4,039 research outputs found
Causality in Time-Neutral Cosmologies
Gell-Mann and Hartle (GMH) have recently considered time-neutral cosmological
models in which the initial and final conditions are independently specified,
and several authors have investigated experimental tests of such models.
We point out here that GMH time-neutral models can allow superluminal
signalling, in the sense that it can be possible for observers in those
cosmologies, by detecting and exploiting regularities in the final state, to
construct devices which send and receive signals between space-like separated
points. In suitable cosmologies, any single superluminal message can be
transmitted with probability arbitrarily close to one by the use of redundant
signals. However, the outcome probabilities of quantum measurements generally
depend on precisely which past {\it and future} measurements take place. As the
transmission of any signal relies on quantum measurements, its transmission
probability is similarly context-dependent. As a result, the standard
superluminal signalling paradoxes do not apply. Despite their unusual features,
the models are internally consistent.
These results illustrate an interesting conceptual point. The standard view
of Minkowski causality is not an absolutely indispensable part of the
mathematical formalism of relativistic quantum theory. It is contingent on the
empirical observation that naturally occurring ensembles can be naturally
pre-selected but not post-selected.Comment: 5 pages, RevTeX. Published version -- minor typos correcte
The small RNA locus map for Chlamydomonas reinhardtii.
Small (s)RNAs play crucial roles in the regulation of gene expression and genome stability across eukaryotes where they direct epigenetic modifications, post-transcriptional gene silencing, and defense against both endogenous and exogenous viruses. It is known that Chlamydomonas reinhardtii, a well-studied unicellular green algae species, possesses sRNA-based mechanisms that are distinct from those of land plants. However, definition of sRNA loci and further systematic classification is not yet available for this or any other algae. Here, using data-driven machine learning approaches including Multiple Correspondence Analysis (MCA) and clustering, we have generated a comprehensively annotated and classified sRNA locus map for C. reinhardtii. This map shows some common characteristics with higher plants and animals, but it also reveals distinct features. These results are consistent with the idea that there was diversification in sRNA mechanisms after the evolutionary divergence of algae from higher plant lineages
Quantum corrections to critical phenomena in gravitational collapse
We investigate conformally coupled quantum matter fields on spherically
symmetric, continuously self-similar backgrounds. By exploiting the symmetry
associated with the self-similarity the general structure of the renormalized
quantum stress-energy tensor can be derived. As an immediate application we
consider a combination of classical, and quantum perturbations about exactly
critical collapse. Generalizing the standard argument which explains the
scaling law for black hole mass, , we
demonstrate the existence of a quantum mass gap when the classical critical
exponent satisfies . When our argument is
inconclusive; the semi-classical approximation breaks down in the spacetime
region of interest.Comment: RevTeX, 6 pages, 3 figures included using psfi
Matrix exponential-based closures for the turbulent subgrid-scale stress tensor
Two approaches for closing the turbulence subgrid-scale stress tensor in terms of matrix exponentials are introduced and compared. The first approach is based on a formal solution of the stress transport equation in which the production terms can be integrated exactly in terms of matrix exponentials. This formal solution of the subgrid-scale stress transport equation is shown to be useful to explore special cases, such as the response to constant velocity gradient, but neglecting pressure-strain correlations and diffusion effects. The second approach is based on an Eulerian-Lagrangian change of variables, combined with the assumption of isotropy for the conditionally averaged Lagrangian velocity gradient tensor and with the recent fluid deformation approximation. It is shown that both approaches lead to the same basic closure in which the stress tensor is expressed as the matrix exponential of the resolved velocity gradient tensor multiplied by its transpose. Short-time expansions of the matrix exponentials are shown to provide an eddy-viscosity term and particular quadratic terms, and thus allow a reinterpretation of traditional eddy-viscosity and nonlinear stress closures. The basic feasibility of the matrix-exponential closure is illustrated by implementing it successfully in large eddy simulation of forced isotropic turbulence. The matrix-exponential closure employs the drastic approximation of entirely omitting the pressure-strain correlation and other nonlinear scrambling terms. But unlike eddy-viscosity closures, the matrix exponential approach provides a simple and local closure that can be derived directly from the stress transport equation with the production term, and using physically motivated assumptions about Lagrangian decorrelation and upstream isotropy
Stabilization of high-order solutions of the cubic Nonlinear Schrodinger Equation
In this paper we consider the stabilization of non-fundamental unstable
stationary solutions of the cubic nonlinear Schrodinger equation. Specifically
we study the stabilization of radially symmetric solutions with nodes and
asymmetric complex stationary solutions. For the first ones we find partial
stabilization similar to that recently found for vortex solutions while for the
later ones stabilization does not seem possible
Patterns and predictors of sitting time over ten years in a large population-based Canadian sample: findings from the Canadian Multicentre Osteoporosis Study (CaMos)
Our objective was to describe patterns and predictors of sedentary behavior (sitting time) over 10 years among a large Canadian cohort. Data are from the Canadian Multicentre Osteoporosis Study, a prospective study of women and men randomly selected from the general population. Respondents reported socio-demographics, lifestyle behaviors and health outcomes in interviewer-administered questionnaires; weight and height were measured. Baseline data were collected between 1995 and 1997 (n = 9418; participation rate = 42%), and at 5- (n = 7648) and 10-year follow-ups (n = 5567). Total sitting time was summed across domain-specific questions at three time points and dichotomized into “low” (≤ 7 h/day) and “high” ( > 7 h/day), based on recent meta-analytic evidence on time sitting and all-cause mortality. Ten-year sitting patterns were classified as “consistently high”, “consistently low”, “increased”, “decreased”, and “mixed”. Predictors of sedentary behavior patterns were explored using chi-square tests, ANOVA and logistic regression. At baseline (mean age = 62.1 years � 13.4) average sitting was 6.9 h/day; it was 7.0 at 5- and 10-year follow-ups (p for trend = 0.12). Overall 23% reported consistently high sitting time, 22% consistently low sitting, 14% decreased sitting, 17% increased sitting with 24% mixed patterns. Consistently high sitters were more likely to be men, university educated, full-time employed, obese, and to report consistently low physical activity levels. This is one of the first population-based studies to explore patterns of sedentary behavior (multi-domain sitting) within men and women over years. Risk classification of sitting among many adults changed during follow-up. Thus, studies of sitting and health would benefit from multiple measures of sitting over time
Phase-space structure of two-dimensional excitable localized structures
In this work we characterize in detail the bifurcation leading to an
excitable regime mediated by localized structures in a dissipative nonlinear
Kerr cavity with a homogeneous pump. Here we show how the route can be
understood through a planar dynamical system in which a limit cycle becomes the
homoclinic orbit of a saddle point (saddle-loop bifurcation). The whole picture
is unveiled, and the mechanism by which this reduction occurs from the full
infinite-dimensional dynamical system is studied. Finally, it is shown that the
bifurcation leads to an excitability regime, under the application of suitable
perturbations. Excitability is an emergent property for this system, as it
emerges from the spatial dependence since the system does not exhibit any
excitable behavior locally.Comment: 10 pages, 9 figure
Spin Relaxation in Single Layer Graphene with Tunable Mobility
Graphene is an attractive material for spintronics due to theoretical
predictions of long spin lifetimes arising from low spin-orbit and hyperfine
couplings. In experiments, however, spin lifetimes in single layer graphene
(SLG) measured via Hanle effects are much shorter than expected theoretically.
Thus, the origin of spin relaxation in SLG is a major issue for graphene
spintronics. Despite extensive theoretical and experimental work addressing
this question, there is still little clarity on the microscopic origin of spin
relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to
tune mobility between 2700 and 12000 cm2/Vs, we successfully isolate the effect
of charged impurity scattering on spin relaxation in SLG. Our results
demonstrate that while charged impurities can greatly affect mobility, the spin
lifetimes are not affected by charged impurity scattering.Comment: 13 pages, 5 figure
Evaluating the effects of PIRAC nitrogen-diffusion treatments on the mechanical performance of Ti-6Al-4V alloy
The authors would like to thank the European Regional Development Fund (Malta) for research equipment funded through the application of the project “Developing an Interdisciplinary Material Testing and Rapid Prototyping R&D Facility (Ref. no. 012)”. The authors are also greatly indebted to MATERAþ/ERA-NET Plus for funding support for this research (Project ESM-1935).Powder Immersion Reaction Assisted Coating (PIRAC) is a relatively simple nitrogen diffusion based
process which has been proposed as a technique capable of considerable improvements in the
tribological performance of ceramics and metals alike; however, the necessary exposure of the substrate
material to high temperatures for several hours may have an adverse effect on the bulk properties of
materials such as titanium alloys. The effect of PIRAC treatments on the bulk metallography and
mechanical properties of Ti–6Al–4V has been studied. Following PIRAC nitrogen-diffusion treatment,
studies using X-ray diffraction and cross-sectional microscopy have shown evidence of the formation of
a thin (1.4 mm) TiN/Ti2N layer, together with the presence of some Ti3Al intermetallic phase. Semi-
logarithmic S–N plots show a deleterious effect after PIRAC treatment in terms of material cyclic fatigue
strength, particularly at higher treatment temperatures. Samples processed at 800 1C for 4 h however
exhibit better fatigue performance than others treated at lower temperatures for longer nitriding times.
Fractographic inspection has shown that fatigue cracks originate at (or near) the surface for the
untreated Ti-alloy and from the subsurface regions following diffusion treatment, owing to the build-up
of compressive stresses in the latter, which hinder crack propagation.peer-reviewe
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