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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1971

ESRO study program for a space experiment on gravitation theories

ESRO is considering a space experiment which is the definition phase. A more complete utilization of space techniques, leading to highly accurate acceleration measurements in a heliocentric spacecraft, together with an improved laser signal propagation method (using a space-borne atomic clock), could substantially increase the validity of the gravitational time delay test during solar conjunction. Preliminary investigations of the primary required techniques were carried out. These studies included an orbit analysis, investigation of drag-free techniques, and studies of the time measuring instrument. These studies were used to define the framework of a space experiment on gravitation theories. A preliminary feasibility study of the mission is being undertaken.

Israel, G. M.↗
NASA NTRS1972

Gravitational-wave astronomy

The significance of experimental evidence for gravitational waves is considered for astronomy. Properties, generation, and astrophysical sources of the waves are discussed. Gravitational wave receivers and antennas are described. A review of the Weber experiment is presented.

Press, W. H.↗
NASA NTRS1972

Gravitation research

Balloon measurements were made of the far infrared background radiation. The radiometer used and its calibration are discussed. An electromagnetically coupled broadband gravitational antenna is also considered. The proposed antenna design and noise sources in the antenna are reviewed. A comparison is made between interferometric broadband and resonant bar antennas for the detection of gravitational wave pulses.

Weiss, R.↗
NASA NTRS1972

Interpretation of gravitational-wave observations.

Weber's (1971) interpretation of his observations involving a source at the center of the Galaxy isotropically radiating in the form of gravitational waves is discussed. The gravitational-wave flux at the earth is presumed to be that indicated by Weber's experiments, and directions are suggested in which more satisfactory theories of the source can be sought.

Misner, C. W.↗
NASA NTRS1972

Possible variation of the gravitational constant over the elements.

We re-examine the theory and data of the Kreuzer experiment to measure the relative difference in the gravitational constant between two elements. Significant errors appear in the statistical analysis of Kreuzer which vitiate his conclusions. The upper bound for the relative difference of F and Br from his experimental data correctly becomes .02, about 10 times the limit already set on the general variation of the gravitational constant over the elements by direct use of a Cavendish balance.

Gilvarry, J. J.↗
NASA NTRS1972

Theory of an experiment in an orbiting space laboratory to determine the gravitational constant.

An experiment is discussed for determining the gravitational constant with the aid of an isolated system consisting of an artificial satellite moving around an artificial planet. The experiment is to be conducted in a spherical laboratory traveling in an orbit around the earth. Difficulties due to the gravity-gradient term are considered, and the three-tunnel method proposed by Wilk (1969) is examined. The rotation of the sphere is discussed together with aspects of the reference systems used, the equations of motion of the spacecraft and of the test objects, the field from the earth's gravity gradient at the test object, higher harmonic terms in the gravity gradient force, gravitational effects of the spacecraft itself, and a computer simulation.

Vinti, J. P.↗
NASA NTRS1972

Polarization of gravitational synchrotron radiation.

Development of a method of analysis of gravitational polarization in terms of Stokes parameters similar to those used for the electromagnetic case. Analytical expressions for these parameters are obtained for the gravitational synchrotron radiation corresponding to the Schwarzschild black-hole model. It is shown that interference between odd- and even-parity terms makes a significant contribution to the angular dependence of the polarization.

Breuer, R. A.↗
NASA NTRS1972

The uniform transparent gravitational lens.

An elementary discussion is given of the gravitational deflection of light due to radially and cylindrically symmetric masses. The effect of the deflection on apparent luminosity of distant sources is also considered. All results are limited to weak, static, asymptotically flat gravitational fields. Emphasis here is on observations of sources aligned behind the disc of the deflecting mass so that the possible transparency of this mass is decisive in image formation. Detailed calculations are made for the simple case of the uniformly dense transparent sphere and comparisons are made with the opaque mass sphere. If they have the same mass and radius, the maximum light deflection produced by these lenses is nearly equal. However, their effects on the area of narrow light beams may be quite different. The uniform transparent sphere does not produce multiple images of one source and, to first order, introduces no image distortion.

Clark, E. E.↗
NASA NTRS1973

Gravitational-wave observations as a tool for testing relativistic gravity

Approaches regarding the role of gravitational wave observations in the investigation of relativistic theories of gravity are treated as providing greater potential in the prediction of wave propagation speed and the polarization properties of gravitational waves. The invariant classes of waves discussed have the same post-Newtonian limit as general relativity for a reasonable choice of cosmological models.

Eardley, D. M.↗
NASA NTRS1972

A simplified model for the gravitational potential of the atmosphere and its effect on the geoid

The earth's atmosphere is considered as made up of oblate spheroidal layers of variable density lying over an oblate spheroidal earth. The gravitational attraction of the atmosphere at exterior points is computed and its contribution to the usual spherical harmonic gravitational expansion is assessed. The potential is also found for points at the bottom of the model atmosphere. This latter result is of interest for determination of the potential at the surface of the geoid. The atmospheric correction to the geoid determination from satellite coefficients is given.

Madden, S. J., Jr.↗
NASA NTRS1974

Gravitational effects of process-induced dislocations in silicon

Matters pertaining to semiconductor device fabrication were studied in terms of the influence of gravity on the production of dislocations in silicon wafers during thermal cycling in a controlled ambient where no impurities are present and oxidation is minimal. Both n-type and p-type silicon wafers having a diameter of 1.25 in to 1.5 in, with fixed orientation and resistivity values, were used. The surface dislocation densities were measured quantitatively by the Sirtl etch technique. The results show two significant features of the plastic flow phenomenon as it is related to gravitational stress: (1) the density of dislocations generated during a given thermal cycle is directly related to the duration of the cycle; and (2) the duration of the thermal cycle required to produce a given dislocation density is inversely related to the equilibrium temperature. Analysis of the results indicates that gravitational stress is instrumental in process-induced defect generation.

Porter, W. A.↗
NASA NTRS1973

Gravitational-wave observations as a tool for testing relativistic gravity

Gravitational-wave observations can be powerful tools in the testing of relativistic theories of gravity - perhaps the only tools for distinguishing between certain extant theories in the foreseeable future. In this paper we examine gravitational radiation in the far field using a formalism that encompasses all 'metric theories of gravity.' There are six possible modes of polarization, which can be completely resolved by feasible experiments. We set forth a theoretical framework for classification of waves and theories, based on the Lorentz transformation properties of the six modes. We also show in detail how the six modes may be experimentally identified and to what extent such information limits the 'correct' theory of gravity.

Eardley, D. M.↗
NASA NTRS1974

Modified Brans-Dicke gravitational theory with nonzero divergence of the energy-momentum tensor

The Brans-Dicke theory of gravitation is modified by assuming that the divergence of the energy-momentum tensor is proportional to the covariant derivative of the scalar curvature. No ad hoc additions to the usual Brans-Dicke field equations are required as in Rastall's modification of the Einstein theory or as in the steady-state theories, of which this is a natural possibility. Three parameters emerge from the theory - namely, the unnormalized gravitational constant, the usual Brans-Dicke parameter, and the proportionality constant. In the post-Newtonian approximation, these parameters can be fixed by experiment. However, there exists a certain choice of the parameters for which the theory reduces to an Einstein theory with constant scalar curvature.

Smalley, L. L.↗
NASA NTRS1974

Gravitational Stokes parameters

The electromagnetic and gravitational Stokes parameters are defined in the general theory of relativity. The general-relativistic equation of radiative transfer for polarized radiation is then derived in terms of the Stokes parameters for both high-frequency electromagnetic and gravitational waves. The concept of Stokes parameters is generalized for the most general class of metric theories of gravity, where six (instead of two) independent states of polarization are present.

Anile, A. M.↗
NASA NTRS1975

Gravitational collapse of a turbulent vortex with application to star formation

The gravitational collapse of a rotating cloud or vortex is analyzed by expanding the dependent variables in the equations of motion in two-dimensional Taylor series in the space variables. It is shown that the gravitation and rotation terms in the equations are of first order in the space variables, the pressure gradient terms are of second order, and the turbulent viscosity term is of third order. The presence of a turbulent viscosity insures that the initial rotation is solid-body-like near the origin. The effect of pressure on the collapse process is found to depend on the shape of the initial density disturbance at the origin. Dimensionless collapse times, as well as the evolution of density and velocity, are calculated by solving numerically the system of nonlinear ordinary differential equations resulting from the series expansions. The axial inflow plays an important role and allows collapse to occur even when the rotation is large. An approximate solution of the governing partial differential equations is also given; the equations are used to study the spacial distributions of the density and velocity.

Deissler, R. G.↗
NASA NTRS1975

The generation of gravitational waves. 2: The post-linear formalism revisited

Two different versions of the Green's function for the scalar wave equation in weakly curved spacetime (one due to DeWitt and DeWitt, the other to Thorne and Kovacs) are compared and contrasted; and their mathematical equivalence is demonstrated. The DeWitt-DeWitt Green's function is used to construct several alternative versions of the Thorne-Kovacs post-linear formalism for gravitational-wave generation. Finally it is shown that, in calculations of gravitational bremsstrahlung radiation, some of our versions of the post-linear formalism allow one to treat the interacting bodies as point masses, while others do not.

Crowley, R. J.↗
NASA NTRS1975

Gravitational collapse of a turbulent vortex with application to star formation

The gravitational collapse of a rotating cloud or vortex is analyzed by expanding the dependent variables in the equations of motion in two-dimensional Taylor series in the space variables. It is shown that the gravitation and rotation terms in the equations are of first order in the space variables, the pressure gradient terms are of second order, and the turbulent viscosity term is of third order. The presence of a turbulent viscosity insures that the initial rotation is solid-body-like near the origin. The effect of pressure on the collapse process is found to depend on the shape of the intial density disturbance at the origin. Dimensionless collapse times, as well as the evolution of density and velocity, are calculated by solving numerically the system of nonlinear ordinary differential equations resulting from the series expansions. The axial inflow plays an important role and allows collapse to occur even when the rotation is large. An approximate solution of the governing partial differential equations is also given, in order to study the spacial distributions of the density and velocity.

Deissler, R. G.↗
NASA NTRS1976

Gravitational-wave bursts from the nuclei of distant galaxies and quasars - Proposal for detection using Doppler tracking of interplanetary spacecraft

It is argued that the collapse which forms the supermassive black holes that are likely to exist in the nuclei of many quasars and galaxies, and collisions between those holes, should produce strong broadband bursts of gravitational waves. The mean time between such gravitational-wave bursts at earth is analyzed in terms of the present Hubble expansion rate, the present deceleration parameter of the universe, the redshift z = 2.5 (at which most of the bursts are assumed to have been generated), the present number density of 'centers' where the bursts originated, the mean number of bursts generated in each center during its active life, and the speed of light. The analysis shows that the range from one week to 300 years is reasonable for the time between bursts, although it is admitted that hardly any bursts at all is also reasonable. Expected characteristics of the bursts are deduced, and it is shown that the best detector for these bursts will probably be Doppler tracking of one or more interplanetary spacecraft.

Thorne, K. S.↗