The First William Fairbank Meeting on Relativistic Gravitational Experiments in Space was held at the University of Rome "La Sapienza," 10-14 September 1990, under the auspices of ICRA with support from ASI (Italian Space Agency), ESA (European Space Agency), the Vatican Observatory, Stanford University and the University of Rome. Almost 80 physicists and engineers in widely diversified fields relativistic gravitation, space research, SQUID technology, large scale cryogenics, clock technology, laser and radar science and other fields - came together in the kinds of free technical exchange so characteristic of William Fairbank, in whose honor the meeting was held.
Bill Fairbank of Stanford University was one of the greatest pioneers and most passionate advocates of experimental gravitation and space research. After a long and varied career whose variety and scope is well recorded in the volume Near Zero: New Frontiers of Ph ysics (W.H. Freeman 1988, edited by J.D. Fairbank, B.S. Deaver, C.W.F. Everitt and P.F. Michelson) he died suddenly and unexpectedly at the age of 72 on September 30,1989 while still in the plenitude of intellectual vigor, having influenced many colleagues at Stanford University, the University of Rome and elsewhere to enter and develop this exciting field of research.
The meeting began with informal recollections by Remo Ruffini, President of ICRA, and two lectures respectively by Fang Li-Zhi and Francis Everitt, ICRA Co-Directors of Research, on "Bill Fairbank: a Scientist and a Friend" and "Bill Fairbank and Gravitational Physics." Remo Ruflini recalled his first meeting with Fairbank in 1968 and his subsequent many returns to Stanford as well as Fairbank's equally numerous visits to Italy. This included the exchanges between Fairbank and his colleagues with the Rome-Frascati-CERN group in the field of cryogenic gravity wave detectors. Fairbank's enthusiasm for physics, his open-mindedness, his ability to stimulate other people at Stanford and elsewhere to the great work of science were noble memories. Similar thoughts were echoed by many other speakers throughout the meeting, with Robert Ves sot remarking that for Fairbank the pursuit of physics took the form of almost religious quest.
Fang Li-Zhi recounted his own strong impression on first meeting Fairbank in 1980 when he (Fang) was still relatively unknown in the West while Fairbank was universally known and revered. The meeting took place in Pakistan on the bus from Rawalpindi to Nathiagali where they were both going to participate in one of the meetings on "Physics and Contemporary Needs" organized by Riazuddin and Salam. What instantly impressed Fang was Fairbank's easy introduction of himself and his eagerness to ask a younger, unknown man to explain to him some basic issues in cosmology. Never in Fang's experience had an older Chinese professor shown himself so willing to learn or expose his ignorance, and after some while Fang was struck to recall a saying of Confucius that "The wise man is never above
learning from those younger than him." Fairbank was full of this wisdom. Fang's talk included many other human glimpses of Fairbank, as well as an impressive survey of modern cosmology interspersed with other fascinating quotations from ancient Chinese wisdom.
Francis Everitt sketched Fairbank's career in gravitational physics through a brief description of five very varied experiments in which he had either directly participated or taken a close personal interest. It all began at a meeting in 1957 where Brice DeWitt had taken a piece of chalk, thrown it into the air, caught it as it fell, and said "We know almost nothing about gravitation, there is only one experiment which we do over and over again, and that is what I have just done. In fact we don't even know whether that experiment works for antimatter. It is perfectly possible, and even plausible, that antimatter falls upward!"
DeWitt's statement was, as Francis Everitt remarked, a slight exaggeration because in 1957 we had good evidence for the equivalence principle; some evidence for the gravitational deflection of starlight from the famous, albeit unsatisfactory, Dyson-Eddington-Davidson experiment of 1919; better evidence for relativistic precession of Mercury's peryhelion; and some very dubious indications of the Einstein gravitational redshift from observations on stars. Nevertheless DeWitt's words do serve to calibrate the extraordinary progress in experimental gravitation since 1957 and Fairbank's part therein.
The immediate challenge was to measure the gravitational masses of the electron and the positron, with work on the electron being started by Fred Witteborn, Fairbank's first graduate student at Stanford. This beautiful experiment raised unexpectedly difficult theoretical questions about the behavior of the surrounding electric shield under gravitational forces and led to further experiments both at room temperature and low temperature to resolve these. The positron experiment, important as it was, was never completed though a source of slow positrons was developed in the 1970's by J.M.J. Madey, and Fairbank was seeking funds to revive this work during his retirement at the same time as a group at Los Alamos began work on the freefall of antiprotons. Next chronologically came Fairbank's proposal with L.I. Schiff and R.H. Cannon to perform the Schiff test of general relativity with gyroscopes in an orbiting spacecraft. This experiment, now known as Gravity Probe B, was extensively described during the meeting in lectures by Everitt, M. Taber, B. Parkinson and others. Third was the development with W.O. Hamilton, R. Giffard, M. Taber, P. Michelson and others at Stanford and Louisiana State University in the U.S., as well as by the Rome-Frascati-CERN group, of cryogenic gravitational wave detectors, now in their third generation of development and about to operate at a temperature of 30 mK. Fourth was Fairbank's close interest in the satellite test of the Equivalence Principle (STEP) developed by Worden, Everitt and Bye after discussions with Fairbank and DeBra. Finally there was an insufficiently recognized result, the very remarkable "quantized Newton's bucket" experiment performed in 1967 by Fairbank's student G.B. Hess. The object of the Fairbank-Hess experiment was to investigate the creation of quantized vortices in superconducting helium contained in a small (0.89 mm diameter) magnetically suspended bucket as it is cooled while slowly rotating through the superfluid transition temperature. At sufficiently low initial angular momenta the liquid stops rotating altogether, but with respect to what ... ? The reference has to be to the framework of the fixed stars or at least the local inertial frame, which suggests a fundamental connection between the quantum condition and Mach's principle. This question stimulated a lively discussion at the meeting.
The following account of the meeting is divided into six sections as follows:
1) Tests of underlying principles in gravitational physics and their theoretical rationale
2) Frameworks for testing gravitational theories, present status of theory testing and future prospects
3) Rotational effects in general relativity, frame-dragging and the geodetic effect
4) Experiments and theory of gravitational radiation
5) Advanced technologies: Clocks, drag-free and cryogenics in space
6) Classical gravity
7) Considerations in spacecraft design, program management and the use of Columbus space station
This division is somewhat arbitrary and there were many cross-connections between the presentations.
1. Test of Underlying Principles in Gravitational Physics and Their Theoretical Rationale
The two principal experiments discussed in Fundamental Principles were STEP, the Satellite Test of the Equivalence Principle, in a paper given by P.W. Worden and 0. Pace (in absentia), and first and second order redshift (clock shift) experiments by R. Vessot.
STEP aims to test the equivalence of gravitational and inertial mass for 6 to 9 test bodies to a precision of order 1 part in ~ i.e., 6 orders of magnitude more precise than existing ground-based tests of equivalence. This increase in precision depends on the increased driving acceleration, the reduced background noise obtained by operating in a drag-free satellite, the use of cryogenic techniques to obtain magnetic readout, magnetic shielding, extreme thermal stability, and extremely high vacuum. Operation in a slightly eccentric orbit allows for test of distance dependence of possible violation of equivalence. A variety of theoretical considerations emphasize the importance of pursuing this.
To date the most precise test of the gravitational rate of change has been NASA's Gravity Probe A, a suborbital flight of a hydrogen maser clock on a Scout rocket to an altitude of 10,000 km lasting 18 minutes. The result confirmed the Einstein formula within the experimental error of 1.4 parts in 104. Improvements since 1976 in clock performance, plus the use of an elliptic repeating orbit and better telemetry using Doppler trakking in S or X bands could increase this precision by a factor of 1,000 while the use of Solar Probe with drag-free control could lead to a measurement of second order redshift.
The theoretical background of such experiments and the meaning of mass were discussed in papers by E. Preparata "Mass in the Standard Model" and G. Gibbons "Comments on Long-Range Parity Violating Forces." The latter strongly recommended pursuit of equivalence principle experiments with polarized test bodies.
Preparata presented a critical evaluation of the standard model of electroweak and strong interactions. He pointed out that in any gauge type quantum theory of elementary interactions, it is not possible to define unambiguously the physical vacuum. He then speculated on the possible physical structure of the vacuum state saying that it might be highly nontrivial and complicated.
Gibbons discussed consequences of existence of long-range parity violating forces. Theories proposed so far which attempt to unify all elementary interactions predict the existence of many particles; some of them can possess very low mass. Gibbons pointed out that at present such particles can manifest themselves as long-range parity violating force. He urged to continue and improve sensitivity of testing the Newton's law at different distances.
M. Bochicchio in his paper "Comments on string theory and general relativity" reviewed progress in constructing unified theory of all elementary interactions. Especially interesting is the low energy limit of this theory which allows one to calculate the first order quantum corrections to the Einstein field equations.
A related paper by D. Kaffigas argued that an apparent violation of equivalence for cosmic strings, as described in a paper by M. Rees et al. is erroneous, and arises from a failure to recognize that mass is varying in the situation discussed.
Two other useful observational papers on fundamentals were the paper by I. Goldman on "Limits on G-Variability from Radio Pulsars" and by C. Gundlach on "Nucleosynthesis Constraints on a Time-Dependent G."
2. Frameworks for Testing Gravitational Theories, Present Status of
Testing and Future Prospects
C. Will in his paper "General Relativity at 75: How Right Was Einstein?" presented a thorough overview of the current status of experimental verification of general relativity. He pointed out that all the experiments performed so far are in very good agreement with predictions of general relativity. Will pointed out that the Brans-Dicke theory which has been severely constrained by radar time-delay experiments might be revived because most theories of unification of all elementary interactions predict existence of a scalar field.
K. Nordtvedt in his presentation "Towards a Second Post-Newtonian Test of Relativistic Gravity in the Solar System" claims that the entire field of first order tests has been mapped out because of the large number of new null tests invented by Will, himself and others. He therefore suggested concentrating on trying to perform higher order tests. Nordtve4t presented a general formalism describing first and second order post-Newtonian corrections and urged people inventing alternative theories of gravity to cast their theories into this formalism and confront them with observational data.
Both Will and Nordtvedt pointed out that present knowledge of PPN parameters g and b is to about 2 parts in io3. Gravity Probe B will determine 7 to 2 parts in 105 and therefore in conjunction with improved lunar ranging should effect a similar improvement in 9 though the claims for possible laser lunar ranging (LLR) improvements are almost certainly exaggerated.
T. Damour presented a new approach to post-Newtonian celestial mechanics. Using local inertial frames and relating them to the global Newtonian frame. Damour introduces new field variables and proposes a new definition of multipole moments. In the new variables, equations describing the post-Newtonian corrections become linear and easy to handle.
Strong claims were made in the paper of M. Schneider and J.M. Milller for improved knowledge of PPN parameters 9 and 7 from the University of Münich reanalysis of LLR data. Pressed by questions from the audience he conceded that the errors given were statistical errors only and that the "real errors" were probably at least an order of magnitude larger than these statistical errors.
N. Ashby presented calculations on relativistic effects arising from the quadrupole moment of a moving source. His calculations have not been completed yet. Preliminary results indicate. however, that there might exist new potentially observable relativistic effects.
A very important future experiment is POINTS (Precision Optical INTerferometry in Space) described by R. Reasenberg, which is designed to measure starlight deflection, possibly to the precision-second level, and hence conceivable make a second order starlight deflection measurement by observations with two laser stabilized Michelson stellar interferometers.
Another very interesting related presentation was by M. Shao on a proposed orbital version of his astrometric interferometer at the Wilson observatory
3. Rotational Effects in General Relativity: Frame-dragging and Geodetic Effects
Three experiments were discussed that may or will provide direct evidence for frame-dragging effects in general relativity: Gravity Probe B, LAGEOS III, and the University of Maryland Superconducting Gravity Gradiometer Mission.
Gravity Probe B was described in three papers by Everitt (on error analysis), Taber (technical progress on the prototype flight instrument), and Parkinson (managing the space program and the role of engineering research in it). Related papers were by D.B. DeBra on drag-free control and J. Walker and C. Broughton on spacecraft design. The experiment is now in an advanced state of technology development; the expected precision is 0.2-0.4% in determining frame-dragging and 2 parts in 105 in determining g from the geodetic precession.
LAGEOS III was described in papers by I. Ciufolini, P. Farinella, M. Watkins, and D. Rubincam. It aims to determine frame-dragging on the plane of two satellite orbits in complementary orbits (700 LAGEOS III and 1100 for LAGEOS I) com
pensating thereby for many of the Newtonian disturbances acting on the individual satellites. The results of a University of Texas simulation reported by M. Watkins gave an error of 8% for orbits matched to 0.10 and 12% for orbits matched to 0.30. He indicated that further simulations now underway might yield results as good as 3-4%.
The SGGM mission was described by H.J. Paik and its application to relativity tests by M. Theiss of Munich.
K. Nordtvedt in his presentation argued that the gravitomagnetic field which is closely related to frame-dragging has in effect been observed already with substantial precision indirectly in lunar ranging measurements and rather less indirectly with the Taylor Hulse binary pulsar. The validity of this general point of view was strongly challenged in discussion by I. Ciufolini.
L. Halpern in his paper "Geometry of Spin Motion" presented a new way of describing motion of spinning particles. He considers a multidimensional space and shows that geodesics in this space when projected into a standard four-dimensional space-time turn into equations of motion of spinning particles.
R. Jantzen and P. Carini in their presentation "Parallel Transport of a Spinning Particle" discuss methods of transporting spatial frames along an observer congruence. They notice that spatial Lie transport is incompatible with orthonormality while Fermi-Walker transport of spatial fields exhibits a rotation relative to the observer congruence. By removing this rotation from the spatial Fermi-Walker transport they obtain spatially corotating Fermi-Walker transport. They use this formalism to derive formula for the spin precession.
4. Experiments and Theory of Gravitational Radiation
The supernova 5N1987A created a new wave of interest in the last stages of evolution of stars and in the process of collapse into a neutron star or a black hole. The structure of a pre-supernova star is now understood much better.
Using these results and results of his own investigations, B. Schutz presented an up-to-date review of possible astrophysical sources of gravitational radiation. In the case of supernova explosion, it is now possible to estimate not only the rate in our and nearby galaxies but also to predict the shape of the signal. Coalescence of compact binary neutron star systems is another interesting example of a source of gravitational waves. As more and more binary pulsars are discovered by radioastronomers, this source of gravity waves has also been studied with great detail and care. Schutz presented current estimates and pointed out that with the constant improvement in sensitivity of gravity wave antennas we are approaching the regime of detectability.
M. Bassan gave a presentation on the ground-based cryogenic gravitational wave detectors at the University of Rome and Stanford University, and of his experience of working with Bill Fairbank. A related presentation on the Rome work was given by G. Pallotini.
In the low frequency band (approximately 10-3 Hz) two major experiments, using Doppler tracking of interplanetary spacecrafts, are now in preparation for themission ULYSSES and GALILEO. Their main limitation - the noise produced by plasma fluctuations - will be overcome in the CASSINI mission if K-bank will be implemented in its radio system, thereby making a very sensitive gravitational wave experiment possible.
Three long-range proposals for space-based gravitational wave detectors were discussed by Robert Vessot, based on clock and ranging technology by A.J. Anderson and R.W. Hellings, based on microwave ranging between three spacecrafts in solar orbit separated by a distance of 106 km (MIGO), and by J. Faller by laser ranging drag-free satellites separated by distance of 107 km (LAGOS). The technology for LAGOS is by far the most ambitious. Thus the drag-free performance required by LAGOS is 10-17 g/Hz1/2 as compared with 10-11 g/Hz1/2 for STEP. Some of the challenges of that technology were discussed by D.B. DeBra in the paper described below.
P. Michelson described gravitational radiation from compact X-ray sources and the possibility of correlating gravitational observations with the output of a large area/fast time X-ray detector on Space Station.
G. Sch£fer presented results of a complicated numerical study of gravitational waves emitted during a collapse of rotating stellar cores. The gravitational dynamics of collapse were considered in the Newtonian approximation since the highest appearing values of the central density and velocity are 2.6 x 10-14g cm3 and 8 x 109 cm/s. Schitfer concluded that in comparison to spherically symmetric models the dynamics of collapse of rotating models can be qualitatively different, because the core may bounce before nuclear densities are reached and the inner part of the core could be significantly deformed before the bounce. However, even in the most promising case the dimensionless quadrupole wave amplitude does not exceed 10-23 at a distance of 10 Mpc. The higher order multipole amplitudes are weaker by at least two orders of magnitude. The temporal structure of the signal strongly depends on the central density of the core at the bounce. Scha~fer presented also detailed expected shapes of the signal.
5. Advanced Technologies: Clock Technology, Cryogenic, Drag-free
SQUIDS and New Rotation Sensors
Progress in clock technology with hydrogen masers at low temperature was discussed by R. Vessot. A new general account of clocks of various kinds was provided by S. Leschiutta. L. Maleski discussed the prospects for trapped ion clocks.
Three papers were presented on cryogenic systems in space. (1) The advanced 2,200 liter dewar for ISO (Infrared Space Observatory) developed by MBB, Germany, was described by A. Seidel. It is scheduled for launch in late 1993 and has a thermal performance agreeing well with theoretical predictions, (2) the small (250 liter) I.D. dewar developed by Lockheed and used by ESA-NASA as a candidate dewar for STEP was discussed with its advanced technology by T. Nast, (3) the 2,000 liter GP-B dewar which involves the most sophisticated technology of all was described by R. Parmley also of Lockheed.
D.B. DeBra described past orbital tests of drag-dree technology and current plans with special emphasis on STEP as a technology development mission as well as a science mission.
C. Pegrum and P. Carelli gave thorough reviews of the present status and future prospects of SQUID technology, a topic of great personal interest of Bill Fairbank besides being important in many of the proposed experiments.
M. Cerdonio proposed two new precise rotation sensors, one based on using SQUIDS to observe the Barnett effect in a rotating ferromagnet, the other based on a superfluid helium analog of the SQUID. A different kind of rotation sensor described by W. Schleich uses a passive/active laser gyro conceptually designed at the Max-Planck Institute for Quantum Optics at Munich.
6. Classical Gravity
F. Fuligni and H.J. Paik respectively gave accounts of room temperature cryogenic gravity gradiometers currently under development and their applications to satellite geodesy. J. Faller described the absolute Colorado gravimeter based on falling bodies surrounded by a "drag-free" shield. Several authors discussed the status of experiments on possible violation of the Newtonian inverse square law.
7. Considerations in Spacecraft Design, Program Management and the Use of Columbus Space Station
Papers on spacecraft design were presented by C. Broughton of Lockheed and J. Walker of Fairchild in the United States and Bradford Parkinson gave an account of the subtle questions of managing a flight program involving universities, industry and NASA with special emphasis on the Gravity Probe B "management experiment."
A special session was held at the request of the European Space Agency (ESA) on future uses of Columbus Space Station for gravitational physics.
All in all this was a wonderful and good spirited meeting, one that William Fairbank would have thoroughly delighted in being at.
- C.W.F. Everitt, ICRA Co-Director of Research, and
M. Demianski, University of Warsaw,
on behalf of the international organizing committee