558 research outputs found
Randomized Polypill Crossover Trial in People Aged 50 and Over
PMCID: PMC3399742This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Reconciling the Evidence on Serum Homocysteine and Ischaemic Heart Disease: A Meta-Analysis
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
The Internal Spin Angular Momentum of an Asymptotically Flat Spacetime
In this paper we investigate the manner in which the internal spin angular
momentum of a spinor field is encoded in the gravitational field at asymptotic
infinity. The inclusion of internal spin requires us to re-analyze our notion
of asymptotic flatness. In particular, the Poincar\'{e} symmetry at asymptotic
infinity must replaced by a spin-enlarged Poincar\'{e} symmetry. Likewise, the
generators of the asymptotic symmetry group must be supplemented to account for
the internal spin. In the Hamiltonian framework of first order Einstein-Cartan
gravity, the extra generator comes from the boundary term of the Gauss
constraint in the asymptotically flat context. With the additional term, we
establish the relations among the Noether charges of a Dirac field, the Komar
integral, and the asymptotic ADM-like geometric integral. We show that by
imposing mild restraints on the generating functionals of gauge transformations
at asymptotic infinity, the phase space is rendered explicitly finite. We
construct the energy-momentum and the new total (spin+orbital) angular momentum
boundary integrals that satisfy the appropriate algebra to be the generators of
the spin-enlarged Poincar\'{e} symmetry. This demonstrates that the internal
spin is encoded in the tetrad at asymptotic infinity. In addition, we find that
a new conserved and (spin-enlarged) Poincar\'{e} invariant charge emerges that
is associated with the global structure of a gauge transformation.Comment: V2: No major changes, journal reference adde
Generalized entropy and Noether charge
We find an expression for the generalized gravitational entropy of Hawking in
terms of Noether charge. As an example, the entropy of the Taub-Bolt spacetime
is calculated.Comment: 6 pages, revtex, reference correcte
QCD uncertainties at the LHC and the implications of HERA
Strong interaction physics will be ubiquitous at the Large Hadron Collider
since the colliding beams consist of confined quarks and gluons. Although the
main purpose of the LHC is to study the mechanism of electroweak symmetry
breaking and to search for physics beyond the Standard Model, to maximise the
precision and sensitivity of such anaylses it is necessary to understand in
detail various perturbative, semi-perturbative and non-perturbative QCD
effects. Many of these effects have been extensively studied at HERA and will
be studied further at HERA II. We discuss the impact of the knowledge thus
gained on physics at the LHC.Comment: Contributed to the Proceedings of DIS04, Strbske Pleso, Slovaki
Unitarity and the Hilbert space of quantum gravity
Under the premises that physics is unitary and black hole evaporation is
complete (no remnants, no topology change), there must exist a one-to-one
correspondence between states on future null and timelike infinity and on any
earlier spacelike Cauchy surface (e.g., slices preceding the formation of the
hole). We show that these requirements exclude a large set of semiclassical
spacetime configurations from the Hilbert space of quantum gravity. In
particular, the highest entropy configurations, which account for almost all of
the volume of semiclassical phase space, would not have quantum counterparts,
i.e. would not correspond to allowed states in a quantum theory of gravity.Comment: 7 pages, 3 figures, revtex; minor changes in v2 (version published in
Class. Quant. Grav.
Black Holes Surrounded by Uniformly Rotating Rings
Highly accurate numerical solutions to the problem of Black Holes surrounded
by uniformly rotating rings in axially symmetric, stationary spacetimes are
presented. The numerical methods developed to handle the problem are discussed
in some detail. Related Newtonian problems are described and numerical results
provided, which show that configurations can reach an inner mass-shedding limit
as the mass of the central object increases. Exemplary results for the full
relativistic problem for rings of constant density are given and the
deformation of the event horizon due to the presence of the ring is
demonstrated. Finally, we provide an example of a system for which the angular
momentum of the central Black Hole divided by the square of its mass exceeds
one.Comment: 12 pages, 14 figures, revtex, v4: minor changes, Eq. (17) corrected,
corresponds to version in PR
Geometric structure of the generic static traversable wormhole throat
Traversable wormholes have traditionally been viewed as intrinsically
topological entities in some multiply connected spacetime. Here, we show that
topology is too limited a tool to accurately characterize a generic traversable
wormhole: in general one needs geometric information to detect the presence of
a wormhole, or more precisely to locate the wormhole throat. For an arbitrary
static spacetime we shall define the wormhole throat in terms of a
2-dimensional constant-time hypersurface of minimal area. (Zero trace for the
extrinsic curvature plus a "flare-out" condition.) This enables us to severely
constrain the geometry of spacetime at the wormhole throat and to derive
generalized theorems regarding violations of the energy conditions-theorems
that do not involve geodesic averaging but nevertheless apply to situations
much more general than the spherically symmetric Morris-Thorne traversable
wormhole. [For example: the null energy condition (NEC), when suitably weighted
and integrated over the wormhole throat, must be violated.] The major technical
limitation of the current approach is that we work in a static spacetime-this
is already a quite rich and complicated system.Comment: 25 pages; plain LaTeX; uses epsf.sty (four encapsulated postscript
figures
Simple Quantum Systems in Spacetimes with Closed Timelike Curves
Three simple examples illustrate properties of path integral amplitudes in
fixed background spacetimes with closed timelike curves: non-relativistic
potential scattering in the Born approximation is non-unitary, but both an
example with hard spheres and the exact solution of a totally discrete model
are unitary.Comment: 15 pages, CALT-68-180
Spacetime Energy Decreases under World-sheet RG Flow
We study renormalization group flows in unitary two dimensional sigma models
with asymptotically flat target spaces. Applying an infrared cutoff to the
target space, we use the Zamolodchikov c-theorem to demonstrate that the target
space ADM energy of the UV fixed point is greater than that of the IR fixed
point: spacetime energy decreases under world-sheet RG flow. This result
mirrors the well understood decrease of spacetime Bondi energy in the time
evolution process of tachyon condensation.Comment: 25 pages, 4 figures, harvma
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