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Hyper-fast interstellar travel via a modification of spacetime geometry
We analyze difficulties with proposals for hyper-fast interstellar travel via modifying the spacetime geometry, using as illustrations the Alcubierre warp drive and the Krasnikov tube. As it is easy to see, no violations of local causality or any other known physical principles are involved as far as motion of spacecrafts is concerned. However, the generation and support of the appropriate spacetime geometry configurations does create problems, the most significant of which are a violation of the weak energy condition, a violation of local causality, and a violation of the global causality protection. The violation of the chronology protection is the most serious of them as it opens a possibility of time travel. We trace the origin of the difficulties to the classical nature of the gravity field. This strongly indicates that hyper-fast interstellar travel should be transferred to the realm of a fully quantized gravitational theory. We outline an approach to further the research in this direction
Caching and Interpolated Likelihoods: Accelerating Cosmological Monte Carlo Markov Chains
We describe a novel approach to accelerating Monte Carlo Markov Chains. Our
focus is cosmological parameter estimation, but the algorithm is applicable to
any problem for which the likelihood surface is a smooth function of the free
parameters and computationally expensive to evaluate. We generate a high-order
interpolating polynomial for the log-likelihood using the first points gathered
by the Markov chains as a training set. This polynomial then accurately
computes the majority of the likelihoods needed in the latter parts of the
chains. We implement a simple version of this algorithm as a patch (InterpMC)
to CosmoMC and show that it accelerates parameter estimatation by a factor of
between two and four for well-converged chains. The current code is primarily
intended as a "proof of concept", and we argue that there is considerable room
for further performance gains. Unlike other approaches to accelerating
parameter fits, we make no use of precomputed training sets or special choices
of variables, and InterpMC is almost entirely transparent to the user.Comment: v2 Trivial Latex change. Source code:
http://easther.physics.yale.edu/interpmc.htm
Muon spin relaxation study of the magnetism in unilluminated Prussian Blue analogue photomagnets
We present longitudinal field muon spin relaxation (SR) measurements in
the unilluminated state of the photo-sensitive molecular magnetic Co-Fe
Prussian blue analogues MCo[Fe(CN)] HO, where
M=K and Rb with and , respectively. These results are
compared to those obtained in the stoichiometric limit,
Co[Fe(CN)] HO, which is not photo-sensitive. We find
evidence for correlation between the range of magnetic ordering and the value
of in the unilluminated state which can be explained using a site
percolation model.Comment: 7 pages, 12 figure
Local Magnetic Susceptibility of the Positive Muon in the Quasi 1D S=1/2 Antiferromagnet KCuF
We report muon spin rotation measurements of the local magnetic
susceptibility around a positive muon in the paramagnetic state of the quasi
one-dimensional spin 1/2 antiferromagnet KCuF. Signals from two distinct
sites are resolved which have a temperature dependent frequency shift which is
different than the magnetic susceptibility. This difference is attributed to a
muon induced perturbation of the spin 1/2 chain.Comment: 13 pages, 4 figures, The 2002 International Conference on Muon Spin
Rotation, Relaxation and Resonance, Virginia. US
Technological Devices in the Archives: A Policy Analysis
Doing research in the archive is the cornerstone of humanities scholarship.
Various archives institute policies regarding the use of technological
devices, such as mobile phones, laptops, and cameras in their reading rooms.
Such policies directly affect the scholars as the devices mediate the nature of
their interaction with the source materials in terms of capturing, organizing,
note taking, and record keeping for future use of found materials. In this paper,
we present our analysis of the policies of thirty archives regarding the use of
technology in their reading rooms. This policy analysis, along with data from
interviews of scholars and archivists, is intended to serve as a basis for developing
mobile applications for assisting scholars in their research activities. In this
paper we introduce an early prototype of such a mobile application—
AMTracker.Informatio
Density of states "width parity" effect in d-wave superconducting quantum wires
We calculate the density of states (DOS) in a clean mesoscopic d-wave
superconducting quantum wire, i.e. a sample of infinite length but finite width
. For open boundary conditions, the DOS at zero energy is found to be zero
if is even, and nonzero if is odd. At finite chemical potential, all
chains are gapped but the qualtitative differences between even and odd
remain.Comment: 7 pages, 8 figures, new figures and extended discussio
Maximally incompressible neutron star matter
Relativistic kinetic theory, based on the Grad method of moments as developed
by Israel and Stewart, is used to model viscous and thermal dissipation in
neutron star matter and determine an upper limit on the maximum mass of neutron
stars. In the context of kinetic theory, the equation of state must satisfy a
set of constraints in order for the equilibrium states of the fluid to be
thermodynamically stable and for perturbations from equilibrium to propagate
causally via hyperbolic equations. Application of these constraints to neutron
star matter restricts the stiffness of the most incompressible equation of
state compatible with causality to be softer than the maximally incompressible
equation of state that results from requiring the adiabatic sound speed to not
exceed the speed of light. Using three equations of state based on experimental
nucleon-nucleon scattering data and properties of light nuclei up to twice
normal nuclear energy density, and the kinetic theory maximally incompressible
equation of state at higher density, an upper limit on the maximum mass of
neutron stars averaging 2.64 solar masses is derived.Comment: 8 pages, 2 figure
Probleme bei sozialen Experimenten: Lehren aus US-amerikanischen Arbeitsmarktprogrammen
When evaluating a pilot or demonstration program, there are risks from drawing inferences from a single test. This paper reviews the experiences of replication efforts from demonstrations using randomized controlled trials in the initial evaluation and the replications. Although replications of promising programs are primarily gathered to increase sample size, replications are also used to learn if the intervention is successful for other target groups and geographic locations, and to vary some of the intervention's features. In many cases, replications fail to achieve the same success as the original evaluation, and the paper reviews reasons that have been suggested for such failures. The paper reviews what has been learned from replications where random assignment was used in six instances: income maintenance experiments, unemployment insurance bonus experiments, the Center for Employment Training program, job clubs, job search experiments, and the Quantum Opportunity Program. The paper concludes by summarizing lessons learned from the review and areas where more research is needed
Breakdown of correspondence in chaotic systems: Ehrenfest versus localization times
Breakdown of quantum-classical correspondence is studied on an experimentally
realizable example of one-dimensional periodically driven system. Two relevant
time scales are identified in this system: the short Ehrenfest time t_h and the
typically much longer localization time scale T_L. It is shown that
surprisingly weak modification of the Hamiltonian may eliminate the more
dramatic symptoms of localization without effecting the more subtle but
ubiquitous and rapid loss of correspondence at t_h.Comment: 4 pages, 5 figures, replaced with a version submitted to PR
International, collaborative assessment of 146 000 prenatal karyotypes: expected limitations if only chromosome-specific probes and fluorescent in-situ hybridization are used
The development of chromosome-specific probes (CSP) and fluorescent in-situ hybridization (FISH) has allowed for very rapid identification of selected numerical abnormalities. We attempt here to determine, in principle, what percentage of abnormalities would be detectable if only CSP-FISH were performed without karyotype for prenatal diagnosis. A total of 146 128 consecutive karyotypes for prenatal diagnosis from eight centres in four countries for 5 years were compared with predicted detection if probes for chromosomes 13, 18, 21, X and Y were used, and assuming 100% detection efficiency. A total of 4163 abnormalities (2.85%) were found including 2889 (69.4%) (trisomy 21, trisomy 18, trisomy 13, numerical sex chromosome abnormalities, and triploidies) which were considered detectable by FISH. Of these, 1274 were mosaics, translocations, deletions, inversions, rings, and markers which would not be considered detectable. CSP-FISH is a useful adjunct to karyotype for high risk situations, and may be appropriate in low risk screening, but should not be seen as a replacement for karyotype as too many structural chromosome abnormalities will be misse
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