14,881 research outputs found
Solar wind test of the de Broglie-Proca's massive photon with Cluster multi-spacecraft data
Our understanding of the universe at large and small scales relies largely on
electromagnetic observations. As photons are the messengers, fundamental
physics has a concern in testing their properties, including the absence of
mass. We use Cluster four spacecraft data in the solar wind at 1 AU to estimate
the mass upper limit for the photon. We look for deviations from Amp\`ere's
law, through the curlometer technique for the computation of the magnetic
field, and through the measurements of ion and electron velocities for the
computation of the current. We show that the upper bound for lies
between and kg, and thereby discuss
the currently accepted lower limits in the solar wind.Comment: The paper points out that actual photon mass upper limits (in the
solar wind) are too optimistic and model based. We instead perform a much
more experiment oriented measurement. This version matches that accepted by
Astroparticle Physic
Satellite measurement of the Hannay angle
The concept of a measurement of the yet unevaluated Hannay angle, by means of
an Earth-bound satellite, adiabatically driven by the Moon, is shown herein.
Numerical estimates are given for the angles, the orbital displacements, the
shortening of the orbital periods, for different altitudes. It is concluded
that the Hannay effect is measurable in high Earth orbits, by means of atomic
clocks, accurate Time & Frequency transfer system and precise positioning.Comment: Lette
A source-free integration method for black hole perturbations and self-force computation: Radial fall
Perturbations of Schwarzschild-Droste black holes in the Regge-Wheeler gauge
benefit from the availability of a wave equation and from the gauge invariance
of the wave function, but lack smoothness. Nevertheless, the even perturbations
belong to the C\textsuperscript{0} continuity class, if the wave function and
its derivatives satisfy specific conditions on the discontinuities, known as
jump conditions, at the particle position. These conditions suggest a new way
for dealing with finite element integration in time domain. The forward time
value in the upper node of the ) grid cell is obtained by the linear
combination of the three preceding node values and of analytic expressions
based on the jump conditions. The numerical integration does not deal directly
with the source term, the associated singularities and the potential. This
amounts to an indirect integration of the wave equation. The known wave forms
at infinity are recovered and the wave function at the particle position is
shown. In this series of papers, the radial trajectory is dealt with first,
being this method of integration applicable to generic orbits of EMRI (Extreme
Mass Ratio Inspiral).Comment: This arXiv version differs from the one to be published by Phys. Rev.
D for the use of British English and other minor editorial difference
Gauge dependence and self-force from Galilean to Einsteinian free fall, compact stars falling into black holes, Hawking radiation and the Pisa tower at the general relativity centennial
(Short abstract). In Galilean physics, the universality of free fall implies
an inertial frame, which in turns implies that the mass m of the falling body
is omitted. Otherwise, an additional acceleration proportional to m/M would
rise either for an observer at the centre of mass of the system, or for an
observer at a fixed distance from the centre of mass of M. These elementary,
but overlooked, considerations fully respect the equivalence principle and the
identity of an inertial or a gravitational pull for an observer in the Einstein
cabin. They value as fore-runners of the self-force and gauge dependency in
general relativity. The approximate nature of Galilei's law of free fall is
explored herein. When stepping into general relativity, we report how the
geodesic free fall into a black hole was the subject of an intense debate again
centred on coordinate choice. Later, we describe how the infalling mass and the
emitted gravitational radiation affect the free fall motion of a body. The
general relativistic self-force might be dealt with to perfectly fit into a
geodesic conception of motion. Then, embracing quantum mechanics, real black
holes are not classical static objects any longer. Free fall has to handle the
Hawking radiation, and leads us to new perspectives on the varying mass of the
evaporating black hole and on the varying energy of the falling mass. Along the
paper, we also estimate our findings for ordinary masses being dropped from a
Galilean or Einsteinian Pisa-like tower with respect to the current state of
the art drawn from precise measurements in ground and space laboratories, and
to the constraints posed by quantum measurements. The appendix describes how
education physics and high impact factor journals discuss the free fall.
Finally, case studies conducted on undergraduate students and teachers are
reviewed
Signatures of axion-like particles in the spectra of TeV gamma-ray sources
One interpretation of the unexplained signature observed in the PVLAS
experiment invokes a new axion-like particle (ALP) with a two-photon vertex,
allowing for photon-ALP oscillations in the presence of magnetic fields. In the
range of masses and couplings suggested by PVLAS, the same effect would lead to
a peculiar dimming of high-energy photon sources. For typical parameters of the
turbulent magnetic field in the galaxy, the effect sets in at E_gamma >~ 10
TeV, providing an ALP signature in the spectra of TeV gamma sources that can be
probed with Cherenkov telescopes. A dedicated search will be strongly motivated
if the ongoing photon regeneration experiments confirm the PVLAS particle
interpretation.Comment: 8 pages, 1 eps figure; typos corrected, matches published versio
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
Algorithms for Graph-Constrained Coalition Formation in the Real World
Coalition formation typically involves the coming together of multiple,
heterogeneous, agents to achieve both their individual and collective goals. In
this paper, we focus on a special case of coalition formation known as
Graph-Constrained Coalition Formation (GCCF) whereby a network connecting the
agents constrains the formation of coalitions. We focus on this type of problem
given that in many real-world applications, agents may be connected by a
communication network or only trust certain peers in their social network. We
propose a novel representation of this problem based on the concept of edge
contraction, which allows us to model the search space induced by the GCCF
problem as a rooted tree. Then, we propose an anytime solution algorithm
(CFSS), which is particularly efficient when applied to a general class of
characteristic functions called functions. Moreover, we show how CFSS can
be efficiently parallelised to solve GCCF using a non-redundant partition of
the search space. We benchmark CFSS on both synthetic and realistic scenarios,
using a real-world dataset consisting of the energy consumption of a large
number of households in the UK. Our results show that, in the best case, the
serial version of CFSS is 4 orders of magnitude faster than the state of the
art, while the parallel version is 9.44 times faster than the serial version on
a 12-core machine. Moreover, CFSS is the first approach to provide anytime
approximate solutions with quality guarantees for very large systems of agents
(i.e., with more than 2700 agents).Comment: Accepted for publication, cite as "in press
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