811 research outputs found
Entropic-gravity derivation of MOND
A heuristic entropic-gravity derivation has previously been given of the
gravitational two-body force of modified Newtonian dynamics (MOND). Here, it is
shown that also another characteristic of MOND can be recovered, namely, the
external field effect (implying a violation of the Strong Equivalence
Principle). In fact, the derivation gives precisely the modified Poisson
equation which Bekenstein and Milgrom proposed as a consistent nonrelativistic
theory of MOND.Comment: 6 pages; v5: published versio
Observational Signatures of Quantum Gravity in Interferometers
We consider the uncertainty in the arm length of an interferometer due to
metric fluctuations from the quantum nature of gravity, proposing a concrete
microscopic model of energy fluctuations in holographic degrees of freedom on
the surface bounding a causally connected region of spacetime. In our model,
fluctuations longitudinal to the beam direction accumulate in the infrared and
feature strong long distance correlation in the transverse direction. This
leads to a signal that could be observed in a gravitational wave
interferometer. We connect the positional uncertainty principle arising from
our calculations to the 't Hooft gravitational S-matrix.Comment: 6 pages, 1 figur
New perspective on space and time from Lorentz violation
I present a brief review on space and time in different periods of physics,
and then talk on the nature of space and time from physical arguments. I
discuss the ways to test such a new perspective on space and time through
searching for Lorentz violation in some physical processes. I also make an
introduce to a newly proposed theory of Lorentz violation from basic
considerations.Comment: 10 latex pages. Plenary talk at First LeCosPA Symposium: Towards
Ultimate Understanding of the Universe (LeCosPA2012), National Taiwan
University, Taipei, Taiwan, February 6-9, 201
Black Extended Objects, Naked Singularities and P-Branes
We treat the horizons of charged, dilaton black extended objects as quantum
mechanical objects. We show that the S matrix for such an object can be written
in terms of a p-brane-like action. The requirements of unitarity of the S
matrix and positivity of the p-brane tension equivalent severely restrict the
number of space-time dimensions and the allowed values of the dilaton parameter
a. Generally, black objects transform at the extremal limit into p-branes.Comment: 9 pages, REVTE
Entropic gravity, minimum temperature, and modified Newtonian dynamics
Verlinde's heuristic argument for the interpretation of the standard
Newtonian gravitational force as an entropic force is generalized by the
introduction of a minimum temperature (or maximum wave length) for the
microscopic degrees of freedom on the holographic screen. With the simplest
possible setup, the resulting gravitational acceleration felt by a test mass m
from a point mass M at a distance R is found to be of the form of the modified
Newtonian dynamics (MOND) as suggested by Milgrom. The corresponding MOND-type
acceleration constant is proportional to the minimum temperature, which can be
interpreted as the Unruh temperature of an emerging de-Sitter space. This
provides a possible explanation of the connection between local MOND-type
two-body systems and cosmology.Comment: 12 pages, v6: published versio
Quantum Black Hole Evaporation
We investigate a recently proposed model for a full quantum description of
two-dimensional black hole evaporation, in which a reflecting boundary
condition is imposed in the strong coupling region. It is shown that in this
model each initial state is mapped to a well-defined asymptotic out-state,
provided one performs a certain projection in the gravitational zero mode
sector. We find that for an incoming localized energy pulse, the corresponding
out-going state contains approximately thermal radiation, in accordance with
semi-classical predictions. In addition, our model allows for certain acausal
strong coupling effects near the singularity, that give rise to corrections to
the Hawking spectrum and restore the coherence of the out-state. To an
asymptotic observer these corrections appear to originate from behind the
receding apparent horizon and start to influence the out-going state long
before the black hole has emitted most of its mass. Finally, by putting the
system in a finite box, we are able to derive some algebraic properties of the
scattering matrix and prove that the final state contains all initial
information.Comment: 37 pages (figs 2 and 3 included as uuencoded compressed tar file),
Latex, needs epsf.tex, PUPT-1395, IASSNS-HEP-93/25 (revised version has minor
corrections, one reference added
Wall Crossing, Discrete Attractor Flow and Borcherds Algebra
The appearance of a generalized (or Borcherds-) Kac-Moody algebra in the
spectrum of BPS dyons in N=4, d=4 string theory is elucidated. From the
low-energy supergravity analysis, we identify its root lattice as the lattice
of the T-duality invariants of the dyonic charges, the symmetry group of the
root system as the extended S-duality group PGL(2,Z) of the theory, and the
walls of Weyl chambers as the walls of marginal stability for the relevant
two-centered solutions. This leads to an interpretation for the Weyl group as
the group of wall-crossing, or the group of discrete attractor flows.
Furthermore we propose an equivalence between a "second-quantized multiplicity"
of a charge- and moduli-dependent highest weight vector and the dyon
degeneracy, and show that the wall-crossing formula following from our proposal
agrees with the wall-crossing formula obtained from the supergravity analysis.
This can be thought of as providing a microscopic derivation of the
wall-crossing formula of this theory.Comment: This is a contribution to the Special Issue on Kac-Moody Algebras and
Applications, published in SIGMA (Symmetry, Integrability and Geometry:
Methods and Applications) at http://www.emis.de/journals/SIGMA
Dilatonic Black Holes, Naked Singularities and Strings
We extend a previous calculation which treated Schwarschild black hole
horizons as quantum mechanical objects to the case of a charged, dilaton black
hole. We show that for a unique value of the dilaton parameter `a', which is
determined by the condition of unitarity of the S matrix, black holes transform
at the extremal limit into strings.Comment: 8 pages, REVTE
A Black Hole Levitron
We study the problem of spatially stabilising four dimensional extremal black
holes in background electric/magnetic fields. Whilst looking for stationary
stable solutions describing black holes kept in external fields we find that
taking a continuum limit of Denef et al's multi-center solutions provides a
supergravity description of such backgrounds within which a black hole can be
trapped in a given volume. This is realised by levitating a black hole over a
magnetic dipole base. We comment on how such a construction resembles a
mechanical Levitron.Comment: 5 pages, 1 figur
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