15,684 research outputs found

### Lorentz invariance violation and charge (non--)conservation: A general theoretical frame for extensions of the Maxwell equations

All quantum gravity approaches lead to small modifications in the standard
laws of physics which lead to violations of Lorentz invariance. One particular
example is the extended standard model (SME). Here, a general phenomenological
approach for extensions of the Maxwell equations is presented which turns out
to be more general than the SME and which covers charge non--conservation
(CNC), too. The new Lorentz invariance violating terms cannot be probed by
optical experiments but need, instead, the exploration of the electromagnetic
field created by a point charge or a magnetic dipole. Some scalar--tensor
theories and higher dimensional brane theories predict CNC in four dimensions
and some models violating Special Relativity have been shown to be connected
with CNC and its relation to the Einstein Equivalence Principle has been
discussed. Due to this upcoming interest, the experimental status of electric
charge conservation is reviewed. Up to now there seem to exist no unique tests
of charge conservation. CNC is related to the precession of polarization, to a
modification of the $1/r$--Coulomb potential, and to a time-dependence of the
fine structure constant. This gives the opportunity to describe a dedicated
search for CNC.Comment: To appear in Physical Review

### Q & A Experiment to Search for Vacuum Dichroism, Pseudoscalar-Photon Interaction and Millicharged Fermions

A number of experiments are underway to detect vacuum birefringence and
dichroism -- PVLAS, Q & A, and BMV. Recently, PVLAS experiment has observed
optical rotation in vacuum by a magnetic field (vacuum dichroism). Theoretical
interpretations of this result include a possible pseudoscalar-photon
interaction and the existence of millicharged fermions. Here, we report the
progress and first results of Q & A (QED [quantum electrodynamics] and Axion)
experiment proposed and started in 1994. A 3.5-m high-finesse (around 30,000)
Fabry-Perot prototype detector extendable to 7-m has been built and tested. We
use X-pendulums and automatic control schemes developed by the
gravitational-wave detection community for mirror suspension and cavity
control. To polarize the vacuum, we use a 2.3-T dipole permanent magnet, with
27-mm-diameter clear borehole and 0.6-m field length,. In the experiment, the
magnet is rotated at 5-10 rev/s to generate time-dependent polarization signal
with twice the rotation frequency. Our
ellipsometer/polarization-rotation-detection-system is formed by a pair of
Glan-Taylor type polarizing prisms with extinction ratio lower than 10-8
together with a polarization modulating Faraday Cell with/without a quarter
wave plate. We made an independent calibration of our apparatus by performing a
measurement of gaseous Cotton-Mouton effect of nitrogen. We present our first
experimental results and give a brief discussion of our experimental limit on
pseudo-scalar-photon interaction and millicharged fermions.Comment: 21 pages, 13 figures, submitted to Modern Physics Letter

### The effect of the motion of the Sun on the light-time in interplanetary relativistic experiments

In 2002 a measurement of the effect of solar gravity upon the phase of
coherent microwave beams passing near the Sun has been carried out with the
Cassini mission, allowing a very accurate measurement of the PPN parameter
$\gamma$. The data have been analyzed with NASA's Orbit Determination Program
(ODP) in the Barycentric Celestial Reference System, in which the Sun moves
around the centre of mass of the solar system with a velocity $v_\odot$ of
about 10 m/sec; the question arises, what correction this implies for the
predicted phase shift. After a review of the way the ODP works, we set the
problem in the framework of Lorentz (and Galilean) transformations and evaluate
the correction; it is several orders of magnitude below our experimental
accuracy. We also discuss a recent paper \cite{kopeikin07}, which claims wrong
and much larger corrections, and clarify the reasons for the discrepancy.Comment: Final version accepted by Classical and Quantum Gravity (8 Jan. 2008

### Constraint on intermediate-range gravity from earth-satellite and lunar orbiter measurements, and lunar laser ranging

In the experimental tests of gravity, there have been considerable interests
in the possibility of intermediate-range gravity. In this paper, we use the
earth-satellite measurement of earth gravity, the lunar orbiter measurement of
lunar gravity, and lunar laser ranging measurement to constrain the
intermediate-range gravity from lambda=1.2*10^{7}m - 3.8*10^{8}m. The limits
for this range are alpha=10^{-8}-5*10^{-8}, which improve previous limits by
about one order of magnitude in the range lambda=1.2*10^{7}m-3.8*10^{8}m.Comment: 8 pages, International Journal of Modern Physics D, in press (World
Scientific, 2005

### Multicast broadcast services support in OFDMA-based WiMAX systems [Advances in mobile multimedia]

Multimedia stream service provided by broadband wireless networks has emerged as an important technology and has attracted much attention. An all-IP network architecture with reliable high-throughput air interface makes orthogonal frequency division multiplexing access (OFDMA)-based mobile worldwide interoperability for microwave access (mobile WiMAX) a viable technology for wireless multimedia services, such as voice over IP (VoIP), mobile TV, and so on. One of the main features in a WiMAX MAC layer is that it can provide'differentiated services among different traffic categories with individual QoS requirements. In this article, we first give an overview of the key aspects of WiMAX and describe multimedia broadcast multicast service (MBMS) architecture of the 3GPP. Then, we propose a multicast and broadcast service (MBS) architecture for WiMAX that is based on MBMS. Moreover, we enhance the MBS architecture for mobile WiMAX to overcome the shortcoming of limited video broadcast performance over the baseline MBS model. We also give examples to demonstrate that the proposed architecture can support better mobility and offer higher power efficiency

### Suspension of the fiber mode-cleaner launcher and measurement of the high extinction-ratio (10^{-9}) ellipsometer for the Q & A experiment

The Q & A experiment, first proposed and started in 1994, provides a feasible
way of exploring the quantum vacuum through the detection of vacuum
birefringence effect generated by QED loop diagram and the detection of the
polarization rotation effect generated by photon-interacting (pseudo-)scalar
particles. Three main parts of the experiment are: (1) Optics System (including
associated Electronic System) based on a suspended 3.5-m high finesse
Fabry-Perot cavity, (2) Ellipsometer using ultra-high extinction-ratio
polarizer and analyzer, and (3) Magnetic Field Modulation System for generating
the birefringence and the polarization rotation effect. In 2002, the Q & A
experiment achieved the Phase I sensitivity goal. During Phase II, we set (i)
to improve the control system of the cavity mirrors for suppressing the
relative motion noise, (ii) to enhance the birefringence signal by setting-up a
60-cm long 2.3 T transverse permanent magnet rotatable to 10 rev/s, (iii) to
reduce geometrical noise by inserting a polarization-maintaining optical fiber
(PM fiber) as a mode cleaner, and (iv) to use ultra-high extinction-ratio
(10^{-9}) polarizer and analyzer for ellipsometry. Here we report on (iii) &
(iv); specifically, we present the properties of the PM-fiber mode-cleaner, the
transfer function of its suspension system, and the result of our measurement
of high extinction-ratio polarizer and analyzer.Comment: 8 pages, 6 figures, presented in the 6th Edoardo Amaldi Conference on
Gravitational Waves, Okinawa, Japan, June 2005, and accepted by "Journal of
Physics: Conference Series". Modifications from version 2 were made based on
the referees' comments on figures. Ref. [31] were update

### Acceleration disturbances due to local gravity gradients in ASTROD I

The Astrodynamical Space Test of Relativity using Optical Devices (ASTROD)
mission consists of three spacecraft in separate solar orbits and carries out
laser interferometric ranging. ASTROD aims at testing relativistic gravity,
measuring the solar system and detecting gravitational waves. Because of the
larger arm length, the sensitivity of ASTROD to gravitational waves is
estimated to be about 30 times better than Laser Interferometer Space Antenna
(LISA) in the frequency range lower than about 0.1 mHz. ASTROD I is a simple
version of ASTROD, employing one spacecraft in a solar orbit. It is the first
step for ASTROD and serves as a technology demonstration mission for ASTROD. In
addition, several scientific results are expected in the ASTROD I experiment.
The required acceleration noise level of ASTROD I is 10^-13 m s^-2 Hz^{-1/2} at
the frequency of 0.1 mHz. In this paper, we focus on local gravity gradient
noise that could be one of the largest acceleration disturbances in the ASTROD
I experiment. We have carried out gravitational modelling for the current
test-mass design and simplified configurations of ASTROD I by using an
analytical method and the Monte Carlo method. Our analyses can be applied to
figure out the optimal designs of the test mass and the constructing materials
of the spacecraft, and the configuration of compensation mass to reduce local
gravity gradients.Comment: 6 pages, presented at the 6th Edoardo Amaldi Conference (Okinawa
Japan, June 2005); to be published in Journal of Physics: Conference Serie

### Non-minimal coupling of photons and axions

We establish a new self-consistent system of equations accounting for a
non-minimal interaction of gravitational, electromagnetic and axion fields. The
procedure is based on a non-minimal extension of the standard
Einstein-Maxwell-axion action. The general properties of a ten-parameter family
of non-minimal linear models are discussed. We apply this theory to the models
with pp-wave symmetry and consider propagation of electromagnetic waves
non-minimally coupled to the gravitational and axion fields. We focus on exact
solutions of electrodynamic equations, which describe quasi-minimal and
non-minimal optical activity induced by the axion field. We also discuss
empirical constraints on coupling parameters from astrophysical birefringence
and polarization rotation observations.Comment: 31 pages, 2 Tables; replaced with the final version published in
Classical and Quantum Gravit

### Numerical simulation of time delay interferometry for eLISA/NGO

eLISA/NGO is a new gravitational wave detection proposal with arm length of
10^6 km and one interferometer down-scaled from LISA. Just like LISA and
ASTROD-GW, in order to attain the requisite sensitivity for eLISA/NGO, laser
frequency noise must be suppressed to below the secondary noises such as the
optical path noise, acceleration noise etc. In previous papers, we have
performed the numerical simulation of the time delay interferometry (TDI) for
LISA and ASTROD-GW with one arm dysfunctional by using the CGC 2.7 ephemeris.
The results are well below their respective limits which the laser frequency
noise is required to be suppressed. In this paper, we follow the same procedure
to simulate the time delay interferometry numerically. To do this, we work out
a set of 1000-day optimized mission orbits of the eLISA/NGO spacecraft starting
at January 1st, 2021 using the CGC 2.7 ephemeris framework. We then use the
numerical method to calculate the residual optical path differences in the
second-generation TDI solutions as in our previous papers. The maximum path
length difference, for all configurations calculated, is below 13 mm (43 ps).
It is well below the limit which the laser frequency noise is required to be
suppressed for eLISA/NGO. We compare and discuss the resulting differences due
to the different arm lengths for various mission proposals -- eLISA/NGO, an
NGO-LISA-type mission with a nominal arm length of 2 x 10^6 km, LISA and
ASTROD-GW.Comment: 17 pages, 13 figures, 3 tables, minor changes in description to match
the accepted version of Classical and Quantum Gravity. arXiv admin note: text
overlap with arXiv:1102.496

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