5,335 research outputs found
A Correspondence Principle for Black Holes and Strings
For most black holes in string theory, the Schwarzschild radius in string
units decreases as the string coupling is reduced. We formulate a
correspondence principle, which states that (i) when the size of the horizon
drops below the size of a string, the typical black hole state becomes a
typical state of strings and D-branes with the same charges, and (ii) the mass
does not change abruptly during the transition. This provides a statistical
interpretation of black hole entropy. This approach does not yield the
numerical coefficient, but gives the correct dependence on mass and charge in a
wide range of cases, including neutral black holes.Comment: 24 pages, one typo correcte
Counting States of Black Strings with Traveling Waves
We consider a family of solutions to string theory which depend on arbitrary
functions and contain regular event horizons. They describe six dimensional
extremal black strings with traveling waves and have an inhomogeneous
distribution of momentum along the string. The structure of these solutions
near the horizon is studied and the horizon area computed. We also count the
number of BPS string states at weak coupling whose macroscopic momentum
distribution agrees with that of the black string. It is shown that the number
of such states is given by the Bekenstein-Hawking entropy of the black string
with traveling waves.Comment: 21 pages RevTex. One equation correcte
Breaking stress of neutron star crust
The breaking stress (the maximum of the stress-strain curve) of neutron star
crust is important for neutron star physics including pulsar glitches, emission
of gravitational waves from static mountains, and flares from star quakes. We
perform many molecular dynamic simulations of the breaking stress at different
coupling parameters (inverse temperatures) and strain rates. We describe our
results with the Zhurkov model of strength. We apply this model to estimate the
breaking stress for timescales ~1 s - 1 year, which are most important for
applications, but much longer than can be directly simulated. At these
timescales the breaking stress depends strongly on the temperature. For
coupling parameter <200, matter breaks at very small stress, if it is applied
for a few years. This viscoelastic creep can limit the lifetime of mountains on
neutron stars. We also suggest an alternative model of timescale-independent
breaking stress, which can be used to estimate an upper limit on the breaking
stress.Comment: 5 pages, 2 figures. Accepted for publication in MNRAS Letter
The Kramers equation simulation algorithm I. Operator analysis
Using an operatorial formalism, we study the Kramers equation and its
applications to numerical simulations. We obtain classes of algorithms which
may be made precise at every desired order in the time step and with
a set of free parameters which can be used to reduce autocorrelations. We show
that it is possible to use a global Metropolis test to restore Detailed
Balance.Comment: 32 pages, REVTeX 3.0, IFUP-TH-2
Black Holes with Multiple Charges and the Correspondence Principle
We consider the entropy of near extremal black holes with multiple charges in
the context of the recently proposed correspondence principle of Horowitz and
Polchinski, including black holes with two, three and four Ramond-Ramond
charges. We find that at the matching point the black hole entropy can be
accounted for by massless open strings ending on the D-branes for all cases
except a black hole with four Ramond-Ramond charges, in which case a possible
resolution in terms of brane-antibrane excitations is considered.Comment: 26 pages, harvmac, minor correction
Localized Branes and Black Holes
We address the delocalization of low dimensional D-branes and NS-branes when
they are a part of a higher dimensional BPS black brane, and the homogeneity of
the resulting horizon. We show that the effective delocalization of such branes
is a classical effect that occurs when localized branes are brought together.
Thus, the fact that the few known solutions with inhomogeneous horizons are
highly singular need not indicate a singularity of generic D- and NS-brane
states. Rather, these singular solutions are likely to be unphysical as they
cannot be constructed from localized branes which are brought together from a
finite separation.Comment: 13 pages, RevTex, no figures, few references and comments adde
The reality conditions for the new canonical variables of General Relativity
We examine the constraints and the reality conditions that have to be imposed
in the canonical theory of 4--d gravity formulated in terms of Ashtekar
variables. We find that the polynomial reality conditions are consistent with
the constraints, and make the theory equivalent to Einstein's, as long as the
inverse metric is not degenerate; when it is degenerate, reality conditions
cannot be consistently imposed in general, and the theory describes complex
general relativity.Comment: 11
Space--Time Tradeoffs for Subset Sum: An Improved Worst Case Algorithm
The technique of Schroeppel and Shamir (SICOMP, 1981) has long been the most
efficient way to trade space against time for the SUBSET SUM problem. In the
random-instance setting, however, improved tradeoffs exist. In particular, the
recently discovered dissection method of Dinur et al. (CRYPTO 2012) yields a
significantly improved space--time tradeoff curve for instances with strong
randomness properties. Our main result is that these strong randomness
assumptions can be removed, obtaining the same space--time tradeoffs in the
worst case. We also show that for small space usage the dissection algorithm
can be almost fully parallelized. Our strategy for dealing with arbitrary
instances is to instead inject the randomness into the dissection process
itself by working over a carefully selected but random composite modulus, and
to introduce explicit space--time controls into the algorithm by means of a
"bailout mechanism"
Comments on Black Holes in Matrix Theory
The recent suggestion that the entropy of Schwarzschild black holes can be
computed in matrix theory using near-extremal D-brane thermodynamics is
examined. It is found that the regime in which this approach is valid actually
describes black strings stretched across the longitudinal direction, near the
transition where black strings become unstable to the formation of black holes.
It is argued that the appropriate dynamics on the other (black hole) side of
the transition is that of the zero modes of the corresponding super Yang-Mills
theory. A suggestive mean field theory argument is given for the entropy of
black holes in all dimensions. Consequences of the analysis for matrix theory
and the holographic principle are discussed.Comment: 15 pages, harvmac, minor errors correcte
Comment on Counting Black Hole Microstates Using String Dualities
We discuss a previous attempt at a microscopic counting of the entropy of
asymptotically flat non-extremal black-holes. This method used string dualities
to relate 4 and 5 dimensional black holes to the BTZ black hole. We show how
the dualities can be justified in a certain limit, equivalent to a near horizon
limit, but the resulting spacetime is no longer asymptotically flat.Comment: 10 pages, harvmac. v(2) typo correcte
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