Search

The coupling of the electromagnetic field to gravity is an age-old problem. Presently, there is a resurgence of interest in it, mainly for two reasons: (i) Experimental investigations are under way with ever increasing precision, be it in the laboratory or by observing outer space. (ii) One desires to test out alternatives to Einstein's gravitational theory, in particular those of a gauge-theoretical nature, like Einstein-Cartan theory or metric-afine gravity.— A clean discussion requires a reflection on the foundations of electrodynamics. If one bases electrodynamics on the conservation laws of electric charge and magnetic flux, one finds Maxwell's equations expressed in terms of the excitation H = (D,H) and the field strength F = (E,B) without any intervention of the metric or the linear connection of spacetime. In other words, there is still no coupling to gravity. Only the constitutive law H = functional(F) mediates such a coupling. We discuss the different ways of how metric, nonmetricity, torsion, and curvature can come into play here. Along the way, we touch on non-local laws (Mashhoon), non-linear ones (Born-Infeld, Heisenberg-Euler, Plebaśki), linear ones, including the Abelian axion (Ni), and fid a method for deriving the metric from linear electrodynamics (Toupin, Schönberg). Finally, we discuss possible non-minimal coupling schemes. [ABSTRACT FROM AUTHOR]

The dynamical equations which are basic for the description of the dynamics of quantum felds in arbitrary space-time geometries, can be derived from the requirements of a unique deterministic evolution of the quantum fields, the superposition principle, a finite propagation speed, and probability conservation. We suggest and describe observations and experiments which are able to test the unique deterministic evolution and analyze given experimental data from which restrictions of anomalous terms violating this basic principle can be concluded. One important point is, that such anomalous terms are predicted from loop gravity as well as from string theories. Most accurate data can be obtained from future astrophysical observations. Also, laboratory tests like spectroscopy give constraints on the anomalous terms. [ABSTRACT FROM AUTHOR]

Spin is the ultimate gyroscope. The smallest possible amount of angular momentum is ħ/Π- that possessed by a spin 1/2 particle. When the day comes that it becomes realistic to theorise about and to measure the precession of a spin 1/2 particle it will be necessary to have to hand the relevant theoretical tools; in other words, to be able to give a description of spin one-half particles which is consistent with Special Relativity, and to generalise that description to General Relativity. In general terms, then, this is an exercise in relativistic quantum mechanics, and in the case of General Relativity, in quantum mechanics in a curved space. This latter is, of course, different from quantum gravity. Quantum gravity is a theory describing the quantum nature of the gravitational field itself, for example in terms of gravitons. For our purposes the gravitational field is treated classically, as a curved space-time. The only thing to be quantised is the spin 1/2 particle. It may be thought that this exercise has already been performed, since the Dirac equation is nothing other than a relativistic equation for spin 1/2 particles. It turns out, however, that the Dirac equation itself does not automatically yield a relativistic spin operator. The problems connected with finding such an operator were already identified in 1950b y Foldy and Wouthuysen. After outlining these problems, we shall describe how this operator is constructed. The paper concludes with some remarks about the extent to which it makes sense to talk about spin in the context of general relativity. In particular it will be pointed out that spin precession is inevitable in GR; there is no such thing as conserved spin in curved spacetime. [ABSTRACT FROM AUTHOR]

In these two talks, we report on the efforts to probe the role of spin and polarization in gravitation. After reviewing the motivation and historical background, we focus the talks on the experimental searches. These experimental searches are mainly of two categories: (i) laboratory searches (torsion-balance experiments, magnetic resonance experiments, SQUID experiments), and (ii) astrophysical and cosmological searches (pulsar observations, radio-galaxy observations, gamma-ray observations). We first discuss experimental searches for photon polarization coupling and then discuss experimental searches for electron spin-coupling. In the discussion of photon polarization coupling, we review the astrophysical and cosmological electromagnetic propagation observations. In the discussion of electron spin-coupling, we review the weak equivalence principle experiments, the finite-range spin coupling experiments, the spin-spin coupling experiments and the cosmic-spin coupling experiments. We discuss two recent laboratory experiments, a SQUID experiment and a torsion-balance experiment in detail to illustrate the experimental techniques. The ultimate searches for the role of spin in gravitation is to measure the gyrogravitational ratio. A discussion of the strategies to perform such experiments conclude these two talks. [ABSTRACT FROM AUTHOR]

We present a second-quantized field theory of massive spin one-half particles or antiparticles in the presence of a weak gravitational field treated as a spin two external field in a flat Minkowski background. We solve the difficulties which arise from the derivative coupling and we are able to introduce an interaction picture. We derive expressions for the scattering amplitude and for the outgoing spinor to first-order. In several appendices, the link with the canonical approach in General Relativity is established and a generalized stationary phase method is used to calculate the outgoing spinor. We show how our expressions can be used to calculate and discuss phase shifts in the context of matter-wave interferometry (especially atom or antiatom interferometry). In this way, many effects are introduced in a unified relativistic framework, including spin-gravitation terms: gravitational red shift, Thomas precession, Sagnac effect, spin-rotation effect, orbital and spin Lense-Thirring effects, de Sitter geodetic precession and finally the effect of gravitational waves. A new analogy with the electromagnetic interaction is pointed out. [ABSTRACT FROM AUTHOR]

25 years ago, in summer 1974, Joseph Taylor and Russell Hulse discovered the first binary pulsar, a pulsar in orbit with a compact companion which itself is most likely a neutron star. This pulsar, denoted PSR B1913+16, turned out to be the most exciting laboratory for testing relativistic gravity theories. Before the discovery of PSR B1913+16 all gravity experiments were confined to our solar system with its very weak gravitational fields. Hence, it has been possible to test gravity theories only in the first post-Newtonian approximation. Binary pulsars take us beyond the weak-field context because of their high orbital velocity and/or the strong self-gravitational fields of neutron stars. To date, more than 70 binary pulsars have been discovered, most of them in orbit with a neutron star or a white dwarf. Many binary pulsars belong to a group of so-called millisecond pulsars which have very short rotational periods (< 20 ms) and slowdown rates of typically 10p-20, proving to be extremely accurate clocks. This gave rise to a variety of new gravity experiments, like tests for the strong equivalence principle. After a brief introduction to pulsars, the technical and theoretical aspects of binary- pulsar gravity experiments are reviewed. The latest results are presented and an outlook is given to future improvements of these experiments. [ABSTRACT FROM AUTHOR]

SpaceTime is a mission concept developed to test the Equivalence Principle. The mission is based on a clock experiment that will search for a violation of the Equivalence Principle through the observation of a variation of the fine structure constant, α. A spatio-temporal variation of α is expected in some string theories aimed at unifying gravity with other forces in nature. SpaceTime uses a special tri- clock instrument on a spacecraft which approaches the sun to within four solar radii. The instrument consists of three trapped ion clocks based on mercury, cadmium, and ytterbium ions, in the same environment. This configuration allows for a differential measurement of the frequency of the clocks and the cancellation of perturbations common to the three. The observation of any frequency drift between each of the clocks, as the tri-clock instrument approaches the sun, signals the existence of a scalar partner to the tensor gravity. Some relevant details of the mission design are discussed in the paper. [ABSTRACT FROM AUTHOR]

The evolution of various fields of science, technology, trade or legal metrology is intimately connected with the ability to relate measurements with each other that were performed at different places and different instants of time. For this purpose a practical system of units of measurement i.e. the International System of Units (SI) has been established by international cooperation [1]. In this SI, the metre and the second represent the base units of length and time, respectively. From all units these two can be realized with by far the highest accuracy since they are based on frequency measurements and most accurate clocks. In contrast to clocks based on mechanical properties of macroscopic bodies, e.g., pendulum clocks, quartz clocks or pulsars, the frequency reference for a suitable oscillator in atomic clocks is mainly determined by the intrinsic properties of suitable absorbers like atoms, molecules or ions. These atomic properties are determined by fundamental constants resulting from the basic interactions between the elementary constituents of matter. Following the generally accepted idea that the properties of each atomic absorber of a selected species are the same, identical clocks can be set up in any desired number and at any desired place. [ABSTRACT FROM AUTHOR]

For last thirty years, great advances in the testing of relativistic gravity have come from interplanetary radio ranging and lunar laser ranging. With optical mission concepts in the interplanetary space, testing relativistic gravity can be improved by 3 - 6 orders of magnitude and many solar-system parameters can be measured either for the first time or more precisely. After reviewing briefly dedicated optical mission concepts — SORT, IPLR and ASTROD together with other optical mission or mission concepts which have important implications on testing relativity and astrodynamics — HIPPARCOS, GAIA and LISA, we concentrate on a specific mission concept — ASTROD to discuss various mission goals and capabilities in detail. ASTROD is an optical interferometry mission concept. Optical interferometry missions hold great promises for the testing of relativistic gravity and for the measuring of solar-system parameters. We discuss the determination of relativistic parameters γ, β and the solar quadrupole moment parameter J2, the measurements of solar Lense-Thirring effect together with the application of laser astrodynamics to solar system studies — solar angular momentum, solar g-modes, asteroid masses, etc. [ABSTRACT FROM AUTHOR]

30 years of lunar laser ranging (LLR) data has been modeled and fit with several millimeters precision using the general relativistic equations of motion for solar system dynamics. This produces several key tests of that tensor theory of gravity and strongly constrains presence of any supplementary interactions. Earth and Moon fall toward the Sun at rates equal to a couple parts in 1013, confirming both the universal coupling of gravity to matter's stress-energy tensor, and gravity's specific non-linear coupling to itself. The expected deSitter precession (with respect to the distant ‘fixed' stars) of the local inertial frame moving with the Earth-Moon system is confirmed to 3.5 parts in 103 precision, and Newton's constant indeed shows no cosmological time variation at the few parts in 1012 per year level. All the types of post-Newtonian terms in the N-body equation of motion—motional, gravito-magnetic, non-linear, inductive, etc.—contribute to the measured details of the lunar orbit, so LLR achieves ‘near- completeness' as a gravity experiment and probe. The precision of these measurements, especially those connected with lunar orbit frequencies and their rates of change, should further improve as LLR observations continue into the future. [ABSTRACT FROM AUTHOR]

The main general relativistic effects in the motion of the Moon are briefly reviewed. The possibility of detection of the solar gravitomagnetic contributions to the mean motions of the lunar node and perigee is discussed. [ABSTRACT FROM AUTHOR]

The appearance of new fundamental forces and extra-dimensional modifications to gravity in extensions of the Standard Model has motivated considerable interest in testing Newtonian gravity at short distances (≲ 10-3 m). Presently a number of new gravity experiments are searching for non-Newtonian effects in the ranges ~ 10-4-10-3 m. However, as challenging as these experiments are, formidable new obstacles await the next generation of experiments which will probe gravity at distances ≲10-4 m where Casimir/van der Waals forces become dominant. Here we will review the motivation for conducting such very short distance gravity experiments, and discuss some of the new problems that may arise in future experiments. Finally, we suggest schematic designs for null experiments which would address some of these problems using the "iso-electronic" and "finite-size" effects. [ABSTRACT FROM AUTHOR]

In view of space missions, for accurate recovery of the Earth gravity field, for the test of the equivalence principle and for the observation of gravity waves in particular, specific inertial sensors are developed exhibiting very high resolution and limited full scale range suited for in orbit operation. These sensors are constructed around a high density proof-mass with a very fine and stable silica gold coated core. The proof-mass position and attitude are measured with highly sensitive capacitive sensors and are controlled with electrostatic actuators. The configuration and the major design parameters of these instruments are described in relation to the expected performances. The present status of the development of these instruments is shown together with the associated space mission scientific objectives. The main experimental results obtained during the ground qualification of these accelerometers are also presented. [ABSTRACT FROM AUTHOR]

Free fall tests for proving the Weak Equivalence Principle (WEP) have been rarely be done in history. Although they seem to be the natural experiments to test the equivalence of inertial and gravitational mass, best results for proofs of the WEP could be attained with torsion pendulum tests to an accuracy of 10-12. Pendulum tests are long term periodic experiments, whereas free fall tests on Earth can be carried out only for seconds causing certain limitations in principle. Nevertheless, very precise fall tests in the 10-12 to 10-13 range are possible and under preparation to be carried out on the Drop Tower Bremen for a free fall over 110 m. These tests require position detectors with an extremely high resolution in order to measure tiny displacements of freely falling test masses. Using SQUID-based sensing technique, the displacements can be determined with an accuracy of 2 x 10-14 m/√Hz. The SQUID system, developed and manufactured at Jena University, provides high sensitivity and extremely low intrinsic noise, especially at low frequencies. Some recent results are discussed. [ABSTRACT FROM AUTHOR]

This paper presents an overview of the current technical status of STEP, the Satellite Test of the Equivalence Principle. STEP was originally presented as a candidate for ESA's M2 mission as a joint mission with NASA, and has since been studied as an M3 candidate, and under NASA as QuickSTEP and MiniSTEP. Studies especially during the last two years have resolved some long standing issues such as control of helium tide, improved the mission definition and error analysis, and have resulted in an improved baseline design which should be capable of comparing rates of fall to an accuracy approaching 10-18. [ABSTRACT FROM AUTHOR]

Modern formulations of equivalence principles provide the foundation for an efficient approach to understanding and organizing the structural features of gravitation field theories. Since theories' predictions reflect differences in their structures, principles of equivalence also support an efficient experimental strategy for testing gravitation theories and for exploring the range of conceivable gravitation physics. These principles focus attention squarely on empirical consequences of the fundamental structural differences that distinguish one gravitation theory from another. Interestingly, the variety of such consequences makes it possible to design and perform experiments that test equivalence principles stringently but do so in markedly different ways than the most familiar experimental tests. [ABSTRACT FROM AUTHOR]

The range of expected amplitudes and spectral slopes of relic (squeezed) gravitational waves, predicted by theory and partially supported by observations, is within the reach of sensitive gravity-wave detectors. In the most favorable case, the detection of relic gravitational waves can be achieved by the cross-correlation of outputs of the initial laser interferometers in LIGO, VIRGO, GEO600. In the more realistic case, the sensitivity of advanced ground-based and space-based laser interferometers will be needed. The specific statistical signature of relic gravitational waves, associated with the phenomenon of squeezing, is a potential reserve for further improvement of the signal to noise ratio. [ABSTRACT FROM AUTHOR]

The paper gives an introduction to the gravitational radiation theory of isolated sources and to the propagation properties of light rays in radiative gravitational fields. It presents a theoretical study of the generation, propagation, back-reaction, and detection of gravitational waves from astrophysical sources. After reviewing the various quadrupole-moment laws for gravitational radiation in the Newtonian approximation, we show how to incorporate post-Newtonian corrections into the source multipole moments, the radiative multipole moments at infinity, and the back-reaction potentials. We further treat the light propagation in the linearized gravitational field outside a gravitational wave emitting source. The effects of time delay, bending of light, and moving source frequency shift are presented in terms of the gravitational lens potential. Time delay results are applied in the description of the procedure of the detection of gravitational waves. Pacs Numbers : 04.25.-g, 04.25.Nx [ABSTRACT FROM AUTHOR]

The last few years have brought a great break-through in the quest for earth-bound detection of gravitational waves: at five sites, laser-interferometric detectors, of armlengths from 0.3 to 4 km, are being built. These projects have in common that one prominent noise source, the shot noise, is reduced by the use of power recycling. By using advanced optical technologies early on, the German-British project GEO 600, although only intermediate in size (600 m), has good chances for a competitive sensitivity, at least with the first versions of the larger detectors. Particularly the use of the so-called signal recycling technique will allow to search for faint sources of only slowly varying frequency (pulsars, close binaries). The talk will describe the particular topology of the GEO 600 interferometer, characterized by the use of a four-pass delay line and signal recycling. The major noise sources, and the experimental effort aiming at their reduction, will be discussed. The current status of the construction of GEO 600 will be outlined (civil engineering, vacuum, optics). The research and development activities at the experimental sites (Garching, Glasgow, Hannover) will be given broad emphasis. First science runs of GEO 600, well in time with those of other ground-based interferometers, are expected in the year 2001. [ABSTRACT FROM AUTHOR]

The motion of spinning relativistic particles in external electromagnetic and gravitational fields is considered. The noncovariant spin formalism is crucial for the correct description of the influence of the spin on the particle trajectory. It is shown that the true coordinate of a relativistic spinning particle is the naive, common coordinate r. A simple derivation is presented for the gravitational interaction of first order in spin, for a relativistic particle. The equations of motion obtained for a relativistic spinning particle in an external gravitational field differ essentially from the Papapetrou equations. Effects of higher order in spin are discussed, including the gravimagnetic moment, a special spin effect in general relativity. We consider also the contributions of the spin interactions of first and second order to the gravitational radiation of compact binary stars. [ABSTRACT FROM AUTHOR]

The main theoretical aspects of gravitomagnetism are reviewed. It is shown that the gravitomagnetic precession of a gyroscope is intimately connected with the special temporal structure around a rotating mass that is revealed by the gravitomagnetic clock effect. This remarkable effect, which involves the difference in the proper periods of a standard clock in prograde and retrograde circular geodesic orbits around a rotating mass, is discussed in detail. The implications of this effect for the notion of "inertial dragging" in the general theory of relativity are presented. The theory of the clock effect is developed within the PPN framework and the possibility of measuring it via spaceborne clocks is examined. [ABSTRACT FROM AUTHOR]

NASA's Gravity Probe B Mission is a test of two predictions of Einstein's General Theory of Relativity based on observations on very precise cryogenic gyroscopes in a satellite in a 650 km polar orbit about the Earth. Construction and the first round of testing of the flight payload was completed in December 1999. Of the 32 planned qualification tests 28 were passed with complete success, meeting or in several instances surpassing the program requirements. However, one test very unexpectedly revealed a problem in the thermal performance of the Dewar/Probe system which has required a significant redesigin and rework, now successfully completed. Gravity Probe B is scheduled for launch on April 1, 2002. This article reviews from the physicist's viewpoint the experience of living through a space flight program. [ABSTRACT FROM AUTHOR]

A pedagogical derivation is given of the Lense-Thirring effect using basic notions from the motion of point particles and light rays. First, the notion of rotation is introduced using the properties of light rays only. Second, two realizations for a non- rotating propagation of space-like directions are presented: the gyroscope and the spin of elementary particles. Then the gravitational field around a rotating body is specified which is taken for determining the various effects connected with a point particle or a gyroscope: the deSitter precession (geodesic precession) and the Lense-Thirring effect (‘frame dragging'). The results are applied to the precession of gyroscopes and to the motion of satellites around the earth. [ABSTRACT FROM AUTHOR]

The Newtonian gravitational constant G was the first known fundamental constant of physics. Nevertheless, the measurement of its value still seem to be in a rather sad shape. Recently, the CODATA Task Group on Fundamental Constants recommended a preliminary value of G with a relative uncertainty of 0.15 %. This is more than ten times larger as the previous recommendation! In the first part of this lecture, a brief summary is given of recent experimental efforts to determine G. The second part is a description of our experiment at the University of Zürich. We use a beam balance to measure the gravitational forces of large field masses (13.5 × 103 kg mercury) on 1 kg test masses. A first result with an uncertainty of 220 × 10?6 has been published recently. Presently we are working to improve the experiment. [ABSTRACT FROM AUTHOR]

If the gravitational metric tensor field of Einstein's General Relativity is supplemented by other long range, very weakly coupled interaction fields, then General Relativity's Equivalence Principle foundations are violated, or its post-Newtonian (1/c2order) structure is altered, or both. Space experiments test for and measure such possibilities; presently universality of free fall is confirmed to about a part in 1012, and no deviations of post-Newtonian metric gravity from general relativity are seen down to the few parts in 104 level. Future experiments in space can significantly increase the precision to which fundamental physical law is probed. In particular, transponded interplanetary laser ranging can measure presence of metrically coupled scalar fields in gravity with two or three orders of magnitude higher precision than past experiments, and can begin to measure the second post-Newtonian (1/c4) structure of gravity. A space-based experiment of the universality of free-fall (STEP) can detect additional interactions of a non-metric nature with five or six orders of magnitude higher precision than today's experiments. [ABSTRACT FROM AUTHOR]