Gravitational Physics: Exploring the Structure of Space and Time

Abstract

This book contains a report on the goals and hopes of the physics of gravitation, prepared by the Committee on Gravitational Physics of the National Research Council of USA, and is a part of the decadal survey Physics in a New Era. The Committee is formed of fifteen very distinguished scientists and is chaired by J B Hartle. Its charges were (1) to assess the achievements in gravitational physics over the last decade and (2) to identify the most promising opportunities for research in order to describe the resources necessary to realize these opportunities. It received proposals on different aspects of gravitation and requested input from the community of gravitational physics in a number of ways before preparing the report, which was reviewed later by other prestigious physicists. It begins with an Executive Summary that states the conclusions in just six pages and has three chapters: 1. Introduction, overview and recommendations; 2. Ideas and phenomena of general relativity; and 3. Achievements and opportunities in general relativity. The book is really interesting, as might be expected from the institutions and scientists that prepared it. It is easily read and states its points in a direct, expressive and clear language. Although it cannot be excluded that some lonely hunter could provide a conceptual breakthrough, the complexity of the issues involved, needing the constructive interference of approaches from many fields, as well as the necessity of taking account of new data obtained in large astrophysical observatories, suggest that very probably the recommendations of the book will articulate most of the progress over the next ten or twenty years. Chapter 2 contains a terse review of the key ideas of general relativity and of the key phenomena of gravitational physics. Although it does not offer any new ideas for people who know about these topics, it is pedagogical and appealing and is a pleasure to read. Chapter 3, with 68 pages, is divided into five parts, dedicated to: I. Gravitational waves, II. Black holes, III. Origin, evolution and fate of the universe, IV. General relativity and beyond: experimental exploration, and V. Unifying gravity and quantum theory. Each part examines successively the key questions, the achievements up to now and the opportunities that are perceived. In the 17 pages of chapter 1, the book states the goals and recommendations for the next ten years, after reviewing the achievements of the past decade and the opportunities that can be appreciated now. These goals are: to receive gravitational waves and use them to study regions of strong gravity; to explore the conditions near black holes; to measure the geometry of the universe at cosmological scales; to test the limits of Einstein's general relativity and explore for new physics; and to unify gravity and quantum theory. The recommendations are as follows. (1) Develop the LIGO scientific program with respect to the high-frequency gravitational wave window and a space-based laser interferometry facility to detect the low-frequency gravitational waves produced by the merging of supermassive black holes. (2) Study the classical and quantum theory of strong gravitational fields (both by developing analytic and numerical tools to study strong field solutions of Einstein's equations and by supporting research in quantum gravity). (3) Precision measurements: improve dramatically the tests of the equivalence principle and of the gravitational inverse square law. (4) Astronomical observations: use gamma-ray, x-ray, optical, infrared and radio telescopes to study the environments of black holes; measure the temperature and polarization fluctuations of the cosmic background radiation at scales down to the arcminute; search for relativistic binary systems; launch detectors of gamma-rays and x-ray bursts capable of detecting counterparts of LIGO events; and use supernovae and gravitational lenses to infer the distribution of dark matter. The book contains a useful and highly reliable summary of the state of the art of gravitational physics and a most authoritative set of recommendations. It is very advisable reading for researchers and teachers in gravitational physics and for all those who wish to maintain a good general knowledge of the evolution of the main ideas of physics, even outside of their particular field.