Beverly Berger, Oakland University
For the second year, the Topical Group in Gravitation (GTG) has had a significant presence at this meeting, which took place in Washington, DC, April 18-21, with sponsorship of three invited sessions (two jointly with other groups), two focus (special topics) sessions, and two contributed sessions. The annual GTG business meeting was also held. For those of you who were unable to attend but wish more information than can be found in the summary given below, the abstracts of most of the contributed and invited talks can be found at http://www.aps.org/BAPSAPR97/.
The GTG co-sponsored with the Division of Particles and Fields an invited session ``Frontiers of Theoretical Physics.'' Bob Wald spoke on cosmic censorship. While this topic is normally of interest only to gravitational theorists, you may recall that Kip Thorne's bet with Steven Hawking on this subject made the front page of The New York Times. Wald was able to capitalize on this excitement to present an excellent review of the meaning of (primarily weak) cosmic censorship that and whether known counter-examples of naked singularities (including the Choptuik solution) were generic. Again fortuitously, Abhay Ashtekar was able to report on very recent results in which geometrical operators in non-perturbative gravity could be used to compute the quantum states of a black hole. This approach could then immediately be compared with Juan Maldacena's discussion of the microscopic calculation of black hole entropy in string theory using duality and D-branes.
The focus session on ``Analyzing Data from Gravitational Wave Detectors'' was organized by Bill Hamilton. This represents an important area of interaction between theorists and experimentalists: How to interpret the response of current and future gravitational wave detectors. Sam Finn gave an invited talk on data analysis for gravitational wave detectors emphasizing that one could improve the statistical measure by considering all on-line detectors as a single unit rather than considering each singlely and then looking for coincidences. This new measure would be the probability or likelihood of the combined signal profile. Warren Johnson then gave an invited talk on the lessons learned with regard to data analysis from the Allegro bar detector at LSU. He emphasized that all gravitational wave detectors to date have found non-stationary, non-Gaussian noise sources whose level can be reduced but not eliminated and that these should be treated as a ``background'' source of signal in the data analysis. Even coincidences between two detectors may not be sufficient to rule out such events. Other topics discussed in this session were searching for burst gravitational waves using nonlinear filtering methods (E. Flanagan), a pulsar search with Allegro (E. Mauceli), a rigorous way to characterize observed coincidences in the absence of signal (A. Morse et al), some novel ways to extend the frequency range of gravitational wave interferometers (R. Drever), using the anelastic aftereffect to study thermal noise in interferometer test masses (M.A. Beilby et al), and elimination of some systematic error sources in Gravity Probe B (G.M. Kaiser et al).
Leonard Parker organized a focus session on black hole formation, evaporation and entropy with several invited talks. Matt Choptuik described critical phenomena in black hole formation. First discovered by Choptuik numerically, the past few years have seen a growth of understanding of the nature of the transition between initial data which collapse to a black hole and those which disperse to infinity. Bob Wald, in an invited talk, argued that the ``loss of information'' in the Hawking radiation process---pure state to mixed state---was not a violation of quantum theory because the black hole formation followed by evaporation creates a spacetime diagram that is not equivalent to the one in which the black hole never existed. Ted Jacobson continues to consider the issue of the role of field modes above the Planck scale in the Hawking process. Standard derivations require such modes to be present. However, Jacobson reports on calculations showing that it is possible for energy that would in principle come from such modes to appear at lower energies (mode conversion) without seriously altering the thermal spectrum of black hole evaporation. In his invited talk, Larry Ford discussed the role of quantum fluctuations in the stability of black hole horizons. He showed that while quantum effects could perturb the horizon, the effect on the Hawking radiation is small for black holes with masses above the Planck mass. Leonard Parker, meanwhile, showed that in his exactly solvable 1 + 1 dilaton gravity model, there is a threshold mass for black hole formation (in contrast to the 3 + 1 Choptuik result). Contributions by Eric Martinez on a thermodynamic formalism that incorporates strong gravitational fields and a discussion by David Brown on the role of boundary states in black hole entropy completed this session.
The joint invited session between GTG and the Topical Group on Fundamental Constants and Precision Measurements was again very successful. This time, the emphasis was on ``Sensitive Mechanical Measurements and the Detection of Gravitational Radiation.'' Peter Saulson led off with an overview of how to detect a feeble signal amidst the noise. He emphasized lessons learned from Bob Dicke---perform a null measurement and use modulation to enhance the effect of the signal. Jennifer Logan then described the efforts made over the past several years to reduce the mechanical noise in the LIGO 40 meter detector. She also discussed the use of this prototype in the development of power recycling and other advanced LIGO techniques. Bill Hamilton then gave a talk in which he reviewed the development and progress of the Allegro detector. This device has operated almost continuously for the past 5 years and has given great insight into noise reduction and the problems associated with continuous operation. He also discussed proposed spherical detectors. Finally, Mark Bocko reviewed the state of the art in quantum non-demolition techniques and how they might be applied to the detection of weak forces.
The GTG invited session was entitled ``Sources and Detection of Gravitational Waves.'' Jorge Pullin discussed the ``close approximation''---the treatment of two black holes as a single distorted black hole. In second order perturbation theory, the accuracy of the approximation can be studied. Perhaps surprisingly, the analytic results agree quite well with numerical results---for separations larger than one would expect. Bruce Allen reported on calculations of how a stochastic background of gravitational waves might be detectable. He considered the sensitivity of LIGO to such a background. Joan Centrella described recent work on numerical simulations of infalling neutron star binaries and the gravitational radiation they produce. Smooth particle hydrodynamics with Newtonian gravity was used to describe the neutron stars while the quadrupole approximation was used to calculate the gravitational radiation. These approximations allowed many simulations to be run in order to study the dependence of the waveforms and spectra on the parameters of the binary and infall. Finally, David Shoemaker reported on the status of the LIGO construction. The highlights of his talks were photographs of one completed arm at the Hanford site and of the construction progress at both sites. He also described some of the nuts and bolts issues of operation and the development of the laboratory and users group.
There were also two sessions of contributed papers. Highlights included a series of experimental talks on the proposed space-based LISA interferometer (R.T. Stebbins et al), noise reduction using active vibration isolation (J.A. Giaime et al and S.J. Richman et al), a balanced heterodyne detection scheme for signal extraction (K.-X. Sun), and using VLBI for solar system gravitation tests (T.M Eubanks et al). There were also several talks on numerical simulations of close compact binaries (New and Tohline), inspiraling neutron star binaries (Matthews and Marronetti), critical phenomena in a harmonic map model (Liebling and Choptuik), and velocity dominance in Gowdy cosmologies (Berger and Garfinkle). In addition, there were analytic discussions on a prescription for relativistic quantization (C. Vuille) and Cauchy horizon stability in plane-wave spacetimes (Konkowski and Helliwell). Several other experimental and theoretical talks were also given.
Kip Thorne chaired the GTG business meeting which was followed by the business meeting of the LIGO Research Community chaired by Sam Finn.