Riley Newman, UC Irvine and Peter Saulson, Syracuse University
rdnewman@uci.edu -
saulson@suhep.phy.syr.edu
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.