arXiv subjects

Wei-Tou Ni

Publications and source records attributed to Wei-Tou Ni.

100 records · Page 6Linked to original sources

arXiv2005

Ellipsometry noise spectrum, suspension transfer function measurement and closed-loop control of the suspension system in the Q & A experiment

The Q & A experiment, aiming at the detection of vacuum birefringence predicted by quantum electrodynamics, consists mainly of a suspended 3.5 m Fabry-Perot cavity, a rotating permanent dipole magnet and an ellipsometer. The 2.3 T magnet can rotate up to 10 rev/s, introducing an ellipticity signal at twice the rotation frequency. The X-pendulum gives a good isolation ratio for seismic noise above its main resonant frequency 0.3 Hz. At present, the ellipsometry noise decreases with frequency, from 1*10^{-5} rad Hz^{-1/2} at 5 Hz, 2*10^{-6} rad Hz^{-1/2} at 20 Hz to 5*10^{-7} rad Hz^{-1/2} at 40 Hz. The shape of the noise spectrum indicates possible improvement can be made by further reducing the movement between the cavity mirrors. From the preliminary result of yaw motion alignment control, it can be seen that some peaks due to yaw motion of the cavity mirror was suppressed. In this paper, we first give a schematic view of the Q & A experiment, and then present the measurement of transfer function of the compound X-pendulum-double pendulum suspension. A closed-loop control was carried out to verify the validity of the measured transfer functions. The ellipsometry noise spectra with and without yaw alignment control and the newest improvement is presented.

physics.ins-det↗

arXiv2005

Empirical Foundations of Relativistic Gravity

In 1859, Le Verrier discovered the mercury perihelion advance anomaly. This anomaly turned out to be the first relativistic-gravity effect observed. During the 141 years to 2000, the precisions of laboratory and space experiments, and astrophysical and cosmological observations on relativistic gravity have been improved by 3 orders of magnitude. In 1999, we envisaged a 3-6 order improvement in the next 30 years in all directions of tests of relativistic gravity. In 2000, the interferometric gravitational wave detectors began their runs to accumulate data. In 2003, the measurement of relativistic Shapiro time-delay of the Cassini spacecraft determined the relativistic-gravity parameter gammaγ with a 1.5-order improvement. In October 2004, Ciufolini and Pavlis reported a measurement of the Lense-Thirring effect on the LAGEOS and LAGEOS2 satellites to 10 percent of the value predicted by general relativity. In April 2004, Gravity Probe B was launched and has been accumulating science data for more than 170 days now. MICROSCOPE is on its way for a 2007 launch to test Galileo equivalence principle to 10-15. STEP (Satellite Test of Equivalence Principle), and ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) are in the good planning stage. Various astrophysical tests and cosmological tests of relativistic gravity will reach precision and ultra-precision stages. Clock tests and atomic interferometry tests of relativistic gravity will reach an ever-increasing precision. These will give revived interest and development both in experimental and theoretical aspects of gravity, and may lead to answers to some profound questions of gravity and the cosmos.

gr-qc↗

arXiv2004

Propagation of Dirac Wave Functions in Accelerated Frames of Reference

The first-order gravity effects of Dirac wave functions are found from the inertial effects in the accelerated frames of reference. Derivations and discussions about Lense-Thirring effect and the gyrogravitational ratio for intrinsic spin are presented. We use coordinate transformations among reference frames to study and understand the Lense-Thirring effect of a scalar particle. For a Dirac particle, the wave-function transformation operator from an inertial frame to a moving accelerated frame is obtained. From this, the Dirac wave function is solved and its change of polarization gives the gyrogravitational ratio 1 for the first-order gravitational effects. The eikonal approach to this problem is presented in the end for ready extension to investigations involving curvature terms.

gr-qc↗

arXiv2004

Asteroid Perturbations and Mass Determination for the ASTROD Space Mission

Due to the high-precision nature of the ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) mission concept, the asteroid perturbations on the ASTROD spacecraft is crucial. These perturbations need to be modelled and determined together with relativistic effects and other solar-system parameters. In a previous investigation (Su et al., Planetary and Space Science, 47, 339-43[1999]), we used the mass estimation of Ceres, Pallas and Vesta in the literature to calculate their perturbations on the ASTROD spacecraft. Recently, we established an ephemeris framework (CGC 1) including the 3 big asteroids and used this ephemeris framework to simulate the determination of their masses together with other solar-system parameters and relativistic-gravity parameters. In this paper, we extend the CGC 1 to CGC 2 ephemeris framework to include 492 asteroids (with diameter > 65 km) . We then use CGC 2 to simulate the determination of ten parameters -- the masses of Ceres, Pallas and Vesta, the six average densities for the other 489 asteroids, classified into C, S,M,E,G and U types, and the rate of change of the gravitational constant G. The fractional mass uncertainties for Ceres, Pallas and Vesta are about 10**(-3)-10**(-4); the fractional density uncertainties for six types of asteroids are about 10**(-2)-10**(-3); that for G-dot/G is about 10**(-14)-10**(-15)/yr. This mass and density determination will be useful for the determination and understanding of the structure and origin of asteroids.

astro-ph↗

arXiv2004

Probing the Microscopic Origin of Gravity via Precision Polarization and Spin Experiments

As in other parts of physics, we advocate the interaction approach: experiments <--> phenomenology <--> low-energy effective (field) theory <--> microscopic theory to probe the microscopic origin of gravity. Using chi-g phenomenological framework, we discuss the tests of equivalence principles. The only experimentally unconstrained degree of freedom is the axion freedom. It has effects on the long-range astrophysical/cosmological propagation of electromagnetic waves and can be tested/measured using future generation of polarization measurement of cosmic background radiation. The verification or refutal of this axionic effect will be a crucial step for constructing effective theory and probing the microscopic origin of gravity. The interaction of spin with gravity is another important clue for probing microscopic origin of gravity. The interplay of experiments, phenomenology and effective theory is expounded. An ideal way to reveal the microscopic origin of gravity is to measure the gyrogravitational ratio of particles. Three potential experimental methods are considered.

gr-qc↗

arXiv2003

Improving ellipticity detection sensitivity for the Q & A vacuum birefringence experiment

Q & A (quantum electrodynamics test and search for axion) experiment was first proposed in 1994 and a 3.5 m high-finesse Fabry-Perot prototype detector extendable to 7 m has been built and tested. We now enter into the 2nd phase of improving the ellipticity detection sensitivity. We present our scheme, compare with other ongoing experiments, analyze the displacement spectra of our suspension system, and use them for designing automatic alignment control to improve the ellipticicy detection sensitivity. To improve ellipsometry resolution, we use new polarizers of extinction ratio smaller than 10^{-8}.

hep-ex↗

arXiv2003

ASTROD, ASTROD I and their gravitational-wave sensitivities

ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) is a mission concept with three spacecraft -- one near L1/L2 point, one with an inner solar orbit and one with an outer solar orbit, ranging coherently with one another using lasers to test relativistic gravity, to measure the solar system and to detect gravitational waves. ASTROD I with one spacecraft ranging optically with ground stations is the first step toward the ASTROD mission. In this paper, we present the ASTROD I payload and accelerometer requirements, discuss the gravitational-wave sensitivities for ASTROD and ASTROD I, and compare them with LISA and radio-wave PDoppler-tracking of spacecraft.

gr-qc↗

arXiv2003

Building a 3.5 m prototype interferometer for the Q & A vacuum birefringence experiment and high precision ellipsometry

We have built and tested a 3.5 m high-finesse Fabry-Perot prototype inteferometer with a precision ellipsometer for the QED test and axion search (Q & A) experiment. We use X-pendulum-double-pendulum suspension designs and automatic control schemes developed by the gravitational-wave detection community. Verdet constant and Cotton-Mouton constant of the air are measured as a test. Double modulation with polarization modulation 100 Hz and magnetic-field modulation 0.05 Hz gives 10^{-7} rad phase noise for a 44-minute integration.

physics.ins-det↗

arXiv2000

Test of Cosmic Spatial Isotropy for Polarized Electrons Using a Rotatable Torsion Balance

To test the cosmic spatial isotropy, we use a rotatable torsion balance carrying a transversely spin-polarized ferrimagnetic Dy_{6}Fe_{23} mass. With a rotation period of one hour, the period of anisotropy signal is reduced from one sidereal day by about 24 times, and hence the 1/f noise is greatly reduced. Our present experimental results constrain the cosmic anisotropy Hamiltonian H = C_{1} sigma_{1} + C_{2} sigma_{2} + C_{3} sigma_{3} (sigma_{3} is in the axis of earth rotation) to (C_{1}^{2} +C_{2}^{2})^{1/2} = (1.8 +- 5.3) X 10^{-21} eV and | C_{3} | = (1.2 +- 3.5) X 10^{-19} eV. This improves the previous limits on (C_{1},C_{2}) by 120 times and C_{3} by a factor of 800.

physics.space-ph↗

arXiv2000

Rotatable-Torsion-Balance Equivalence Principleexperiment for the Spin-Polarized Hofe_{3}

We use a rotatable torsion balance to perform an equivalence principle test on a magnetically shielded spin-polarized body of HoFe_{3}. With a rotation period of one hour, the period of possible signal is reduced from one solar day by 24 times, and hence the 1/f noise is greatly reduced. Our present experimental results gives a limit (0.25 +- 1.26) X 10^{-9} on the Eotvos parameter ηof equivalence of the polarized body compared with unpolarized aluminium-brass cylinders in the solar gravitional field, and a limit (0.34 +- 0.52) X 10^{-9} in the earth gravitional fields. This improves the previous limit on polarized bodies by a factor of 45 for solar field and by a factor of 11 for earth field.

physics.space-ph↗