30,497 research outputs found
An analysis and simulation of landings utilizing stored-energy lift Final report, May 12 - Nov. 30, 1967
Computerized simulation of aircraft landing deceleration with stored energy lif
First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices
Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent {100} interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions
Quantum Decoherence in a D-Foam Background
Within the general framework of Liouville string theory, we construct a model
for quantum D-brane fluctuations in the space-time background through which
light closed-string states propagate. The model is based on monopole and vortex
defects on the world sheet, which have been discussed previously in a treatment
of 1+1-dimensional black-hole fluctuations in the space-time background, and
makes use of a T-duality transformation to relate formulations with Neumann and
Dirichlet boundary conditions. In accordance with previous general arguments,
we derive an open quantum-mechanical description of this D-brane foam which
embodies momentum and energy conservation and small mean energy fluctuations.
Quantum decoherence effects appear at a rate consistent with previous
estimates.Comment: 16 pages, Latex, two eps figures include
Search for Quantum Gravity
A satisfactory theory of quantum gravity may necessitate a drastic
modification of our perception of space-time, by giving it a foamy structure at
distances comparable to the Planck length. It is argued in this essay that the
experimental detection of such structures may be a realistic possibility in the
foreseeable future. After a brief review of different theoretical approaches to
quantum gravity and the relationships between them, we discuss various possible
experimental tests of the quantum nature of space-time. Observations of photons
from distant astrophysical sources such as Gamma-Ray Bursters and laboratory
experiments on neutral kaon decays may be sensitive to quantum-gravitational
effects if they are only minimally suppressed. Experimental limits from the
Whipple Observatory and the CPLEAR Collaboration are already probing close to
the Planck scale, and significant increases in sensitivity are feasible.Comment: 7 pages LATEX, no figures, Awarded First Prize in the Gravity
Research Foundation Essay Competition for 199
Quantum Decoherence in a Four-Dimensional Black Hole Background
We display a logarithmic divergence in the density matrix of a scalar field
in the presence of an Einstein-Yang-Mills black hole in four dimensions. This
divergence is related to a previously-found logarithmic divergence in the
entropy of the scalar field, which cannot be absorbed into a renormalization of
the Hawking-Bekenstein entropy of the black hole. As the latter decays, the
logarithmic divergence induces a non-commutator term \nd{\delta H}\rho in the
quantum Liouville equation for the density matrix of the scalar field,
leading to quantum decoherence. The order of magnitude of \nd{\delta H} is
, where is the mass of the scalar particle.Comment: 13-pages LATE
Liouville Cosmology
Liouville string theory is a natural framework for discussing the
non-equilibrium evolution of the Universe. It enables non-critical strings to
be treated in mathematically consistent manner, in which target time is
identified with a world-sheet renormalization-group scale parameter, preserving
target-space general coordinate invariance and the existence of an S-matrix. We
review our proposals for a unified treatment of inflation and the current
acceleration of the Universe. We link the current acceleration of the Universe
with the value of the string coupling. In such a scenario, the dilaton plays an
essential background role, driving the acceleration of the Universe during the
present era after decoupling as a constant during inflation.Comment: 23 pages latex, 2 eps figures, contribution to the proceedings of the
Dark 2004 conference, College Station, October 200
M-Theory Model-Building and Proton Stability
We study the problem of baryon stability in M theory, starting from realistic
four-dimensional string models constructed using the free-fermion formulation
of the weakly-coupled heterotic string. Suitable variants of these models
manifest an enhanced custodial gauge symmetry that forbids to all orders the
appearance of dangerous dimension-five baryon-decay operators. We exhibit the
underlying geometric (bosonic) interpretation of these models, which have a
orbifold structure similar, but not identical, to the class of
Calabi-Yau threefold compactifications of M and F theory investigated by Voisin
and Borcea. A related generalization of their work may provide a solution to
the problem of proton stability in M theory.Comment: 14 pages. Standard Late
Impact of Low-Energy Constraints on Lorentz Violation
We extend previous analyses of the violation of Lorentz invariance induced in
a non-critical string model of quantum space-time foam, discussing the
propagation of low-energy particles through a distribution of non-relativistic
D-particles.We argue that nuclear and atomic physics experiments do not
constitute sensitive probes of this approach to quantum gravity due to a
difference in the dispersion relations for massive probes as compared to those
for massless ones, predicted by the model.Comment: 4 pages revte
Testing Quantum Mechanics in the Neutral Kaon System
The neutral kaon system is a sensitive probe of quantum mechanics. We revive
a parametrization of non-quantum-mechanical effects that is motivated by
considerations of the nature of space-time foam, and show how it can be
constrained by new measurements of and
semileptonic decays at LEAR or a factory.Comment: 10 page
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