1,932 research outputs found
Comment on 'Helmholtz theorem and the v-gauge in the problem of superluminal and instantaneous signals in classical electrodynamics,' by Chubykalo et al [Found. of Phys. Lett, 19, 37-46 (2006)]
Fundamental errors in the Chubykalo et al paper [Found. of Phys. Lett, 19,
37-46 (2006)] are highlighted. Contrary to their claim that "... the
irrotational component of the electric field has a physical meaning and can
propagate exclusively instantaneously," it is shown that this instantaneous
component is physically irrelevant because it is always canceled by a term
contained into the solenoidal component. This result follows directly from the
solution of the wave equation that satisfies the solenoidal component.
Therefore the subsequent inference of these authors that there are two
mechanisms of transmission of energy and momentum in classical electrodynamics,
one retarded and the other one instantaneous, has no basis. The example given
by these authors in which the full electric field of an oscillating charge
equals its instantaneous irrotational component on the axis of oscillations is
proved to be false.Comment: An alternative discussion can be found in the paper: Jose A. Heras,
"How potentials in different gauges yield the same retarded electric and
magnetic fields," Am. J. Phys. 75, 176-183 (2007
A short proof that the Coulomb-gauge potentials yield the retarded fields
A short demonstration that the potentials in the Coulomb gauge yield the
retarded electric and magnetic fields is presented. This demonstration is
relatively simple and can be presented in an advanced undergraduate curse of
electromagnetic theory
Quantum Estimation Methods for Quantum Illumination
Quantum illumination consists in shining quantum light on a target region
immersed in a bright thermal bath, with the aim of detecting the presence of a
possible low-reflective object. If the signal is entangled with the receiver,
then a suitable choice of the measurement offers a gain with respect to the
optimal classical protocol employing coherent states. Here, we tackle this
detection problem by using quantum estimation techniques to measure the
reflectivity parameter of the object, showing an enhancement in the
signal-to-noise ratio up to 3 dB with respect to the classical case when
implementing only local measurements. Our approach employs the quantum Fisher
information to provide an upper bound for the error probability, supplies the
concrete estimator saturating the bound, and extends the quantum illumination
protocol to non-Gaussian states. As an example, we show how Schrodinger's cat
states may be used for quantum illumination.Comment: Published versio
HESS J1632-478: an energetic relic
HESS J1632-478 is an extended and still unidentified TeV source in the
galactic plane. In order to identify the source of the very high energy
emission and to constrain its spectral energy distribution, we used a deep
observation of the field obtained with XMM-Newton together with data from
Molonglo, Spitzer and Fermi to detect counterparts at other wavelengths. The
flux density emitted by HESS J1632-478 peaks at very high energies and is more
than 20 times weaker at all other wavelengths probed. The source spectrum
features two large prominent bumps with the synchrotron emission peaking in the
ultraviolet and the external inverse Compton emission peaking in the TeV. HESS
J1632-478 is an energetic pulsar wind nebula with an age of the order of 10^4
years. Its bolometric (mostly GeV-TeV) luminosity reaches 10% of the current
pulsar spin down power. The synchrotron nebula has a size of 1 pc and contains
an unresolved point-like X-ray source, probably the pulsar with its wind
termination shock.Comment: A&A accepted, 9 pages, 5 figures, 4 table
Probing Clumpy Stellar Winds in SFXTs
Quantitative constraints on the wind clumping of massive stars can be
obtained from the study of the hard X-ray variability of SFXTs. In these
systems, a large fraction of the hard X-ray emission is emitted in the form of
flares with typical duration of 3 ksec, frequency of 7 days and luminosity of
ergs/s. Such flares are most probably emitted by the interaction of a
compact object orbiting at R with wind clumps ( g).
The density ratio between the clumps and the inter-clump medium is .
The parameters of the clumps and of the inter-clump medium are in good
agreement with macro-clumping scenario and line-driven instability simulations.Comment: 3 pages, A Population Explosion: The Nature and Evolution of X-ray
Binaries in Diverse Environment
A Universal Machine for Biform Theory Graphs
Broadly speaking, there are two kinds of semantics-aware assistant systems
for mathematics: proof assistants express the semantic in logic and emphasize
deduction, and computer algebra systems express the semantics in programming
languages and emphasize computation. Combining the complementary strengths of
both approaches while mending their complementary weaknesses has been an
important goal of the mechanized mathematics community for some time. We pick
up on the idea of biform theories and interpret it in the MMTt/OMDoc framework
which introduced the foundations-as-theories approach, and can thus represent
both logics and programming languages as theories. This yields a formal,
modular framework of biform theory graphs which mixes specifications and
implementations sharing the module system and typing information. We present
automated knowledge management work flows that interface to existing
specification/programming tools and enable an OpenMath Machine, that
operationalizes biform theories, evaluating expressions by exhaustively
applying the implementations of the respective operators. We evaluate the new
biform framework by adding implementations to the OpenMath standard content
dictionaries.Comment: Conferences on Intelligent Computer Mathematics, CICM 2013 The final
publication is available at http://link.springer.com
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