381 research outputs found
Relativistic positioning and Sagnac-like measurements for fundamental physics in space
The paper concerns the use of satellites of the Galileo constellation for
relativistic positioning and for measurements of the gravito-magnetic effects
induced by the angular momentum both of the Earth and of the dark halo of the
Milky Way. The experimental approach is based on the generalized Sagnac effect,
induced both by the rotation of the device and the fact that the observer is
located within the gravitational field of a spinning mass. Among the possible
sources there is also the angular momentum of the dark halo of the Milky Way.
Time modulation of the expected signal would facilitate its disentanglement
from the other contributions. The modulation could be obtained using satellites
located on different orbital planes.Comment: 15 pages, 4 figures. To appear on Advances in Space Researc
Testing General Relativity
This lecture will present a review of the past and present tests of the
General Relativity theory. The essentials of the theory will be recalled and
the measurable effects will be listed and analyzed. The main historical
confirmations of General Relativity will be described. Then, the present
situation will be reviewed presenting a number of examples. The opportunities
given by astrophysical and astrometric observations will be shortly discussed.
Coming to terrestrial experiments the attention will be specially focused on
ringlasers and a dedicated experiment for the Gran Sasso Laboratories, named by
the acronym GINGER, will be presented. Mention will also be made of
alternatives to the use of light, such as particle beams and superfluid rings.Comment: The paper will appear on Proceedings of Science: Gran Sasso Summer
Institute 201
Experimental determination of gravitomagnetic effects by means of ring lasers
A new experiment aimed to the detection of the gravito-magnetic Lense-Thirring effect at the surface of the Earth will be presented; the name of the experiment is GINGER. The proposed technique is based on the behavior of light beams in ring lasers, also known as gyrolasers. A three-dimensional array of ringlasers will be attached to a rigid monument; each ring will have a different orientation in space. Within the space-time of a rotating mass the propagation of light is indeed anisotropic; part of the anisotropy is purely kinematical (Sagnac effect), part is due to the interaction between the gravito-electric field of the source and the kinematical motion of the observer (de Sitter effect), finally there is a contribution from the gravito-magnetic component of the Earth (gravito-magnetic frame dragging or Lense-Thirring effect). In a ring laser a light beam traveling counterclockwise is superposed to another beam traveling in the opposite sense. The anisotropy in the propagation leads to standing waves with slightly different frequencies in the two directions; the final effect is a beat frequency proportional to the size of the instrument and its effective rotation rate in space, including the gravito-magnetic drag. Current laser techniques and the performances of the best existing ring lasers allow at the moment a sensitivity within one order of magnitude of the required accuracy for the detection of gravito-magnetic effects, so that the objective of GINGER is in the range of feasibility and aims to improve the sensitivity of a couple of orders of magnitude with respect to present. The experiment will be underground, probably in the Gran Sasso National Laboratories in Italy, and is based on an international collaboration among four Italian groups, the Technische Universitaet Muenchen and the University of Canterbury in Christchurch (NZ
A tensor theory of space-time as a strained material continuum
The classical theory of strain in material continua is reviewed and
generalized to space-time. Strain is attributed to "external" (matter/energy
fields) and intrinsic sources fixing the global symmetry of the universe
(defects in the continuum). A Lagrangian for space-time is worked out, adding
to the usual Hilbert term an "elastic" contribution from intrinsic strain. This
approach is equivalent to a peculiar tensor field, which is indeed part of the
metric tensor. The theory gives a configuration of space-time accounting both
for the initial inflation and for the late acceleration. Considering also the
contribution from matter the theory is used to fit the luminosity data of type
Ia supernovae, giving satisfactory results.Comment: Revised to match the version accepted for publication in Class.
Quantum Gra
Massive gravitational waves from the Cosmic Defect theory
The Cosmic Defect theory (CD), which is presented elsewhere in this
conference, introduces in the standard Einstein-Hilbert Lagrangian an elastic
term accounting for the strain of space-time viewed as a four-dimensional
physical continuum. In this framework the Ricci scalar acts as the kinetical
term of the strain field whose potential is represented by the additional
terms. Here we are presenting the linearised version of the theory in order to
analyze its implications in the weak field limit. First we discuss the recovery
of the Newtonian limit. We find that the typical static weak field limit
imposes a constraint on the values of the two parameters (Lame coefficients) of
the theory. Once the constraint has been implemented, the typical gravitational
potential turns out to be Yukawa-like. The value for the Yukawa parameter is
consistent with the constraints coming from the experimental data at the Solar
system and galactic scales. We then come to the propagating solutions of the
linearised Einstein equations in vacuo, i.e. to gravitational waves. Here,
analogously with other alternative or extended theories of gravity, the
presence of the strain field produces massive waves, where massive (in this
completely classical context) means subluminal. Furthermore longitudinal
polarization modes are allowed too, thus lending, in principle, a way for
discriminating these waves from the plane GR ones.Comment: Proceedings of 'Invisible Universe International Conference', Paris,
June 29- July 3, 200
Test of gravitomagnetism with satellites around the Earth
We focus on the possibility of measuring the gravitomagnetic effects due to
the rotation of the Earth, by means of a space-based experiment that exploits
satellites in geostationary orbits. Due to the rotation of the Earth, there is
an asymmetry in the propagation of electromagnetic signals in opposite
directions along a closed path around the Earth. We work out the delays between
the two counter-propagating beams for a simple configuration, and suggest that
accurate time measurements could allow, in principle, to detect the
gravitomagnetic effect of the EarthComment: 6 pages, 3 figures; revised to match the version accepted for
publication in EPJ
Lorentz contraction and accelerated systems
The paper discusses the problem of the Lorentz contraction in accelerated
systems, in the context of the special theory of relativity. Equal proper
accelerations along different world lines are considered, showing the
differences arising when the world lines correspond to physically connected or
disconnected objects. In all cases the special theory of relativity proves to
be completely self-consistentComment: 7 pages, LaTeX, to be published in European Journal of Physic
Mapping Cartesian Coordinates into Emission Coordinates: some Toy Models
After briefly reviewing the relativistic approach to positioning systems
based on the introduction of the emission coordinates, we show how explicit
maps can be obtained between the Cartesian coordinates and the emission
coordinates, for suitably chosen set of emitters, whose world-lines are
supposed to be known by the users. We consider Minkowski space-time and the
space-time where a small inhomogeineity is introduced (i.e. a small
"gravitational" field), both in 1+1 and 1+3 dimensions.Comment: 13 pages, 7 figures, Accepted for publication in International
Journal of Modern Physics
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