838 research outputs found
Radio Tests of GR
Since VLBI techniques give microarcsecond position accuracy of celestial
objects, tests of GR using radio sources as probes of a gravitational field
have been made. We present the results from two recent tests using the VLBA: In
2005, the measurement of the classical solar deflection; and in 2002, the
measurement of the retarded gravitational deflection associated with Jupiter.
The deflection experiment measured PPN-gamma to an accuracy of 0.0003; the
Jupiter experiment measured the retarded term to 20% accuracy. The controversy
over the interpretation of the retarded term is summarized.Comment: 4 pages: IAU24
Progress in Measurements of the Gravitational Bending of Radio Waves Using the VLBA
We have used the Very Long Baseline Array (VLBA) at 43, 23 and 15 GHz to
measure the solar gravitational deflection of radio waves among four radio
sources during an 18-day period in October 2005. Using phase-referenced radio
interferometry to fit the measured phase delay to the propagation equation of
the parameterized post-Newtonian (PPN) formalism, we have determined the
deflection parameter gamma = 0.9998 +/- 0.0003$ (68% confidence level), in
agreement with General Relativity. The results come mainly from 43 GHz
observations where the refraction effects of the solar corona were negligible
beyond 3 degrees from the sun. The purpose of this experiment is three-fold: to
improve on the previous results in the gravitational bending experiments near
the solar limb; to examine and evaluate the accuracy limits of terrestrial VLBI
techniques; and to determine the prospects and outcomes of future experiments.
Our conclusion is that a series of improved designed experiments with the VLBA
could increase the presented accuracy by at least a factor of 4.Comment: 22 pages, 3 figure
From Reference Frames to Relativistic Experiments: Absolute and Relative Radio Astrometry
Reference systems and frames are crucial for high precision absolute
astrometric work, and their foundations must be well-defined. The current
frame, the International Celestial Reference Frame, will be discussed: its
history, the use of the group delay as the measured quantity, the positional
accuracy of 0.3 milliarcsec, and possible future improvements. On the other
hand, for the determination of the motion of celestial objects, accuracies
approaching 0.01 milliarcsec can be obtained by measuring the differential
position between the target object and nearby stationary sources. This
astrometric technique uses phase referencing, and the current techniques and
limitations are discussed, using the results from four experiments. Brief
comments are included on the interpretation of the Jupiter gravity deflection
experiment of September 2002.Comment: 25 pages, 7 figures--Presented at JENAM meeting in Budapest, 27-30
August 200
Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA
We present Very Long Baseline Array phase-referenced measurements of the
parallax and proper motion of two pulsars, B0919+06 and B1857-26.
Sub-milliarcsecond positional accuracy was obtained by simultaneously observing
a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz.
We discuss the merits of using weak close calibrator sources for VLBI
observations at low frequencies, and outline a method of observation and data
reduction for these type of measurements. For the pulsar B1919+06 we measure a
parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is
approximately 0.5 mas, an order of magnitude improvement over most previous
determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa
General relativistic model for experimental measurement of the speed of propagation of gravity by VLBI
A relativistic sub-picosecond model of gravitational time delay in radio
astronomical observations is worked out and a new experimental test of general
relativity is discussed in which the effect of retardation of gravity
associated with its finite speed can be observed. As a consequence, the speed
of gravity can be measured by differential VLBI observations. Retardation in
propagation of gravity is a central part of the Einstein theory of general
relativity which has not been tested directly so far. The idea of the proposed
gravitational experiment is based on the fact that gravity in general
relativity propagates with finite speed so that the deflection of light caused
by the body must be sensitive to the ratio of the body's velocity to the speed
of gravity. The interferometric experiment can be performed, for example,
during the very close angular passage of a quasar by Jupiter. Due to the finite
speed of gravity and orbital motion of Jupiter, the variation in its
gravitational field reaches observer on Earth not instantaneously but at the
retarded instant of time and should appear as a velocity-dependent excess time
delay in addition to the well-known Shapiro delay, caused by the static part of
the Jupiter's gravitational field. Such Jupiter-QSO encounter events take place
once in a decade. The next such event will occur on September 8, 2002 when
Jupiter will pass by quasar J0842+1835 at the angular distance 3.7 arcminutes.
If radio interferometric measurement of the quasar coordinates in the sky are
done with the precision of a few picoseconds (about 5 microarcseconds) the
effect of retardation of gravity and its speed of propagation may be measured
with an accuracy about 10%.Comment: 4 pages, Proceedings of the 6th European VLBI Network Symposium, Ros,
E., Porcas, R.W., Zensus, J.A. (eds.), MPIfR, Bonn, Germany (2002
Recent VLBA/VERA/IVS Tests of General Relativity
We report on recent VLBA/VERA/IVS observational tests of General Relativity.
First, we will summarize the results from the 2005 VLBA experiment that
determined gamma with an accuracy of 0.0003 by measuring the deflection of four
compact radio sources by the solar gravitational field. We discuss the limits
of precision that can be obtained with VLBA experiments in the future. We
describe recent experiments using the three global arrays to measure the
aberration of gravity when Jupiter and Saturn passed within a few arcmin of
bright radio sources. These reductions are still in progress, but the
anticipated positional accuracy of the VLBA experiment may be about 0.01 mas.Comment: 5 pages, 3 figures, IAU261 conference proceedings. IAU 261 Symposium
Proceedings, in pres
On the Speed of Gravity and Relativistic v/c Corrections to the Shapiro Time Delay
Recent papers by Samuel declared that the linearized post-Newtonian v/c
effects are too small to have been measured in the recent experiment involving
Jupiter and quasar J0842+1845 that was used to measure the ultimate speed of
gravity defined as a fundamental constant entering in front of each time
derivative of the metric tensor in the Einstein gravity field equations. We
describe our Lorentz-invariant formulation of the Jovian deflection experiment
and confirm that v/c effects are do observed, as contrasted to the erroneous
claim by Samuel, and that they vanish if and only if the speed of gravity is
infinite.Comment: 7 pages. Final version published in Physics Letters
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