387 research outputs found
On the Gravitomagnetic Time Delay
We study the gravitational time delay in ray propagation due to rotating
masses in the linear approximation of general relativity. Simple expressions
are given for the gravitomagnetic time delay that occurs when rays of radiation
cross a slowly rotating shell and propagate in the field of a distant rotating
source. Moreover, we calculate the local gravitational time delay in the Goedel
universe. The observational consequences of these results in the case of weak
gravitational lensing are discussed.Comment: 15 pages, 1 figure, revised version submitted to Phys. Lett.
Quantum Zeno effect and the detection of gravitomagnetism
In this work we introduce two experimental proposals that could shed some
light upon the inertial properties of intrinsic spin. In particular we will
analyze the role that the gravitomagnetic field of the Earth could have on a
quantum system with spin 1/2. We will deduce the expression for Rabi
transitions, which depend, explicitly, on the coupling between the spin of the
quantum system and the gravitomagnetic field of the Earth. Afterwards, the
continuous measurement of the energy of the spin 1/2 system is considered, and
an expression for the emerging quantum Zeno effect is obtained. Thus, it will
be proved that gravitomagnetism, in connection with spin 1/2 systems, could
induce not only Rabi transitions but also a quantum Zeno effect.Comment: Essay awarded with an ``honorable mention'' in the Annual Essay
Competition of the Gravity Research Foundation for the year 2000, four new
references, discussion enlarged, 9 pages. Accepted in International Journal
of Modern Physics
On the Earth's tidal perturbations for the LARES satellite
Frame dragging, one of the outstanding phenomena predicted by General
Relativity, is efficiently studied by means of the laser-ranged satellites
LARES, LAGEOS and LAGEOS 2. The accurate analysis of the orbital perturbations
of Earth's solid and ocean tides has been relevant for increasing the accuracy
in the test of frame-dragging using these three satellites. The Earth's tidal
perturbations acting on the LARES satellite are obtained for the 110
significant modes of corresponding Doodson number and are exhibited to enable
the comparison to those of the LAGEOS and LAGEOS-2 satellites. For LARES we
represent 29 perturbation modes for l=2,3,4 for ocean tides.Comment: 14 pages, 9 figures, to appear in Eur Phys J Plus (subm. Sept 20,
2017
Focus Point on Tests of General Relativity and Alternative Gravity Theories
Testing of fundamental physical theories at ever higher accuracy is a continuous process and hence General Rela-tivity and the development of alternative gravity theories have always been among the interests of experimentalists,astronomers and theoreticians. The empirical basis of General Relativity is linked to an immense scope of areas, fromthe equivalence principle up to, say, the variation of the gravitational constant and of other physical constants.In 1960s the General Relativity was transferred from abstract mathematical construction to a driving force in anew area of research –relativistic astrophysics— dealing with a sequence of impressive observational discoveries, suchas quasars, cosmic microwave background radiation, pulsars, compact binary stars, etc. Practically at the same time,the development of rockets and space technologies provided means for new types of tests of General Relativity, suchas Lunar laser ranging and signal time delays. Let us recall a remark attributed to Einstein at the end of a discussionwith Thirring in 1918 in regard to a test of frame-dragging:What a pity the earth has no moon in an orbit just outsideits atmosphere!Currently, laser-ranged satellites are among the important tools for testing tiny predictions of GeneralRelativity, such as frame-dragging. Some alternative gravitational theories agree with all the current tests of GeneralRelativity but predict different results for frame-dragging.A new phase of studies on extensions of General Relativity and alternative gravity theories started with thediscovery of the dark sector –dark energy and dark matter— as of dominating content of the Universe. Cosmology,therefore, became an area of broad applications of alternative gravity theories. Informative constraints on gravitytheories are also obtained monitoring S2 and other stars in the vicinity of the massive black hole in the Galacticcenter. Thus, it is now hardly possible to mention cosmic structures, from the Solar system up to the dynamics ofgalaxies and clusters of galaxies, for which alternative gravity theories have not been probed.The discovery of gravitational waves, one hundred years after the formulation of General Relativity, further stim-ulated the consideration of alternative gravity theories. The particular importance of this new window is in that itenables to test the strong-field General Relativity,i.e.the effects close to the event horizon.Finally, testing the Equivalence Principle is a probe for gravitational theories at fundamental level and a main steptowards any quantum gravity theory.The papers included in thisFocus Pointnaturally cover a minor fraction of the above-mentioned areas; neverthelessthey contain timely accounts on broad topics, from tests on the Equivalence Principle, to orbital dynamics of extendedbodies in inspiraling binary systems, up to cosmology
Constraining spacetime torsion with LAGEOS
We compute the corrections to the orbital Lense-Thirring effect (or
frame-dragging) in the presence of spacetime torsion. We derive the equations
of motion of a test body in the gravitational field of a rotating axisymmetric
massive body, using the parametrized framework of Mao, Tegmark, Guth and Cabi.
We calculate the secular variations of the longitudes of the node and of the
pericenter. We also show how the LAser GEOdynamics Satellites (LAGEOS) can be
used to constrain torsion parameters. We report the experimental constraints
obtained using both the nodes and perigee measurements of the orbital
Lense-Thirring effect. This makes LAGEOS and Gravity Probe B (GPB)
complementary frame-dragging and torsion experiments, since they constrain
three different combinations of torsion parameters
LARES/WEBER-SAT and the equivalence principle
It has often been claimed that the proposed Earth artificial satellite
LARES/WEBER-SAT-whose primary goal is, in fact, the measurement of the general
relativistic Lense-Thirring effect at a some percent level-would allow to
greatly improve, among (many) other things, the present-day (10^-13) level of
accuracy in testing the equivalence principle as well. Recent claims point
towards even two orders of magnitude better, i.e. 10^-15. In this note we show
that such a goal is, in fact, unattainable by many orders of magnitude being,
instead, the achievable level of the order of 10^-9.Comment: LaTex, 4 pages, no figures, no tables, 26 references. Proofs
corrections included. To appear in EPL (Europhysics Letters
Testing gravity at the Second post-Newtonian level through gravitational deflection of massive particles
Expression for second post-Newtonian level gravitational deflection angle of
massive particles is obtained in a model independent framework. Several of its
important implications including the possibility of testing gravitational
theories at that level are discussed.Comment: 5 pages, couple of equations of the previous version are correcte
Gravitomagnetism in the Kerr-Newman-Taub-NUT spacetime
We study the motion of test particles and electromagnetic waves in the
Kerr-Newman-Taub-NUT spacetime in order to elucidate some of the effects
associated with the gravitomagnetic monopole moment of the source. In
particular, we determine in the linear approximation the contribution of this
monopole to the gravitational time delay and the rotation of the plane of the
polarization of electromagnetic waves. Moreover, we consider "spherical" orbits
of uncharged test particles in the Kerr-Taub-NUT spacetime and discuss the
modification of the Wilkins orbits due to the presence of the gravitomagnetic
monopole.Comment: 12 pages LaTeX iopart style, uses PicTex for 1 Figur
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Geodetic precession and strong gravitational lensing in the dynamical Chern-Simons modified gravity
We have investigated the geodetic precession and the strong gravitational
lensing in the slowly-rotating black hole in the dynamical Chern-Simons
modified gravity theory. We present the formulas of the orbital period and
the geodetic precession angle for the timelike particles in the
circular orbits around the black hole, which shows that the change of the
geodetic precession angle with the Chern-Simons coupling parameter is
converse to the change of the orbital period with for fixed . We also
discuss the effects of the Chern-Simons coupling parameter on the strong
gravitational lensing when the light rays pass close to the black hole and
obtain that for the stronger Chern-Simons coupling the prograde photons may be
captured more easily, and conversely, the retrograde photons is harder to be
captured in the slowly-rotating black hole in the dynamical Chern-Simons
modified gravity. Supposing that the gravitational field of the supermassive
central object of the Galaxy can be described by this metric, we estimated the
numerical values of the main observables for gravitational lensing in the
strong field limit.Comment: 19 pages, 5 figures, more clarifications and references added,
accepted for publication in Classical and Quantum Gravit
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