Riley Newman, UC Irvine and Peter Saulson, Syracuse University
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Here is an idiosyncratic selection of highlights of the experimental sessions of the 8th Marcel Grossmann meeting, held in Jerusalem 23-27 June 1997.
Ken Nordtvedt, speaking as chair of a session on gravitational experiments in space, gave a graceful review. He focused on the aspect of experimental gravity that consists of the search for (new) long range fields. This he placed in the context of the general paradigm of physics that all interactions are carried by bosonic fields. From this point of view, the questions facing experimental gravity include the possible existence of non-linear gravity, or of scalar, vector, or tensor fields in excess of those included in general relativity. Nordtvedt then reminded his listeners of the tremendous success Lunar Laser Ranging has had since the Apollo 11 astronauts installed the retroreflector array on the Moon. It is now fully competitive with laboratory experiments as a test of the Equivalence Principle, and is expected to keep pace with improvements of lab experiments to reach sensitivity to possible variations in free-fall at the 1 part in level. Excellent prospects for future improvements in our empirical knowledge could come from two new classes of experiments: high-precision clocks carried through the solar system (especially to the vicinity of the Sun), and laser ranging (instead of radar ranging) to the planets. Coupled with further studies of binary pulsars, Nordtvedt predicted a long life ahead for this branch of experimental gravity.
Experiments to observe the Lense-Thirring effect (dragging of inertial frames by rotating masses) were discussed at several events during the meeting. Ignazio Ciufolini described what could be done with the series of LAGEOS satellites, dense spherical bodies studded with hundreds of corner-cube reflectors that have been placed in high Earth orbit. LAGEOS I and II are already in orbit, with a proposed launch of LAGEOS III sometime in the near future. The L-T effect should make the planes of the orbits of these satellites precess in a characteristic way; Ciufolini has now claimed to have detected such orbit precession at the level. Classical effects from the non-sphericity of the Earth also cause precession, so the claim for the detection of the relativistic effect rests on the assertion that these less interesting effects can be accurately modeled. A lively discussion among the attendees was dominated by a sense of optimism that such modeling could be done well. A plenary talk by Francis Everitt described progress on Gravity Probe-B, by all accounts to be the definitive test of the Lense-Thirring effect. The experiment, originally proposed by Leonard Schiff, involves a slightly different version of frame dragging. GP-B will carry four or five gyroscopes of unprecedented symmetry, the precession of whose spin axes will reveal the dragging of inertial frames. The experimental plan includes a rich mix of diagnostic tests that should give one confidence that precession is due to relativity, and not to unmodelled classical effects. The satellite is now making rapid progress toward completion, with a launch expected some time in the interval Dec 1999 to Oct 2000.
A number of interesting papers were presented at the session on experimental tests of gravity, chaired by Cliff Will. Progress reports on G measurements were presented by three groups. The Wuppertal group has increased its earlier estimates of uncertainty in field mass positioning, and now reports . Their work continues, with a goal of a 50-100 ppm measurement. The Zürich group, which will measure G by measuring the weight changes of 1 kg masses induced by steel tanks containing tons of mercury, reported preliminary results using water instead of mercury: , with systematic error yet to be determined (currently estimated to be <600 ppm). The goal of the group is a 10 ppm measurement. The UC Irvine group, which plans a G measurement with a cryogenic torsion balance using a dynamic ("time of swing") method, reported measurements of the properties of torsion fibers at low temperature suggesting that anelastic fiber properties should not limit the accuracy of such a G measurement at a 10 ppm level or better. Also presented were a test (A. Arnsek and A. Cadez) indicating that the ratio of gravitational forces at distances of 30 cm and 100 cm agrees with Newton to about 1 part/thousand, and a progress report on the TIFR equivalence principle experiment, which anticipates sensitivity to at a level next year and in the future. H.J. Paik described plans for a test for dependent forces such as could be generated by an axion, using a superconducting differential angular accelerometer with target sensitivity more than five orders of magnitude greater than current limits. New space-based equivalence principle tests were suggested by A. Nobili, who suggests that an sensitivity of may be achieved with a spring-tethered test mass system rotating with its capsule at 5 Hz, and by B. Lange, who proposes a system of unconstrained concentric spherical shells in a drag-free satellite. Several talks suggested new types of EP tests in the realm of atomic physics. Ken Nordtvedt discussed GR tests that may be made using clocks in solar orbit or a solar probe where redshift measurements can be made in fields much larger than achieved to date, with some scenarios suggesting sensitivity to at a level as small as or , to below , as well as great sensitivity to the solar J2 and possible EP violation in the form of different rates for clocks with different dependencies on .
A special Memorial Symposium was held in honor of Robert H. Dicke, who passed away in March of this year. Ken Nordtvedt spoke on Dicke's thinking about Mach's Principle, particularly on whether general relativity sufficiently embodies Mach's idea or instead if something like Dicke's scalar-tensor theory is truer to Mach's vision. Symposium organizer Clifford Will gave an overview of the key experiments carried out during Dicke's long career, including his many contributions to microwave physics and astronomy, his improved Eotvos experiment, his early championing of Lunar Laser Ranging, and his measurement of the solar oblateness. Brandon Carter paid tribute to Dicke's proposal of the key idea that became known as the Anthropic Principle. Francis Everitt spoke movingly of the inspiration he had drawn throughout his own career from the work of Dicke, especially the new Eotvos experiment, as reported both in a preliminary account in Scientific American and in the great 1962 treatise by Roll, Krotkov, and Dicke. He also reminded those in attendance of the influence of Dicke's informal discussion group on gravitational physics at Princeton; in 1957 one of its attendees was a Maryland physicist on sabbatical, Joseph Weber. The session was rounded out by impromptu tributes from R. Cowsik, H.J. Paik, and P. Saulson.
A generous portion of time was allotted to work on the detection of gravitational waves, including sessions of contributed talks on resonant mass detectors, interferometers, and on calculations of waveforms from astrophysics sources. There were also invited talks on various aspects of the experiments given by Ken Strain (GEO and LISA), David Blair (UWA), and Piero Rapagnani (VIRGO).
There were descriptions of several fascinating astrophysical phenomena of obvious interest to relativists. Felix Mirabel gave a beautiful review of the properties objects within our Galaxy that exhibit superluminal motion (sometimes called "microquasars".) These objects appear to be wonderful laboratories in which to test the Rees model of apparent superluminal motion as an effect caused by light-travel-time effects when emitting sources move at relativistic velocities in a direction not parallel to the plane of the sky. Two review talks headlined a contributed session on gamma ray bursts. David Band summarized the whole history and phenomenology of the field since the first discovery of the mysterious events in the 1970s. He was followed by E. Costa's outline of the new discoveries made by the Italian satellite Beppo-SAX, whose multi-waveband instrumentation enabled observers to finally find optical, radio, and X-ray counterparts to the enigmatic sources of the bursts. Now that the cosmological distribution of these objects is apparently confirmed, modelers can focus their attention on the luminous end of the phase space of models, most likely binary neutron stars that collide after spiralling together due to gravitational radiation emission.