Following some introductory comments on the fundamentals or first principles governing jointly the emission and absorption of gravitational waves, a list is given of observational targets or goals for gravitational wave astronomy which have been selected from recent critical reviews. Then theoretical studies of plunge radiation and gravitational synchrotron radiation are surveyed, since in this area new techniques are developing rapidly although new observational prospects have not yet been found.

We extend the Weizsäcker-Williams method to the domain of gravitational encounters and correlate collision problems with the corresponding interaction of gravitational radiation. To an ultra-relativistic test particle the field of a Schwarzschild mass appears as a pulse of gravitational plane waves. We consider the scattering of each Fourier component, virtual quanta, by the Newtonian-type field of the test body. The scattered flux at infinity gives us the radiative loss of gravitational energy by a rapidly moving particle.

The claims by J. Weber must be examined in the context of system time-response and signal-to-noise ratio. After briefly discussing the sources and spectral distribution of system noise, we discuss optimal time-domain filtering. Although five independent groups have searched for two-antenna coincidences, only Weber has claimed any excess at zero time lag. We present preliminary results of a high sensitivity two-antenna coincidence search.

The sensitivity of a resonant gravitational wave detector which is necessary for the detection of pulses from asymmetric stellar collapses or from black-hole collisions in nearby galaxies is examined. Limitations on this sensitivity due to the resonating quality of the detector, thermal noise, and reaction of the detection system upon the detector are studied. No severe difficulties for the detection of the above-mentioned pulses are expected. For the detection of pulses from a cluster at the galactic center, a nonresonant system based on Doppler ranging to a drag-free satellite would be more appropriate. At present the sensitivity of Doppler-ranging is still two orders of magnitude below the requirements.

The Maryland Univac 1108 computer has analyzed magnetic tapes, one set having outputs of a 1661 Hz gravitational radiation detector at the University of Maryland and another set having outputs of a 710 Hz gravitational radiation detector at the University of Rochester. For a seven day period beginning August 23, 1973, 391 coincidences were found. Time delay experiments performed by the computer observed 338 accidental coincides. The zero delay excess is therefore 53 coincidences.

These data have not been verified by the Rochester group and any errors in this report are solely the responsibility of the Maryland authors.

A mass loss of 200 M⊙ per year, as conservatively suggested if Weber is detecting gravitational waves from an isotropic source at the galactic centre, is shown to be incompatible with the existence of (a) globular clusters, (b) old wide binaries, if this loss rate has been constant over the past 1010 yr.

From the orbit of ω Centauri in the galactic field and its observed mass distribution and tidal radius an upper limit to the mass loss from the galactic centre is found to be 1 M⊙ yr-1 over the past 1010 yr.

The principles and operation of wide-band gravitational radiation detectors are described. In 7 months of coincidence observations, one signal which fulfills requirements set for a gravitational wave event was recorded, and details are presented. The experiment sets an upper limit to millisecond pulses of gravitational radiation of 0.9±2.1 per month at a 25% threshold of 0.3kT in 300 kG bars, which appears inconsistent with the flux implied by Weber's 1970 results if these are due to such pulses.

Both the theory of optimal evaluation, developed in Munich, and tests with artificial pulses suggest that the Munich-Frascati experiment was more sensitive to short gravitational pulses than the Maryland-Argonne experiment. This was the case even in April 1973, before the Munich sensitivity was increased by another factor of 5. Nevertheless, the coincidences between Munich and Frascati did not show any surplus number for zero time delay.

It is concluded that Weber's events very likely were not due to gravitational radiation. However, a tape with four days of data from Maryland and Argonne (June 1973), which Weber kindly sent to us, does show a peak at approximately zero time delay, about 0.3 s wide and centered at 0.1 s time delay. The significance is not quite 2 standard deviations for the zero delay bin, but higher when the bins are chosen 0.3 s wide. No explanation can be offered at the moment.

A description of Weber's type experiment of gravitational waves detection is given. Double and triple coincidences with Frascati and Munich groups were looked for with negative results. A capacitive passive method of detection has been presented. This method can improve the sensitivity by a factor of 30 when working at room temperature.

Cryogenic detectors for gravitational wave astronomy promise greatly improved sensitivity over room temperature detectors. The 3 mK detector which we have under construction should give an improvement of 106 over existing detectors. The cryogenic antennae are described and the calculated low temperature performance is detailed. New superconducting instrumentation is described.

A general approach to overall system optimization is developed using the concepts of mechanical and electrical signal-to-noise ratios (MSNR and ESNR). These are proportional to and to respectively, where Qsys=mechanical Q of the complete detector, μ = (transducer mass/metal mass), and n = resolving time in units of one-half of the detector fundamental period. The overall SNR becomes a maximum for n = n∗ = optimum resolving time; this procedure yields whereas

Application to ‘strong-coupling’ type antennae (such as divided-cylinder systems) gives a high SNR∗ which depends very little on μ if μ > 0.01. Q-factor and coupling efficiency relations were checked for 22 kg-prototypes using μ = 0.26%, 0.9%, and 1.8%. Two new detector configurations are suggested: the ‘bridged-tube’ allows strong-coupling for very long detectors; the ‘folded-tube’ operates at lower frequencies for a given length.

The structure of weak, plane, null gravitational waves is obtained for any metric theory of gravity. In general, six polarization states are present, which reduce to three (spin 0, ±2) if the theory is to be quantizable. Schemes for obtaining the polarization amplitudes, as well as the direction and velocity of a wave, are presented.

Our group is investigating highfrequency gravitational waves (GW). The most promising approach to detection and laboratory generation of such GW seems to be through the transformation of GW into electromagnetic waves (EMW), and the reverse process: EMW→GW. The effects are small of course.

Maxwell equations in the field of a gravitational wave are linearized by means of the weak field approximation. Then the equations are solved in the case of a uniformly magnetized sphere and the dipole electromagnetic radiation power is calculated. These results are applied to compute the electromagnetic radiation emitted by magnetic neutron stars and by the Earth when hit by gravitational radiation.

I have shown (Westervelt, 1966) that ultrarelativistic bodies do not radiate gravitational waves in the forward direction. This work has been extended so as to apply to circular orbits. Even if low efficiency of generation precludes direct observation of gravitational waves, indirect evidence for their existence is available in a recent analysis (Westervelt, 1969) of Shapiro's fourth test of general relativity.

The stability of relativistic systems is reviewed against the background of what is known in the corresponding contexts of the Newtonian theory. In particular, the importance of determining whether Dedekind-like points of bifurcation occur along given stationary axisymmetric sequences is emphasized: the occurrence of such points of bifurcation may signal the onset of secular instability induced by radiation-reaction. (At a Dedekind-like point of bifurcation, the system can be subject, quasistationarily, to a non-axisymmetric deformation with an e2iϕ-dependence on the azimuthal angle ϕ.)

A formalism is described in terms of which the normal modes of axisymmetric oscillation of axisymmetric systems can be determined. Specialized to neutral modes of oscillation the formalism provides an alternative proof of Carter's theorem and clarifies the minimal requirements for its validity. A parallel formalism is described for ascertaining whether an axisymmetric system can be subject to a quasi-stationary non-axisymmetric deformation. The possibility of applying this latter formalism to determining whether a Dedekind-like point of bifurcation occurs along the Kerr sequence is considered.

In the standard picture of gravitational collapse to a black hole, a key role is played by the hypothesis of cosmic censorship – according to which no naked space-time singularities can result from any collapse. A precise definition of a naked singularity is given here which leads to a strong ‘local’ version of the cosmic censorship hypothesis. This is equivalent to the proposition that a Cauchy hypersurface exits for the space-time. The principle that the surface area of a black hole can never decrease with time is presented in a new and simplified form which generalizes the earlier statements. A discussion of the relevance of recent work to the naked singularity problem is also given.