11,199 research outputs found
Quantum Geometrodynamics I: Quantum-Driven Many-Fingered Time
The classical theory of gravity predicts its own demise -- singularities. We
therefore attempt to quantize gravitation, and present here a new approach to
the quantization of gravity wherein the concept of time is derived by imposing
the constraints as expectation-value equations over the true dynamical degrees
of freedom of the gravitational field -- a representation of the underlying
anisotropy of space. This self-consistent approach leads to qualitatively
different predictions than the Dirac and the ADM quantizations, and in
addition, our theory avoids the interpretational conundrums associated with the
problem of time in quantum gravity. We briefly describe the structure of our
functional equations, and apply our quantization technique to two examples so
as to illustrate the basic ideas of our approach.Comment: 11, (No Figures), (Typeset using RevTeX
Phase field modeling of electrochemistry II: Kinetics
The kinetic behavior of a phase field model of electrochemistry is explored
for advancing (electrodeposition) and receding (electrodissolution) conditions
in one dimension. We described the equilibrium behavior of this model in [J. E.
Guyer, W. J. Boettinger, J.A. Warren, and G. B. McFadden, ``Phase field
modeling of electrochemistry I: Equilibrium'', cond-mat/0308173]. We examine
the relationship between the parameters of the phase field method and the more
typical parameters of electrochemistry. We demonstrate ohmic conduction in the
electrode and ionic conduction in the electrolyte. We find that, despite making
simple, linear dynamic postulates, we obtain the nonlinear relationship between
current and overpotential predicted by the classical ``Butler-Volmer'' equation
and observed in electrochemical experiments. The charge distribution in the
interfacial double layer changes with the passage of current and, at
sufficiently high currents, we find that the diffusion limited deposition of a
more noble cation leads to alloy deposition with less noble species.Comment: v3: To be published in Phys. Rev. E v2: Attempt to work around
turnpage bug. Replaced color Fig. 4a with grayscale 13 pages, 7 figures in 10
files, REVTeX 4, SIunits.sty, follows cond-mat/030817
Hidden Dirac Monopoles
Dirac showed that the existence of one magnetic pole in the universe could
offer an explanation of the discrete nature of the electric charge. Magnetic
poles appear naturally in most grand unified theories. Their discovery would be
of greatest importance for particle physics and cosmology. The intense
experimental search carried thus far has not met with success. I proposed a
universe with magnetic poles which are not observed free because they hide in
deeply bound monopole--anti-monopole states named monopolium. I discuss the
realization of this proposal and its consistency with known cosmological
features. I furthermore analyze its implications and the experimental
signatures that confirm the scenario.Comment: Comments: 15 pages, 3 figure
A Liquid Model Analogue for Black Hole Thermodynamics
We are able to characterize a 2--dimensional classical fluid sharing some of
the same thermodynamic state functions as the Schwarzschild black hole. This
phenomenological correspondence between black holes and fluids is established
by means of the model liquid's pair-correlation function and the two-body
atomic interaction potential. These latter two functions are calculated exactly
in terms of the black hole internal (quasilocal) energy and the isothermal
compressibility. We find the existence of a ``screening" like effect for the
components of the liquid.Comment: 20 pages and 6 Encapsulated PostScript figure
Phase-field modeling of microstructural pattern formation during directional solidification of peritectic alloys without morphological instability
During the directional solidification of peritectic alloys, two stable solid
phases (parent and peritectic) grow competitively into a metastable liquid
phase of larger impurity content than either solid phase. When the parent or
both solid phases are morphologically unstable, i.e., for a small temperature
gradient/growth rate ratio (), one solid phase usually outgrows and
covers the other phase, leading to a cellular-dendritic array structure closely
analogous to the one formed during monophase solidification of a dilute binary
alloy. In contrast, when is large enough for both phases to be
morphologically stable, the formation of the microstructurebecomes controlled
by a subtle interplay between the nucleation and growth of the two solid
phases. The structures that have been observed in this regime (in small samples
where convection effect are suppressed) include alternate layers (bands) of the
parent and peritectic phases perpendicular to the growth direction, which are
formed by alternate nucleation and lateral spreading of one phase onto the
other as proposed in a recent model [R. Trivedi, Metall. Mater. Trans. A 26, 1
(1995)], as well as partially filled bands (islands), where the peritectic
phase does not fully cover the parent phase which grows continuously. We
develop a phase-field model of peritectic solidification that incorporates
nucleation processes in order to explore the formation of these structures.
Simulations of this model shed light on the morphology transition from islands
to bands, the dynamics of spreading of the peritectic phase on the parent phase
following nucleation, which turns out to be characterized by a remarkably
constant acceleration, and the types of growth morphology that one might expect
to observe in large samples under purely diffusive growth conditions.Comment: Final version, minor revisions, 16 pages, 14 EPS figures, RevTe
Finite-time quantum-to-classical transition for a Schroedinger-cat state
The transition from quantum to classical, in the case of a quantum harmonic
oscillator, is typically identified with the transition from a quantum
superposition of macroscopically distinguishable states, such as the
Schr\"odinger cat state, into the corresponding statistical mixture. This
transition is commonly characterized by the asymptotic loss of the interference
term in the Wigner representation of the cat state. In this paper we show that
the quantum to classical transition has different dynamical features depending
on the measure for nonclassicality used. Measures based on an operatorial
definition have well defined physical meaning and allow a deeper understanding
of the quantum to classical transition. Our analysis shows that, for most
nonclassicality measures, the Schr\"odinger cat dies after a finite time.
Moreover, our results challenge the prevailing idea that more macroscopic
states are more susceptible to decoherence in the sense that the transition
from quantum to classical occurs faster. Since nonclassicality is prerequisite
for entanglement generation our results also bridge the gap between
decoherence, which appears to be only asymptotic, and entanglement, which may
show a sudden death. In fact, whereas the loss of coherences still remains
asymptotic, we have shown that the transition from quantum to classical can
indeed occur at a finite time.Comment: 9+epsilon pages, 4 figures, published version. Originally submitted
as "Sudden death of the Schroedinger cat", a bit too cool for APS policy :-
HI Density Distribution Driven by Supernovae: A Simulation Study
We model the complex distribution of atomic hydrogen (HI) in the interstellar
medium (ISM) assuming that it is driven entirely by supernovae (SN). We develop
and assess two different models. In the first approach, the simulated volume is
randomly populated with non-overlapping voids of a range of sizes. This may
relate to a snapshot distribution of supernova-remnant voids, although somewhat
artificially constrained by the non-overlap criterion. In the second approach,
a simplified time evolution (considering momentum conservation as the only
governing constraint during interactions) is followed as SN populate the space
with the associated input mass and energy.
We describe these simulations and present our results in the form of images
of the mass and velocity distributions and the associated power spectra. The
latter are compared with trends indicated by available observations. In both
approaches, we find remarkable correspondence with the observed statistical
description of well-studied components of the ISM, wherein the spatial spectra
have been found to show significant deviations from the Kolmogorov spectrum.
One of the key indications from this study, regardless of whether or not the
SN-induced turbulence is the dominant process in the ISM, is that the apparent
non-Kolmogorov spectral characteristics (of HI and/or electron column density
across thick or thin screens) needed to explain related observations may not at
all be in conflict with the underlying turbulence (i.e. the velocity structure)
being of Kolmogorov nature. We briefly discuss the limitations of our
simulations and the various implications of our results.Comment: To appear in Astrophysical Journal. 21 pages, 6 figure
Extreme Supernova Models for the Superluminous Transient ASASSN-15lh
The recent discovery of the unprecedentedly superluminous transient
ASASSN-15lh (or SN 2015L) with its UV-bright secondary peak challenges all the
power-input models that have been proposed for superluminous supernovae. Here
we examine some of the few viable interpretations of ASASSN-15lh in the context
of a stellar explosion, involving combinations of one or more power inputs. We
model the lightcurve of ASASSN-15lh with a hybrid model that includes
contributions from magnetar spin-down energy and hydrogen-poor circumstellar
interaction. We also investigate models of pure circumstellar interaction with
a massive hydrogen-deficient shell and discuss the lack of interaction features
in the observed spectra. We find that, as a supernova ASASSN-15lh can be best
modeled by the energetic core-collapse of a ~40 Msun star interacting with a
hydrogen-poor shell of ~20 Msun. The circumstellar shell and progenitor mass
are consistent with a rapidly rotating pulsational pair-instability supernova
progenitor as required for strong interaction following the final supernova
explosion. Additional energy injection by a magnetar with initial period of 1-2
ms and magnetic field of 0.1-1 x 10^14 G may supply the excess luminosity
required to overcome the deficit in single-component models, but this requires
more fine-tuning and extreme parameters for the magnetar, as well as the
assumption of efficient conversion of magnetar energy into radiation. We thus
favor a single-input model where the reverse shock formed in a strong SN
ejecta-CSM interaction following a very powerful core-collapse SN explosion can
supply the luminosity needed to reproduce the late-time UV-bright plateau.Comment: 8 pages, 3 figure
Thermal properties of spacetime foam
Spacetime foam can be modeled in terms of nonlocal effective interactions in
a classical nonfluctuating background. Then, the density matrix for the
low-energy fields evolves, in the weak-coupling approximation, according to a
master equation that contains a diffusion term. Furthermore, it is argued that
spacetime foam behaves as a quantum thermal field that, apart from inducing
loss of coherence, gives rise to effects such as gravitational Lamb and Stark
shifts as well as quantum damping in the evolution of the low-energy
observables. These effects can be, at least in principle, experimentally
tested.Comment: RevTeX 3.01, 11 pages, no figure
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