430 research outputs found
"Because it is the Right Thing to Do'': Taking Stock of the Peer Reviewers’ Openness Initiative
Forces between electric charges in motion: Rutherford scattering, circular Keplerian orbits, action-at-a-distance and Newton's third law in relativistic classical electrodynamics
Standard formulae of classical electromagnetism for the forces between
electric charges in motion derived from retarded potentials are compared with
those obtained from a recently developed relativistic classical electrodynamic
theory with an instantaneous inter-charge force. Problems discussed include
small angle Rutherford scattering, Jackson's recent `torque paradox' and
circular Keplerian orbits. Results consistent with special relativity are
obtained only with an instantaneous interaction. The impossiblity of stable
circular motion with retarded fields in either classical electromagnetism or
Newtonian gravitation is demonstrated.Comment: 26 pages, 5 figures. QED and special relativity forbid retarded
electromagnetic forces. See also physics/0501130. V2 has typos corrected,
minor text modifications and updated references. V3 has further typos removed
and added text and reference
On the existence of supersolid helium-4 monolayer films
Extensive Monte Carlo simulations of helium-4 monolayer films adsorbed on
weak substrates have been carried out, aimed at ascertaining the possible
occurrence of a quasi-two-dimensional supersolid phase. Only crystalline films
not registered with underlying substrates are considered. Numerical results
yield strong evidence that helium-4 will not form a supersolid film on {any}
substrate strong enough to stabilize a crystalline layer. On weaker substrates,
continuous growth of a liquid film takes place
Conditioning bounds for traveltime tomography in layered media
This paper revisits the problem of recovering a smooth, isotropic, layered
wave speed profile from surface traveltime information. While it is classic
knowledge that the diving (refracted) rays classically determine the wave speed
in a weakly well-posed fashion via the Abel transform, we show in this paper
that traveltimes of reflected rays do not contain enough information to recover
the medium in a well-posed manner, regardless of the discretization. The
counterpart of the Abel transform in the case of reflected rays is a Fredholm
kernel of the first kind which is shown to have singular values that decay at
least root-exponentially. Kinematically equivalent media are characterized in
terms of a sequence of matching moments. This severe conditioning issue comes
on top of the well-known rearrangement ambiguity due to low velocity zones.
Numerical experiments in an ideal scenario show that a waveform-based model
inversion code fits data accurately while converging to the wrong wave speed
profile
Action at a distance as a full-value solution of Maxwell equations: basis and application of separated potential's method
The inadequacy of Li\'{e}nard-Wiechert potentials is demonstrated as one of
the examples related to the inconsistency of the conventional classical
electrodynamics. The insufficiency of the Faraday-Maxwell concept to describe
the whole electromagnetic phenomena and the incompleteness of a set of
solutions of Maxwell equations are discussed and mathematically proved. Reasons
of the introduction of the so-called ``electrodynamics dualism concept"
(simultaneous coexistence of instantaneous Newton long-range and
Faraday-Maxwell short-range interactions) have been displayed. It is strictly
shown that the new concept presents itself as the direct consequence of the
complete set of Maxwell equations and makes it possible to consider classical
electrodynamics as a self-consistent and complete theory, devoid of inward
contradictions. In the framework of the new approach, all main concepts of
classical electrodynamics are reconsidered. In particular, a limited class of
motion is revealed when accelerated charges do not radiate electromagnetic
field.Comment: ReVTeX file, 24pp. Small corrections which do not have influence
results of the paper. Journal reference is adde
Quantum melting of incommensurate domain walls in two dimensions
Quantum fluctuations of periodic domain-wall arrays in two-dimensional
incommensurate states at zero temperature are investigated using the elastic
theory in the vicinity of the commensurate-incommensurate transition point.
Both stripe and honeycomb structures of domain walls with short-range
interactions are considered. It is revealed that the stripes melt and become a
stripe liquid in a large-wall-spacing (low-density) region due to dislocations
created by quantum fluctuations. This quantum melting transition is of second
order and characterized by the three-dimensional XY universality class.
Zero-point energies of the stripe and honeycomb structures are calculated. As a
consequence of these results, phase diagrams of the domain-wall solid and
liquid phases in adsorbed atoms on graphite are discussed for various
domain-wall masses. Quantum melting of stripes in the presence of long-range
interactions that fall off as power laws is also studied. These results are
applied to incommensurate domain walls in two-dimensional adsorbed atoms on
substrates and in doped antiferromagnets, e.g. cuprates and nickelates.Comment: 11 pages, 5 figure
Recommended from our members
Real time visualization of Quantum Molecular Dynamics
this demonstration displays results of a Quantum Molecular Dynamics (QMD) simulation of the metal cluster Li{sub 6} running on the Intel Touchstone Delta at Caltech
Non-Invasive Microbial Metabolic Activity Sensing at Single Cell Level by Perfusion of Calcein Acetoxymethyl Ester
Krämer CEM, Singh A, Helfrich S, et al. Non-Invasive Microbial Metabolic Activity Sensing at Single Cell Level by Perfusion of Calcein Acetoxymethyl Ester. PLoS one. 2015;10(10): e0141768.Phase contrast microscopy cannot give sufficient information on bacterial metabolic activity, or if a cell is dead, it has the fate to die or it is in a viable but non-growing state. Thus, a reliable sensing of the metabolic activity helps to distinguish different categories of viability. We present a non-invasive instantaneous sensing method using a fluorogenic substrate for online monitoring of esterase activity and calcein efflux changes in growing wild type bacteria. The fluorescent conversion product of calcein acetoxymethyl ester (CAM) and its efflux indicates the metabolic activity of cells grown under different conditions at real-time. The dynamic conversion of CAM and the active efflux of fluorescent calcein were analyzed by combining microfluidic single cell cultivation technology and fluorescence time lapse microscopy. Thus, an instantaneous and non-invasive sensing method for apparent esterase activity was created without the requirement of genetic modification or harmful procedures. The metabolic activity sensing method consisting of esterase activity and calcein secretion was demonstrated in two applications. Firstly, growing colonies of our model organism Corynebacterium glutamicum were confronted with intermittent nutrient starvation by interrupting the supply of iron and carbon, respectively. Secondly, bacteria were exposed for one hour to fatal concentrations of antibiotics. Bacteria could be distinguished in growing and non-growing cells with metabolic activity as well as non-growing and non-fluorescent cells with no detectable esterase activity. Microfluidic single cell cultivation combined with high temporal resolution time-lapse microscopy facilitated monitoring metabolic activity of stressed cells and analyzing their descendants in the subsequent recovery phase. Results clearly show that the combination of CAM with a sampling free microfluidic approach is a powerful tool to gain insights in the metabolic activity of growing and non-growing bacteria
13C-assisted metabolic flux analysis to investigate heterotrophic and mixotrophic metabolism in Cupriavidus necator H16
Introduction. Cupriavidus necator H16 is a gram-negative bacterium, capable of lithoautotrophic growth by utilizing hydrogen as an energy source and fixing carbon dioxide (CO2) through Calvin-Benson-Bassham (CBB) cycle. The potential to utilize synthesis gas (Syngas) and the prospects of rerouting carbon from polyhydroxybutyrate synthesis to value-added compounds makes C. necator an excellent chassis for industrial application.
Objectives. In the context of lack of sufficient quantitative information of the metabolic pathways and to advance in rational metabolic engineering for optimized product synthesis in C. necator H16, we carried out a metabolic flux analysis based on steady-state 13C-labelling.
Methods. In this study, steady-state carbon labelling experiments, using either D-[1-13C]fructose or [1,2-13C]glycerol, were undertaken to investigate the carbon flux through the central carbon metabolism in C. necator H16 under heterotrophic and mixotrophic growth conditions, respectively.
Results. We found that the CBB cycle is active even under heterotrophic condition, and growth is indeed mixotrophic. While Entner-Doudoroff (ED) pathway is shown to be the major route for sugar degradation, tricarboxylic acid (TCA) cycle is highly active in mixotrophic condition. Enhanced flux is observed in reductive pentose phosphate pathway (redPPP) under the mixotrophic condition to supplement the precursor requirement for CBB cycle. The flux distribution was compared to the mRNA abundance of genes encoding enzymes involved in key enzymatic reactions of the central carbon metabolism.
Conclusion. This study leads the way to establishing 13C-based quantitative fluxomics for rational pathway engineering in C. necator H16
Quantum Fluctuations Driven Orientational Disordering: A Finite-Size Scaling Study
The orientational ordering transition is investigated in the quantum
generalization of the anisotropic-planar-rotor model in the low temperature
regime. The phase diagram of the model is first analyzed within the mean-field
approximation. This predicts at a phase transition from the ordered to
the disordered state when the strength of quantum fluctuations, characterized
by the rotational constant , exceeds a critical value . As a function of temperature, mean-field theory predicts a range of
values of where the system develops long-range order upon cooling, but
enters again into a disordered state at sufficiently low temperatures
(reentrance). The model is further studied by means of path integral Monte
Carlo simulations in combination with finite-size scaling techniques,
concentrating on the region of parameter space where reentrance is predicted to
occur. The phase diagram determined from the simulations does not seem to
exhibit reentrant behavior; at intermediate temperatures a pronounced increase
of short-range order is observed rather than a genuine long-range order.Comment: 27 pages, 8 figures, RevTe
- …