1,887 research outputs found
On a modified-Lorentz-transformation based gravity model confirming basic GRT experiments
Implementing Poincar\'e's `geometric conventionalism' a scalar
Lorentz-covariant gravity model is obtained based on gravitationally modified
Lorentz transformations (or GMLT). The modification essentially consists of an
appropriate space-time and momentum-energy scaling ("normalization") relative
to a nondynamical flat background geometry according to an isotropic,
nonsingular gravitational `affecting' function Phi(r). Elimination of the
gravitationally `unaffected' S_0 perspective by local composition of space-time
GMLT recovers the local Minkowskian metric and thus preserves the invariance of
the locally observed velocity of light. The associated energy-momentum GMLT
provides a covariant Hamiltonian description for test particles and photons
which, in a static gravitational field configuration, endorses the four `basic'
experiments for testing General Relativity Theory: gravitational i) deflection
of light, ii) precession of perihelia, iii) delay of radar echo, iv) shift of
spectral lines. The model recovers the Lagrangian of the Lorentz-Poincar\'e
gravity model by Torgny Sj\"odin and integrates elements of the precursor
gravitational theories, with spatially Variable Speed of Light (VSL) by
Einstein and Abraham, and gravitationally variable mass by Nordstr\"om.Comment: v1: 14 pages, extended version of conf. paper PIRT VIII, London,
2002. v2: section added on effective tensorial rank, references added,
appendix added, WEP issue deleted, abstract and other parts rewritten, same
results (to appear in Found. Phys.
Proposed parametric cooling of bilayer cuprate superconductors by terahertz excitation
We propose and analyze a scheme for parametrically cooling bilayer cuprates
based on the selective driving of a -axis vibrational mode. The scheme
exploits the vibration as a transducer making the Josephson plasma frequencies
time-dependent. We show how modulation at the difference frequency between the
intra- and interbilayer plasmon substantially suppresses interbilayer phase
fluctuations, responsible for switching -axis transport from a
superconducting to resistive state. Our calculations indicate that this may
provide a viable mechanism for stabilizing non-equilibrium superconductivity
even above , provided a finite pair density survives between the bilayers
out of equilibrium.Comment: 4 pages + 7 page supplementa
Back and forth from cool core to non-cool core: clues from radio-halos
X-ray astronomers often divide galaxy clusters into two classes: "cool core"
(CC) and "non-cool core" (NCC) objects. The origin of this dichotomy has been
the subject of debate in recent years, between "evolutionary" models (where
clusters can evolve from CC to NCC, mainly through mergers) and "primordial"
models (where the state of the cluster is fixed "ab initio" by early mergers or
pre-heating). We found that in a well-defined sample (clusters in the GMRT
Radio halo survey with available Chandra or XMM-Newton data), none of the
objects hosting a giant radio halo can be classified as a cool core. This
result suggests that the main mechanisms which can start a large scale
synchrotron emission (most likely mergers) are the same that can destroy CC and
therefore strongly supports "evolutionary" models of the CC-NCC dichotomy.
Moreover combining the number of objects in the CC and NCC state with the
number of objects with and without a radio-halo, we estimated that the time
scale over which a NCC cluster relaxes to the CC state, should be larger than
the typical life-time of radio-halos and likely shorter than about 3 Gyr. This
suggests that NCC transform into CC more rapidly than predicted from the
cooling time, which is about 10 Gyr in NCC systems, allowing the possibility of
a cyclical evolution between the CC and NCC states.Comment: Accepted for publication in A&
Terahertz field control of interlayer transport modes in cuprate superconductors
We theoretically show that terahertz pulses with controlled amplitude and
frequency can be used to switch between stable transport modes in layered
superconductors, modelled as stacks of Josephson junctions. We find pulse
shapes that deterministically switch the transport mode between
superconducting, resistive and solitonic states. We develop a simple model that
explains the switching mechanism as a destablization of the centre of mass
excitation of the Josephson phase, made possible by the highly non-linear
nature of the light-matter coupling
A Numerical Method to Predict the RCF Behaviour of PVD-coated Transmission Gears and Experimental Results
This work studies the rolling contact fatigue (RCF) behaviour of case-hardened transmission gears for racing motorcycles both numerically and experimentally. Both as-produced and PVD-WC/C coated conditions are analyzed. Finite element models of the gears were developed and a calculation procedure was applied to predict their RCF life. The Ti-6Al-4 V alloy was also considered to investigate the mass decrease of the components. RCF tests were carried out in both dry and lubricated conditions. The experimental results were compared with the numerical ones to check the reliability of the predictive method proposed
Fatigue behavior of notched Ti-6Al-4V in air and corrosive environment
Abstract The broad use of titanium alloys in naval, automotive and aerospace applications expects the current research to shed light on the fatigue behavior of these materials in corrosive media and notched condition. In this respect, the fatigue behavior in air and NaCl solution of notched Ti-6Al-4 V flat dogbone samples was investigated. A step-loading method was used to generate data points on fatigue limit stress vs. Kt diagrams for a constant life of 200,000 load cycles at R = 0.1. Fracture surfaces were observed using stereoscopic microscope
Magnetic-Field Tuning of Light-Induced Superconductivity in Striped LaBaCuO
Optical excitation of stripe-ordered LaBaCuO has been shown
to transiently enhance superconducting tunneling between the CuO planes.
This effect was revealed by a blue-shift, or by the appearance of a Josephson
Plasma Resonance in the terahertz-frequency optical properties. Here, we show
that this photo-induced state can be strengthened by the application of high
external magnetic fields oriented along the c-axis. For a 7-Tesla field, we
observe up to a ten-fold enhancement in the transient interlayer phase
correlation length, accompanied by a two-fold increase in the relaxation time
of the photo-induced state. These observations are highly surprising, since
static magnetic fields suppress interlayer Josephson tunneling and stabilize
stripe order at equilibrium. We interpret our data as an indication that
optically-enhanced interlayer coupling in LaBaCuO does not
originate from a simple optical melting of stripes, as previously hypothesized.
Rather, we speculate that the photo-induced state may emerge from activated
tunneling between optically-excited stripes in adjacent planes.Comment: 35 pages, 13 figure
Improved Torsion Pendulum for Ground Testing of LISA Displacement Sensors
We discuss a new torsion pendulum design for ground testing of prototype LISA
(Laser Interferometer Space Antenna) displacement sensors. This new design is
directly sensitive to net forces and therefore provides a more representative
test of the noisy forces and parasitic stiffnesses acting on the test mass as
compared to previous ground-based experiments. We also discuss a specific
application to the measurement of thermal gradient effects.Comment: 4 pages 1 figure, to appear in the Proceedings of the 10th Marcel
Grossmann Meeting on General Relativit
A river model of space
Within the theory of general relativity gravitational phenomena are usually
attributed to the curvature of four-dimensional spacetime. In this context we
are often confronted with the question of how the concept of ordinary physical
three-dimensional space fits into this picture. In this work we present a
simple and intuitive model of space for both the Schwarzschild spacetime and
the de Sitter spacetime in which physical space is defined as a specified set
of freely moving reference particles. Using a combination of orthonormal basis
fields and the usual formalism in a coordinate basis we calculate the physical
velocity field of these reference particles. Thus we obtain a vivid description
of space in which space behaves like a river flowing radially toward the
singularity in the Schwarzschild spacetime and radially toward infinity in the
de Sitter spacetime. We also consider the effect of the river of space upon
light rays and material particles and show that the river model of space
provides an intuitive explanation for the behavior of light and particles at
and beyond the event horizons associated with these spacetimes.Comment: 22 pages, 5 figure
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