19,076 research outputs found
Comment on "Bell's Theorem without Inequalities and without Probabilities for Two Observers"
In this Comment we show that Cabello's argument [Phys. Rev. Lett. 86, 1911
(2001)] which proves the nonlocal feature of any classical model of quantum
mechanics based on Einstein-Podolsky-Rosen (EPR) criterion of elements of
reality, must involve at least four distant observers rather than the two
employed by the author. Moreover we raise a remark on the necessity of
performing a real experiment confirming Cabello's argument.Comment: 1 page, REVTex4 fil
A Finite Element Model for Describing the Effect of Muscle Shortening on Surface EMG
A finite-element model for the generation of single fiber action potentials in a muscle undergoing various degrees of fiber shortening is developed. The muscle is assumed fusiform with muscle fibers following a curvilinear path described by a Gaussian function. Different degrees of fiber shortening are simulated by changing the parameters of the fiber path and maintaining the volume of the muscle constant. The conductivity tensor is adapted to the muscle fiber orientation. In each point of the volume conductor, the conductivity of the muscle tissue in the direction of the fiber is larger than that in the transversal direction. Thus, the conductivity tensor changes point-by-point with fiber shortening, adapting to the fiber paths. An analytical derivation of the conductivity tensor is provided. The volume conductor is then studied with a finite-element approach using the analytically derived conductivity tensor. Representative simulations of single fiber action potentials with the muscle at different degrees of shortening are presented. It is shown that the geometrical changes in the muscle, which imply changes in the conductivity tensor, determine important variations in action potential shape, thus affecting its amplitude and frequency content. The model provides a new tool for interpreting surface EMG signal features with changes in muscle geometry, as it happens during dynamic contractions
Static NLO susceptibilities: testing approximation schemes against exact results
The reliability of the approximations commonly adopted in the calculation of
static optical (hyper)polarizabilities is tested against exact results obtained
for an interesting toy-model. The model accounts for the principal features of
typical nonlinear organic materials with mobile electrons strongly coupled to
molecular vibrations. The approximations introduced in sum over states and
finite field schemes are analyzed in detail. Both the Born-Oppenheimer and the
clamped nucleus approximations turn out to be safe for molecules, whereas for
donor-acceptor charge transfer complexes deviations from adiabaticity are
expected. In the regime of low vibrational frequency, static susceptibilities
are strongly dominated by the successive derivatives of the potential energy
and large vibrational contributions to hyperpolarizabilities are found. In this
regime anharmonic corrections to hyperpolarizabilities are very large, and the
harmonic approximation, exact for the linear polarizability, turns out totally
inadequate for nonlinear responses. With increasing phonon frequency the role
of vibrations smoothly decreases, until, in the antiadiabatic (infinite
vibrational frequency) regime, vibrations do not contribute anymore to static
susceptibilities, and the purely electronic responses are regained.Comment: 9 pages, including 3 figure
Entropy theorems in classical mechanics, general relativity, and the gravitational two-body problem
In classical Hamiltonian theories, entropy may be understood either as a
statistical property of canonical systems, or as a mechanical property, that
is, as a monotonic function of the phase space along trajectories. In classical
mechanics, there are theorems which have been proposed for proving the
non-existence of entropy in the latter sense. We explicate, clarify and extend
the proofs of these theorems to some standard matter (scalar and
electromagnetic) field theories in curved spacetime, and then we show why these
proofs fail in general relativity; due to properties of the gravitational
Hamiltonian and phase space measures, the second law of thermodynamics holds.
As a concrete application, we focus on the consequences of these results for
the gravitational two-body problem, and in particular, we prove the
non-compactness of the phase space of perturbed Schwarzschild-Droste
spacetimes. We thus identify the lack of recurring orbits in phase space as a
distinct sign of dissipation and hence entropy production.Comment: 39 pages, 3 figures; v2: version to appear in Phys. Rev. D,
references adde
Testing Primordial Black Holes as Dark Matter through LISA
The idea that primordial black holes (PBHs) can comprise most of the dark
matter of the universe has recently reacquired a lot of momentum. Observational
constraints, however, rule out this possibility for most of the PBH masses,
with a notable exception around . These light PBHs may be
originated when a sizeable comoving curvature perturbation generated during
inflation re-enters the horizon during the radiation phase. During such a
stage, it is unavoidable that gravitational waves (GWs) are generated. Since
their source is quadratic in the curvature perturbations, these GWs are
generated fully non-Gaussian. Their frequency today is about the mHz, which is
exactly the range where the LISA mission has the maximum of its sensitivity.
This is certainly an impressive coincidence. We show that this scenario of PBHs
as dark matter can be tested by LISA by measuring the GW two-point correlator.
On the other hand, we show that the short observation time (as compared to the
age of the universe) and propagation effects of the GWs across the perturbed
universe from the production point to the LISA detector suppress the bispectrum
to an unobservable level. This suppression is completely general and not
specific to our model.Comment: 22 pages, 12 figures. v3: matching published versio
Polarized thermal emission by thin metal wires
We report new measurements of the linear polarization of thermal radiation
emitted by incandescent thin tungsten wires, with thicknesses ranging from five
to hundred microns. Our data show very good agreement with theoretical
predictions, based on Drude-type fits to measured optical properties of
tungsten.Comment: 12 pages, 4 encapsulated figures. This new version matches the one
published in New. J. Phys.. Improved presentation, more references added, and
one new figure include
Radio-quiet and radio-loud pulsars: similar in Gamma-rays but different in X-rays
We present new Chandra and XMM-Newton observations of a sample of eight
radio-quiet Gamma-ray pulsars detected by the Fermi Large Area Telescope. For
all eight pulsars we identify the X-ray counterpart, based on the X-ray source
localization and the best position obtained from Gamma-ray pulsar timing. For
PSR J2030+4415 we found evidence for an about 10 arcsec-long pulsar wind
nebula. Our new results consolidate the work from Marelli et al. 2011 and
confirm that, on average, the Gamma-ray--to--X-ray flux ratios (Fgamma/Fx) of
radio-quiet pulsars are higher than for the radio-loud ones. Furthermore, while
the Fgamma/Fx distribution features a single peak for the radio-quiet pulsars,
the distribution is more dispersed for the radio-loud ones, possibly showing
two peaks. We discuss possible implications of these different distributions
based on current models for pulsar X-ray emission.Comment: Accepted for publication in The Astrophysical Journal; 12 pages, 3
figures, 2 table
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