1,559 research outputs found
An Overview of Gravitational-Wave Sources
We review current best estimates of the strength and detectability of the
gravitational waves from a variety of sources, for both ground-based and
space-based detectors, and we describe the information carried by the waves.Comment: 40 pages, 5 figures, to appear in Proceedings of GR16 (Durban, South
Africa, 2001
An improved, "phase-relaxed" F-statistic for gravitational-wave data analysis
Rapidly rotating, slightly non-axisymmetric neutron stars emit nearly
periodic gravitational waves (GWs), quite possibly at levels detectable by
ground-based GW interferometers. We refer to these sources as "GW pulsars". For
any given sky position and frequency evolution, the F-statistic is the optimal
(frequentist) statistic for the detection of GW pulsars. However, in "all-sky"
searches for previously unknown GW pulsars, it would be computationally
intractable to calculate the (fully coherent) F-statistic at every point of a
(suitably fine) grid covering the parameter space: the number of gridpoints is
many orders of magnitude too large for that. Here we introduce a
"phase-relaxed" F-statistic, which we denote F_pr, for incoherently combining
the results of fully coherent searches over short time intervals. We estimate
(very roughly) that for realistic searches, our F_pr is ~10-15% more sensitive
than the "semi-coherent" F-statistic that is currently used. Moreover, as a
byproduct of computing F_pr, one obtains a rough determination of the
time-evolving phase offset between one's template and the true signal imbedded
in the detector noise. Almost all the ingredients that go into calculating F_pr
are already implemented in LAL, so we expect that relatively little additional
effort would be required to develop a search code that uses F_pr.Comment: 8 pages, 4 figures, submitted to PR
Noncariogenic Sweeteners: Sugar Substitutes for Caries Control
The evidence is clear that the incidence of dental caries is related to the frequency of eating sugar. The use of sugar substitutes is a suggested way of reducing sugar intake. A variety of noncariogenic sweeteners exists, but most have no practical value for caries control because of their technical or safety problems, taste, or cost. Urinary bladder tumorigenic effects have been reported in experimental animals treated with saccharin and cyclamates. Because of concerns for human safety, cyclamates were banned in the U.S., and saccharin use was permitted only by special legislation. The polyalcohols sorbitol and xylitol are important sugar substitutes since they are not efficient substrates for plaque bacteria and therefore produce only minimal plaque pH drop.
Aspartame, with its sugar-like taste, is an excellent low-calorie sweetener now used in over 100 products under the name NutraSweet. Consumption of aspartame by normal humans is safe and does not promote tooth decay. Individuals with a need to control their phenylalanine intake should handle aspartame like any other source of phenylalanine
The generalized F-statistic: multiple detectors and multiple GW pulsars
The F-statistic, derived by Jaranowski, Krolak & Schutz (1998), is the
optimal (frequentist) statistic for the detection of nearly periodic
gravitational waves from known neutron stars, in the presence of stationary,
Gaussian detector noise. The F-statistic was originally derived for the case of
a single detector, whose noise spectral density was assumed constant in time,
and for a single known neutron star. Here we show how the F-statistic can be
straightforwardly generalized to the cases of 1) a network of detectors with
time-varying noise curves, and 2) a population of known sources. Fortunately,
all the important ingredients that go into our generalized F-statistics are
already calculated in the single-source/single-detector searches that are
currently implemented, e.g., in the LIGO Software Library, so implementation of
optimal multi-detector, multi-source searches should require negligible
additional cost in computational power or software development.Comment: 6 pages, 0 figures, submitted to PRD; section IV substantially
enlarged and revised, and a few typos correcte
Performance of thermally excited resonators
A study of electrothermal excitation of micromachined silicon beams is reported. The temperature distribution is calculated as a function of the position of the transducer, resulting in stress in the structure which reduces the resonance frequency. Test samples are realized and measurements of resonance frequency, vibration shape and vibration amplitude are carried out. There is a satisfactory agreement between theory and experiment at small thermal stresses. Near the buckling load we find distinct deviations from theory which are ascribed to mechanical imperfections of the beams
STOCKER CATTLE OWNERSHIP VS. CONTRACT GRAZING: A COMPARISON OF RISK-ADJUSTED RETURNS
Stocker cattle ownership is compared to contract grazing using stochastic simulation. Returns are evaluated for both cattle owners and caretakers in contract grazing agreements. For caretakers, contract grazing is significantly less risky than cattle ownership. For cattle owners, contracting reduces risk only slightly while significantly reducing expected returns.Livestock Production/Industries,
LISA detections of massive black hole inspirals: parameter extraction errors due to inaccurate template waveforms
The planned Laser Interferometer Space Antenna (LISA) is expected to detect
the inspiral and merger of massive black hole binaries (MBHBs) at z <~ 5 with
signal-to-noise ratios (SNRs) of hundreds to thousands. Because of these high
SNRs, and because these SNRs accrete over periods of weeks to months, it should
be possible to extract the physical parameters of these systems with high
accuracy; for instance, for a ~ 10^6 Msun MBHBs at z = 1 it should be possible
to determine the two masses to ~ 0.1% and the sky location to ~ 1 degree.
However, those are just the errors due to noise: there will be additional
"theoretical" errors due to inaccuracies in our best model waveforms, which are
still only approximate. The goal of this paper is to estimate the typical
magnitude of these theoretical errors. We develop mathematical tools for this
purpose, and apply them to a somewhat simplified version of the MBHB problem,
in which we consider just the inspiral part of the waveform and neglect
spin-induced precession, eccentricity, and PN amplitude corrections. For this
simplified version, we estimate that theoretical uncertainties in sky position
will typically be ~ 1 degree, i.e., comparable to the statistical uncertainty.
For the mass and spin parameters, our results suggest that while theoretical
errors will be rather small absolutely, they could still dominate over
statistical errors (by roughly an order of magnitude) for the strongest
sources. The tools developed here should be useful for estimating the magnitude
of theoretical errors in many other problems in gravitational-wave astronomy.Comment: RevTeX4, 16 pages, 2 EPS figures. Corrected typos, clarified
statement
The operator growth hypothesis in open quantum systems
The operator growth hypothesis (OGH) is a technical conjecture about the
behaviour of operators -- specifically, the asymptotic growth of their Lanczos
coefficients -- under repeated action by a Liouvillian. It is expected to hold
for a sufficiently generic closed many-body system. When it holds, it yields
bounds on the high frequency behavior of local correlation functions and
measures of chaos (like OTOCs). It also gives a route to numerically estimating
response functions. Here we investigate the generalisation of OGH to open
quantum systems, where the Liouvillian is replaced by a Lindbladian. For a
quantum system with local Hermitian jump operators, we show that the OGH is
modified: we define a generalisation of the Lanczos coefficient and show that
it initially grows linearly as in the original OGH, but experiences
exponentially growing oscillations on scales determined by the dissipation
strength. We see this behavior manifested in a semi-analytically solvable model
(large-q SYK with dissipation), numerically for an ergodic spin chain, and in a
solvable toy model for operator growth in the presence of dissipation (which
resembles a non-Hermitian single-particle hopping process). Finally, we show
that the modified OGH connects to a fundamental difference between Lindblad and
closed systems: at high frequencies, the spectral functions of the former decay
algebraically, while in the latter they decay exponentially. This is an
experimentally testable statement, which also places limitations on the
applicability of Lindbladians to systems in contact with equilibrium
environments.Comment: 9 pages, 6 figure
Ariel - Volume 6 Number 1
Editors
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Entertainment
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Circulation
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Humorist
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