Peter Saulson, Syracuse University
It wasn't long ago that the knock on General Relativity was that it was a theorist's playground, blissfully disconnected from confrontation with experiment. If there was anyone who was still unaware that things had changed, all she would have had to do was attend the 15th International Conference on General Relativity and Gravitation, held at IUCAA, Pune, India, from December 16-21, 1997.
In this reporter's opinion, GR15's suite of plenary talks was the strongest and most varied of any international meeting in years. Credit must be given to Ted Newman and the Scientific Organizing Committee he chaired for fine choices of topics and speakers. No fewer than ten invited talks were devoted either fully or in large measure to observable phenomena. Five of those were devoted in one way or another to gravitational waves. Talks by Flanagan on the range of possible sources, by Seidel and by Pullin on ways of calculating the gravitational waveforms from the particularly interesting case of black hole coalescences, and by Cerdonio and by Robertson on methods of detection, together gave an unusually complete review of the bustling state of this branch of activity.
But it was the other observationally flavored talks that gave this meeting its most distinctive character. There were two talks on aspects of gravitational lensing: the graceful review of the astrophysical situation with which S.M. Chitre opened the conference, and the whirlwind tour of the optics of caustics and related subjects provided by Michael Berry. The latter included enough novel physics to send everyone away with something new to think about; interferometer jocks will be investigating the phase singularities near the waists of their Gaussian beams with new interest.
There were two other unabashedly astrophysical plenary talks. Malcolm Longair presented a personal overview of the state of our knowledge of astrophysical cosmology, rooted in the remarkable growth of observational knowledge that has occurred in the past few years. All signs point to further dramatic improvement in the situation, including further exploitation of HST's capabilities and the expected detailed maps of the Cosmic Background Radiation from the upcoming satellites MAP and Planck. Ramesh Narayan performed the unlikely feat of interesting a roomful of relativists in the subtleties of energy transport in accretion disks, in the cause of achieving something this reporter would have thought impossible a year ago: demonstrating by conventional (X-ray) astronomy that objects with event horizons inhabit known binary star systems.
Special notice must be given to the talk farthest removed from the ordinary topics of a general relativity meeting, that of Anton Zeilinger on experimental demonstrations of the spooky non-locality of quantum mechanics. This is another subject that has made dramatic progress in the past few years, most recently with the demonstration by Zeilinger's group of teleportation of a quantum state. The breakneck pace of progress was made evident by Zeilinger's remark that the violations of locality of the sort treated by Bell's Theorem were so strong in their experiments (they were detected at the 100-sigma level) that this phenomenon was used as a calibration.
There was, as well, a rich set of contributed papers on experimental topics. These were strongly dominated by talks on gravitational wave detection, which in turn fell into two classes: progress reports on the many interferometers now under construction (LIGO, VIRGO, GEO600) or in the planning stage (LISA, OMEGA), and reports brought back from the trenches by the grizzled veterans already making gravitational wave observations. Among the latter, W. Hamilton and L. Iess emphasized to the raw recruits the tricky issues posed by non-Gaussian noise statistics in, respectively, resonant-mass detectors and spacecraft tracking experiments.
General relativity has undoubtedly been enriched by its new-found observational character. There is every reason to hope that the next GR meeting, slated for summer 2001 in Durban, South Africa, will be an occasion to share further experimental progress in our subject.