetime
    (World scientific, 2012-03-28) Padmanabhan, T.
    Just as the thermal properties of normal matterdemandsthe existence of microscopic degrees of freedom, the thermal properties of null surfaces — perceived as local Rindler horizons by accelerated observers — demands the existence of microscopic degrees of freedom to spacetime. The distortion of the null surfaces, just like the deformation of an elastic solid, costs entropy. I show how, just like in the case of an elastic solid, one can describe the dynamics of thespacetime solid by introducing an entropy density to the distortion of null surfaces in the spacetime.
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    What can classical gravity tell us about quantum structure of spacetime?
    (IOP Publishing, 2011-09-12) Padmanabhan, T.
    Several features of classical gravity, combined with the existence of Davies-Unruh temperature of horizon s, support the following paradigm: Gravitational field equations in a wide class of theories, including Einstein ’s theory, should be viewed as describing the thermodynamic limit of the statistical mechanics of (as yet unknown) atoms of spacetime. I present the conceptual evidence for this emergent paradigm and discuss several facets of this approach.
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    Surface density of spacetime degrees of freedom from equipartition law in theories of gravity
    (American Physical Society, 2010-06-22) Padmanabhan, T.
    I show that the principle of equipartition, applied to area elements of a surface @V which are in equilibrium at the local Davies-Unruh temperature, allows one to determine the surface number density of the microscopic spacetime degrees of freedom in any diffeomorphism invariant theory of gravity. The entropy associated with these degrees of freedom matches with theWald entropy for the theory. This result also allows one to attribute an entropy density to the spacetime in a natural manner. The field equations of the theory can then be obtained by extremizing this entropy. Moreover, when the microscopic degrees of freedom are in local thermal equilibrium, the spacetime entropy of a bulk region resides on its boundary.
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    Entropy density of spacetime and the Navier-Stokes fluid dynamics of null surfaces
    (American Physical Society, 2011-02-24) Padmanabhan, T.
    It has been known for several decades that Einstein’s field equations, when projected onto a null surface, exhibit a structure very similar to the nonrelativistic Navier-Stokes equation. I show that this result arises quite naturally when gravitational dynamics is viewed as an emergent phenomenon. Extremizing the spacetime entropy density associated with the null surfaces leads to a set of equations which, when viewed in the local inertial frame, becomes identical to the Navier-Stokes equation. This is in contrast to the usual description of the Damour-Navier-Stokes equation in a general coordinate system, in which there appears a Lie derivative rather than a convective derivative. I discuss this difference, its importance, and why it is more appropriate to view the equation in a local inertial frame. The viscous force on fluid, arising from the gradient of the viscous stress-tensor, involves the second derivatives of the metric and does not vanish in the local inertial frame, while the viscous stress-tensor itself vanishes so that inertial observers detect no dissipation. We thus provide an entropy extremization principle that leads to the Damour-Navier-Stokes equation, which makes the hydrodynamical analogy with gravity completely natural and obvious. Several implications of these results are discussed.
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    Finite entanglement entropy from the zero-point area of spacetime
    (American Physical Society, 2010-12-13) Padmanabhan, T.
    The calculation of entanglement entropy S of quantum fields in spacetimes with horizon shows that, quite generically, S is (a) proportional to the area A of the horizon and (b) divergent. I argue that this divergence, which arises even in the case of Rindler horizon in flat spacetime, is yet another indication of a deep connection between horizon thermodynamics and gravitational dynamics. In an emergent perspec- tive of gravity, which accommodates this connection, the fluctuations around the equipartition value in the area elements will lead to a minimal quantum of area Oð1ÞL2 P, which will act as a regulator for this divergence. In a particular prescription for incorporating the L2 P as zero-point-area of spacetime, this does happen and the divergence in entanglement entropy is regularized, leading to S / A=L2 P in Einstein gravity. In more general models of gravity, the surface density of microscopic degrees of freedom is different which leads to a modified regularization procedure and the possibility that the entanglement entropy—when appropriately regularized—matches the Wald entropy.