Showing papers on "Gravitation published in 1980"

Gravitational Effects on and of Vacuum Decay

Abstract: It is possible for a classical field theory to have two stable homogeneous ground states, only one of which is an absolute energy minimum. In the quantum version of the theory, the ground state of higher energy is a false vacuum, rendered unstable by barrier penetration. There exists a well-established semiclassical theory of the decay of such false vacuums. In this paper, we extend this theory to include the effects of gravitation. Contrary to naive expectation, these are not always negligible, and may sometimes be of critical importance, especially in the late stages of the decay process.

Test of Relativistic Gravitation with a Space-Borne Hydrogen Maser

Abstract: The results of a test of general relativity with use of a hydrogen-maser frequency standard in a spacecraft launched nearly vertically upward to 10,000 km are reported. The agreement of the observed relativistic frequency shift with prediction is at the 70 x 10 to the -6th level.

QED Vacuum Polarization in a Background Gravitational Field and Its Effect on the Velocity of Photons

Abstract: We calculate in QED the contribution to the photon effective action from one-loop vacuum polarization on a general curved background manifold, and use it to investigate the corrections to the local propagation of photons. We find that the quantum corrections introduce tidal gravitational forces on the photons which in general alter the characteristics of propagation, so that in some cases photons travel at speeds greater than unity. The effect is nondispersive and gauge invariant. We look at a few examples, including a background Schwarzschild geometry, and we argue that although these results are controversial they do not in fact exhibit any obvious inconsistency.

Quantum-Mechanical Radiation-Pressure Fluctuations in an Interferometer

Abstract: The interferometers now being developed to detect gravitational vaves work by measuring small changes in the positions of free masses. There has been a controversy whether quantum-mechanical radiation-pressure fluctuations disturb this measurement. This Letter resolves the controversy: They do.

Where has the fifth dimension gone

Abstract: We show that a simple solution to the vacuum field equations of general relativity in 4 + 1 space-time dimensions leads to a cosmology which at the present epoch has 3 + 1 observable dimensions in which the Einstein-Maxwell equations are obeyed. The large ratio of the electromagnetic to gravitational forces is a consequence of the age of the Universe, in agreement with Dirac's large-number hypothesis.

Physics: Principles with Applications

Abstract: Describing motion - kinematics in one dimension kinematics in two or three dimensions - vectors motion and force - dynamics circular motion - gravitation work and energy linear momentum rotational motion bodies in equilibrium - elasticity and fracture fluids vibrations and waves sound temperature and kinetic theory heat the first and second laws of thermodynamics electric charge and electric field electric potential and electric energy electric currents DC circuits, electromagnetic induction and Faraday's Law - AC circuits electromagnetic waves light - geometric optics the wave nature of light optical instruments special theory of relativity early quantum theory and models of the atom quantum mechanics of atoms molecules and solids nuclear physics and radioactivity nuclear energy effects and uses of radiation elementary particles astrophysics and cosmology. Appendices.

Gravitation, geometry, and nonrelativistic quantum theory

Abstract: In Cartan's description, classical particles freely falling in a Newtonian gravitational field follow geodesics of a curved spacetime. We cast this geodesic motion into generalized Hamiltonian form and quantize it by Dirac's constraint method in a coordinate-independent way. The Dirac constraint takes a simplified form in special noninertial frames (nonrotating, rigid, Galilean, and Gaussian). Transformation theory of the state function allows us to compare descriptions of a given quantum state by two different observers and to illustrate how the principle of equivalence works for quantum systems. In particular, we show that quantum states of a particle moving in a homogeneous gravitational field and of the gravitational harmonic oscillator can be reduced to the study of plane waves in an appropriate frame.

Gravitational-wave research: Current status and future prospects

Abstract: There is a reasonably good change that in the 1980s cosmic gravitational waves will be discovered and will become a powerful tool for astronomy. This prospect has stimulated a three-pronged research effort. First, relativity theorists are developing new mathematical tools for the analysis of gravitational radiation—including (i) methods of analyzing the generation of gravity waves by sources with strong selfgravity and large internal velocities (e.g., collisions of black holes), (ii) methods of computing radiation reaction in sources, and (iii) methods of analyzing how gravitational waves propagate through our lumpy curved-space Universe. Second, astrophysicists are attempting to identify the most promissing sources of gravitational waves, and are using the relativity theorists' mathematical tools to estimate the characteristics of the waves they emit. Third, with the estimated wave characteristics in mind, experimenters are designing and constructing a second generation of gravitational-wave detectors—detectors of three types: Doppler tracking of interplanetary spacecraft, Earth-based laser interferometers, and Earth-based Weber-type resonant bars. This article reviews, in brief, all three prongs of the research effort and gives references to more detailed articles about specialized aspects of gravitational-wave physics.

Parity violation in metric-torsion theories of gravitation

Abstract: The general structure of metric-torsion theories of gravitation is shown to allow a parity-violating contribution to the complete action which is linear in the curvature tensor and vanishes identically in the absence of torsion. The resulting action involves apart from the Newtonian constant a coupling which governs the strength of the predicted parity-nonconserving ''interactions'' mediated by torsion. We consider this theory in the presence of the Proca field and show that it leads to a parity-violating term in the field equations in contrast to the Einstein-Cartan-Sciama-Kibble theory, which we use as a particularly simple example of a metric-torsion theory of gravitation.

Consistency Problems of Spin-2 Gravity Coupling

Abstract: The divergence identities obeyed by the equations of a free linear spin-2 gauge field are no longer satisfied when it is coupled to gravity. In contrast with supergravity, the resulting consistency requirements cannot be satisfied even upon use of the Einstein equations, despite the presence of a supergravitylike «miracle». These and related propagation problems persist for general nonminimal gravitational coupling and also if torsion is included. General relations between consistency requirements, co-ordinate invariance and stress tensor conservation are displayed and applied to strong gravity.

Propagation equations for test bodies with spin and rotation in theories of gravity with torsion

Abstract: We generalize the Papapetrou equations by deriving propagation equations for the energy-momentum and angular momentum of a test body which has both elementary-particle spin and macroscopic rotation and which is moving in background metric and torsion fields. Our results show that the torsion couples to spin but not to rotation. Thus a rotating test body with no net spin will ignore the torsion and move according to the usual Papapetrou equations. Hence the standard tests of gravity are insensitive to a torsion field. We propose experiments (although still infeasible) to compare the motion of a spin-polarized body with the motion of a rotating body. If the spin and rotation precess differently, the theory of gravity cannot be a metric theory but may be a torsion theory.

Gravity from Poincaré Gauge Theory of the Fundamental Particles. III ---Weak Field Approximation---

Abstract: We apply the weak field approximation to the most general gravitational field equations in Poincare gauge theory. The weak gravitational field h., is a multimass field obeying a fourth-order field equation. In the Newtonian approximation we show that there are two routes to arrive at the Newtonian potential. The torsion field is decomposed into six irre­ ducibe building blocks with spinparlty, 2+, z-, 1+, 1-, o+ and o-, each of which obeys the Klein-Gordon equation. Finally, we construct a possible candidate for the massless graviton field which obeys the linearized Einstein equation.

Horizon problem and the broken-symmetric theory of gravity

Abstract: A way of avoiding the standard scenario (which many people find vexing on philosophical grounds) is suggested in which particles are in causal contact with only a limited number of other particles in the early universe.

Semiclassical relativity: The weak-field limit

Abstract: Semiclassical relativity is a theory in which quantum matter fields interact with a classical gravitational field via the semiclassical Einstein equation G/sub a/b=8..pi.. . We consider, for the case of massless quantum fields, the weak-field limit of this theory. It is shown that the linearized quantum stress energy can be determined in a manner which is essentially independent of the details of any regularization prescription. Using this expression for the quantum stress energy, we solve the linearized semiclassical Einstein equation. The solutions found include (1) perturbations that satisfy the linearized classical Einstein equation G/sub a/b=0, (2) perturbations that grow exponentially in time, and (3) perturbations that (in some sense) travel faster than the speed of light.

A harmonic analysis of lunar gravity

Abstract: An improved model of lunar global gravity has been obtained by fitting a sixteenth-degree harmonic series to a combination of Doppler tracking data from Apollo missions 8, 12, 15, and 16, and Lunar Orbiters 1, 2, 3, 4, and 5, and laser ranging data to the lunar surface. To compensate for the irregular selenographic distribution of these data, the solution algorithm has also incorporated a semi-empirical a priori covariance function. Maps of the free-air gravity disturbance and its formal error are presented, as are free-air anomaly and Bouguer anomaly maps. The lunar gravitational variance spectrum has the form V(G; n) = O(n to the -4th power), as do the corresponding terrestrial and martian spectra. The variance spectra of the Bouguer corrections (topography converted to equivalent gravity) for these bodies have the same basic form as the observed gravity; and, in fact, the spectral ratios are nearly constant throughout the observed spectral range for each body. Despite this spectral compatibility, the correlation between gravity and topography is generally quite poor on a global scale.

Conformal invariance, microscopic physics, and the nature of gravitation

Abstract: The question of whether the gravitational "constant" can vary in spacetime has been among the most vexing in physics. The thrust of this paper is that the issue may be fully resolved if one accepts the principle (first proposed by Weyl and lucidly discussed by Hoyle and Narlikar) that all the fundamental equations of physics should be invariant under local (spacetime-dependent) transformations of units (principle of conformal invariance). Theoretical arguments in favor of the principle are discussed. We then show that the presently accepted dynamics for the fundamental particles and their electromagnetic, weak, and strong interactions indeed satisfy the principle. Their conformal invariance is due not least to the indispensable transformation properties of rest masses. Thus in arbitrary units each type of rest mass is a spacetime field. The principle of conformal invariance then demands conformal invariance of the dynamics of each such "mass field." If all rest-mass ratios are strictly constant there is only one mass field. Its dynamics automatically induces dynamics for gravitation. In units defined by particle masses the gravitational action is manifestly that of general relativity, a fact discovered in different guises and independently by several workers. This would seem to forbid the construction of a conformally invariant theory of gravitation with "varying gravitational constant" $G$. Such theories have been proposed by Dirac, and later by Canuto and coworkers, who have argued that, a priori, gravitational (Einstein) units are distinct from those defined by matter (atomic units). We find that to implement such distinction while simultaneously avoiding undetermined elements in the theory, one must introduce conformally invariant dynamics for gravitation and for the mass field separately. We construct this theory; it is a "varying-$G$ theory." We then show that it is definitely ruled out by the solar-system gravitational experiments. We conclude that the principle of conformal invariance requires that gravitation be described by general relativity, and that the dimensionless gravitational constant $\ensuremath{\gamma}$ be strictly constant. We also consider the possibility that gravitation, or the mass field, explicitly break conformal invariance. The corresponding theory, the theory of variable rest masses (VMT), was developed earlier from a different viewpoint. Although it predicts variability of $\ensuremath{\gamma}$, we point out that for a vast majority of cosmological models, the temporal variability of $\ensuremath{\gamma}$ is well below experimental sensitivities.

Gravity theories with propagating torsion

Abstract: We study the propagators for a large class of gravity theories having a nonzero, metric-compatible torsion. The theories are derivable from a Lagrangian containing all possible invariants quadratic or less in the torsion and Riemann curvature tensors, except that invariants are dropped if they do not contribute to the propagator in the linearized limit. Therefore, the torsion in these theories is, in general, a propagating field rather than one which vanishes outside matter. We study the constraints imposed on the propagator by the requirement that the theory have no ghosts or tachyons. In particular, we find that the addition of a spin-${2}^{+}$ torsion multiplet does not remove the spin-${2}^{+}$ ghost contributed by higher-derivative terms (Riemann curvature-squared terms). We discuss the phenomenology of theories with propagating torsion. The torsion must couple to spins with coupling constants much smaller than the electromagnetic fine-structure constant, or the force between two macroscopic ferromagnets, due to torsion exchange, would be huge, far greater than the familiar magnetic force due to photon exchange. We briefly discuss the phenomenology of propagating torsion "potentials." Theories involving such potentials have been proposed recently by several authors.

Isolated systems in general relativity

Abstract: Some problems regarding the relativistic theory of isolated systems are described. The description of bodies as a whole and the derivation of their overall laws of motion are considered along with the asymptotics of gravitational fields near various parts of infinity. Attention is given to the connection between near-zone and asymptotic energy-momentum balances, and to the meanings of the following terms: asymptotically flat, physically reasonable T(ab), and condition for absence of incident radiation. In addition, an improved version of a weak-field, slow-motion approximation method for constructing relativistic models of isolated systems containing more than one body is outlined.

A solution of the Cauchy initial value problem in the nonsymmetric theory of gravitation

Abstract: The Cauchy initial value problem in a theory of gravity based on a nonsymmetric Hermitian metric is investigated. The field equations are solved in terms of a series expansion of the metric gμν. In the nth order the equations take the form of either second‐order hyperbolic partial differential equations for gij (i, j=1,2,3) or a first‐order equation for the vector gauge field Wi. Given initial data on a space‐like surface S (x0=0), the integration forward in time of the equations can be performed, assuming that the field variables are reasonably smooth and analytic.

Radiation from collapsing relativistic stars. III. Second order perturbations of collapse with rotation

Abstract: We study the gravitational waves generated during the collapse of a slowly rotating, pressureless, constant density star. To first order in the rotation rate the perturbation is an odd-parity dipole and generates no gravitational waves. The first order perturbation, however, drives a quadrupole deformation which is second order in the rotation rate and which produces the radiation. Using a gauge invariant description, we derive the equations governing this second order perturbation. These equations are then applied to the case of the collapse of a slowly and uniformly rotating constant-density star following the sudden turnoff of pressure. Numerical results are given for the gravitational waveforms and energies generated.