2,800 research outputs found
Causal structure of acoustic spacetimes
The so-called ``analogue models of general relativity'' provide a number of
specific physical systems, well outside the traditional realm of general
relativity, that nevertheless are well-described by the differential geometry
of curved spacetime. Specifically, the propagation of acoustic disturbances in
moving fluids are described by ``effective metrics'' that carry with them
notions of ``causal structure'' as determined by an exchange of sound signals.
These acoustic causal structures serve as specific examples of what can be done
in the presence of a Lorentzian metric without having recourse to the Einstein
equations of general relativity. (After all, the underlying fluid mechanics is
governed by the equations of traditional hydrodynamics, not by the Einstein
equations.) In this article we take a careful look at what can be said about
the causal structure of acoustic spacetimes, focusing on those containing sonic
points or horizons, both with a view to seeing what is different from standard
general relativity, and to seeing what the similarities might be.Comment: 51 pages, 39 figures (23 colour figures, colour used to convey
physics information.) V2: Two references added, some additional discussion of
maximal analytic extension, plus minor cosmetic change
Noncommutative nonsingular black holes
Adopting noncommutative spacetime coordinates, we determined a new solution
of Einstein equations for a static, spherically symmetric matter source. The
limitations of the conventional Schwarzschild solution, due to curvature
singularities, are overcome. As a result, the line element is endowed of a
regular DeSitter core at the origin and of two horizons even in the considered
case of electrically neutral, nonrotating matter. Regarding the Hawking
evaporation process, the intriguing new feature is that the black hole is
allowed to reach only a finite maximum temperature, before cooling down to an
absolute zero extremal state. As a consequence the quantum back reaction is
negligible.Comment: 10 pages, 2 figures, to appear in Proceedings to the 8th Workshop
`What Comes Beyond the Standard Model', July 19-29, 2005, Bled, Slovenia,
edited by C.D. Froggatt (Glasgow U.), N. Mankoc Borstnik (Ljubljana U.) and
H.B. Nielsen (Bohr Inst.
Quadratic Form Expansions for Unitaries
We introduce techniques to analyze unitary operations in terms of quadratic
form expansions, a form similar to a sum over paths in the computational basis
when the phase contributed by each path is described by a quadratic form over
. We show how to relate such a form to an entangled resource akin to
that of the one-way measurement model of quantum computing. Using this, we
describe various conditions under which it is possible to efficiently implement
a unitary operation U, either when provided a quadratic form expansion for U as
input, or by finding a quadratic form expansion for U from other input data.Comment: 20 pages, 3 figures; (extended version of) accepted submission to TQC
200
Black Holes as Effective Geometries
Gravitational entropy arises in string theory via coarse graining over an
underlying space of microstates. In this review we would like to address the
question of how the classical black hole geometry itself arises as an effective
or approximate description of a pure state, in a closed string theory, which
semiclassical observers are unable to distinguish from the "naive" geometry. In
cases with enough supersymmetry it has been possible to explicitly construct
these microstates in spacetime, and understand how coarse-graining of
non-singular, horizon-free objects can lead to an effective description as an
extremal black hole. We discuss how these results arise for examples in Type II
string theory on AdS_5 x S^5 and on AdS_3 x S^3 x T^4 that preserve 16 and 8
supercharges respectively. For such a picture of black holes as effective
geometries to extend to cases with finite horizon area the scale of quantum
effects in gravity would have to extend well beyond the vicinity of the
singularities in the effective theory. By studying examples in M-theory on
AdS_3 x S^2 x CY that preserve 4 supersymmetries we show how this can happen.Comment: Review based on lectures of JdB at CERN RTN Winter School and of VB
at PIMS Summer School. 68 pages. Added reference
Complexity, action, and black holes
Our earlier paper "Complexity Equals Action" conjectured that the quantum
computational complexity of a holographic state is given by the classical
action of a region in the bulk (the "Wheeler-DeWitt" patch). We provide
calculations for the results quoted in that paper, explain how it fits into a
broader (tensor) network of ideas, and elaborate on the hypothesis that black
holes are the fastest computers in nature.Comment: 55+14 pages, many figures. v2: (so many) typos fixed, references
adde
The Cheshire Cap
A key role in black hole dynamics is played by the inner horizon; most of the
entropy of a slightly nonextremal charged or rotating black hole is carried
there, and the covariant entropy bound suggests that the rest lies in the
region between the inner and outer horizon. An attempt to match this onto
results of the microstate geometries program suggests that a `Higgs branch' of
underlying long string states of the configuration space realizes the degrees
of freedom on the inner horizon, while the `Coulomb branch' describes the
inter-horizon region and beyond. Support for this proposal comes from an
analysis of the way singularities develop in microstate geometries, and their
close analogy to corresponding structures in fivebrane dynamics. These
singularities signal the opening up of the long string degrees of freedom of
the theory, which are partly visible from the geometry side. A conjectural
picture of the black hole interior is proposed, wherein the long string degrees
of freedom resolve the geometrical singularity on the inner horizon, yet are
sufficiently nonlocal to communicate information to the outer horizon and
beyond.Comment: 64 pages, 8 figures. Version 2: References added, together with
substantial elaborations and clarification
Causation does not explain contextuality
Realist interpretations of quantum mechanics presuppose the existence of
elements of reality that are independent of the actions used to reveal them.
Such a view is challenged by several no-go theorems that show quantum
correlations cannot be explained by non-contextual ontological models, where
physical properties are assumed to exist prior to and independently of the act
of measurement. However, all such contextuality proofs assume a traditional
notion of causal structure, where causal influence flows from past to future
according to ordinary dynamical laws. This leaves open the question of whether
the apparent contextuality of quantum mechanics is simply the signature of some
exotic causal structure, where the future might affect the past or distant
systems might get correlated due to non-local constraints. Here we show that
quantum predictions require a deeper form of contextuality: even allowing for
arbitrary causal structure, no model can explain quantum correlations from
non-contextual ontological properties of the world, be they initial states,
dynamical laws, or global constraints.Comment: 18+8 pages, 3 figure
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