Sharon Morsink, University of Wisconsin, Milwaukee
The Raman institute hosted a very pleasant and informative meeting on December 11-12 1997, covering a number of topics of key importance in gravitational wave astrophysics. The meeting's format consisted of ten plenary sessions providing an overview of theoretical and observational aspects of gravitational radiation.
The construction of LIGO, GEO and other interferometric gravitational wave detectors has opened up the possibility of making astronomical observations using gravitational radiation. Harold Lueck presented a historical overview of the development of the experimental techniques and technological improvements which have made these detectors possible. The signal-to-noise ratio for these detectors will be quite low so sophisticated data analysis methods will be needed. Addressing this problem, S. Dhurandhar discussed how the matched filter method will be used to detect gravitational wave signals. He reviewed recent work in detecting signals artificially injected into sample background noise. B. Sathyaprakash provided us with an overview of the types of sources which LIGO may be able to detect, including the inspiral of compact binaries, pulsars and supernovae, to name a few. One of the possible sources are rapidly rotating neutron stars which become unstable due to the CFS mechanism. Nils Anderson provided an introduction to this instability and described his recent work which suggests that axial perturbations may play as important a role as the polar modes. In the discussion of possible sources of gravitational radiation, it is usually assumed that general relativity is, in fact, the correct theory of gravity. Gilles Esposito-Farese reviewed the extent to which general relativity has been tested. He stressed that although GR has been successfully tested in the weak field limit, there are scalar-tensor theories which agree with GR in the weak field but provide different strong field predictions, such as boson stars.
The bulk of our knowledge of gravitational wave sources (e.g. compact binary systems) comes from perturbation theory. The main problem of interest is to find the relation between the outgoing radiation and the matter and motion of the source. The remaining speakers discussed different aspects of this problem with reference to the inspiral of neutron star and black hole binaries. Blanchet discussed a method based on matching of expansions in near, exterior and wave zones which he calls the Multipolar-post-Minkowskian approach. This approach allows one to calculate various non-linear non-local effects such as tail radiation, tails of tails and memory terms. Clifford Will presented a different approach which he has dubbed DIRE (Direct Integration of the Relaxed Einstein Equations). DIRE addresses the problem of divergent integrals in the near and far zones and has been used to compute post-Newtonian corrections to 3.5 order, agreeing with results discussed by Blanchet. Given that we know the radiation emitted by a source, can we predict the backreaction onto the system caused by this emission? This problem, known as radiation reaction, was discussed by Bala Iyer. He presented an approach which assumes the validity of the principle of energy balance: the work done by the reactive force is equal to the negative of the energy flux. It is important to verify that perturbation theory provides the correct results for all problems which can be solved exactly. Misao Sasaki discussed black hole binary inspiral in the limit that one of the black holes is much less massive than the other. This approach has been used to show the validity of the post-Newtonian expansion in this limit. The holy grail of numerical relativity is the exact computation of binary black hole mergers, and Ed Seidel reported on recent progress in this direction. In particular, he focused on highly distorted isolated black holes, and showed that the full non-linear evolution agrees well with the results of perturbation theory in the regimes where perturbation theory should be valid. I would like to thank our hosts, Bala Iyer, Joseph Samuel and all the students at the Raman Institute for organizing such an enjoyable and interesting conference!