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`~ravi~tionai
physics
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Exploring the Structure of
Space arm Time
Committee on Gravitational Physics
Board on Physics and Astronomy
Commission on Physical Sciences, Mathematics, and Applications
National Research Council
NATIONAL ACADEMY PRESS
Washington, D.C.
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NOTICE: The project that is the subject of this report was approved by the Governing Board of the
National Research Council, whose members are drawn from the councils of the National Academy of
Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the
committee responsible for the report were chosen for their special competences and with regard for
appropriate balance.
This project was supported by the National Aeronautics and Space Administration under Grant
No. NAG5-4120, the Department of Energy under Contract No. DE-FG02-97ER41051, and the
National Science Foundation under Grant No. PHY-9722102. Any opinions, findings, and conclu-
sions or recommendations expressed in this material are those of the author(s) and do not necessarily
reflect the views of the sponsors.
Front cover: Gravitational waves are ripples in the curvature of space and time that propagate with
the speed of light through otherwise empty space. Mass in motion is the source of gravitational waves.
The figure shows the predicted gravitational wave pattern from a pair of neutron stars or black holes
spiraling inward toward a final merger. The figure shows one polarization of the waves as seen by
observers stationed throughout the plane of the orbit at the moment of final merger. The waves
measured far away were emitted during the earlier steady inspiral of the objects about one another,
while the peak at the center comes from the final merger. The reception of gravitational waves in the
next decade would not only confirm one of the most basic predictions of Einstein's general relativity,
but also provide a new window on the universe. (Courtesy of Patrick R. Brady, Institute for Theoreti-
cal Physics, University of California at Santa Barbara, and the University of Wisconsin-Milwaukee.)
International Standard Book Number 0-309-06635-2
Additional copies of this report are available from
National Academy Press, 2101 Constitution Avenue, N.W., Lockbox 285, Washington, D.C. 20055;
(800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet
COMMITTEE ON GRAVITATIONAL PHYSICS
JAMES B. HARTLE, University of California at Santa Barbara, Chair
ERIC G. ADELBERGER, University of Washington
AB HAY V. ASHTEKAR, Pennsylvania State University
BEVERLY K. BERGER, Oakland University
GARY T. HOROWITZ, University of California at Santa Barbara
PETER F. MICHELSON, Stanford University
RAMESH NARAYAN, Harvard-Smithsonian Center for Astrophysics
PETER R. SAULSON, Syracuse University
DAVID N. SPERGEL, Princeton University Observatory
JOSEPH H. TAYLOR, Princeton University
SAUL A. TEUKOLSKY, Cornell University
CLIFFORD M. WILL, Washington University
DONALD C. SHAPERO, Director
ROBERT L. RIEMER, Senior Program Officer
JOEL R. PARRIOTT, Program Officer
. . .
BOARD ON PHYSICS AND ASTRONOMY
ROBERT C. DYNES, University of California at San Diego, Chair
ROBERT C. RICHARDSON, Cornell University, Vice Chair
STEVEN CHU, Stanford University
VAL FITCH, Princeton University
IVAR GIAEVER, Rensselaer Polytechnic Institute
RICHARD D. HAZELTINE, University of Texas at Austin
JOHN HUCHRA, Harvard-Smithsonian Center for Astrophysics
JOHN C. MATHER, NASA Goddard Space Flight Center
R.G. HAMISH ROBERTSON, University of Washington
JOSEPH H. TAYLOR, Princeton University
KATHLEEN C. TAYLOR, General Motors Research and Development Center
J. ANTHONY TYSON, Lucent Technologies
GEORGE WHITESIDES, Harvard University
DONALD C. SHAPERO, Director
ROBERT L. RIEMER, Associate Director
KEVIN AYLESWORTH, Program Officer
JOEL R. PARRIOTT, Program Officer
NATASHA CASEY, Senior Administrative Associate
GRACE WANG, Senior Project Associate
MICHAEL LU, Project Assistant
V
COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS,
AND APPLICATIONS
PETER M. BANKS, ERIM International, Inc., Co-chair
W. CARL LINEBERGER, University of Colorado, Co-chair
WILLIAM BROWDER, Princeton University
LAWRENCE D. BROWN, University of Pennsylvania
MARSHALL H. COHEN, California Institute of Technology
RONALD G. DOUGLAS, Texas A&M University
JOHN E. ESTES, University of California at Santa Barbara
JERRY P. GOLLUB, Haverford College
MARTHA P. HAYNES, Cornell University
JOHN L. HENNESSY, Stanford University
CAROL M. JANTZEN, Westinghouse Savannah River Company
PAUL G. KAMINSKI, Technovation, Inc.
KENNETH H. KELLER, University of Minnesota
MARGARET G. KIVELSON, University of California at Los Angeles
DANIEL KLEPPNER, Massachusetts Institute of Technology
JOHN KREICK, Sanders, a Lockheed Martin Company
MARSHA I. LESTER, University of Pennsylvania
M. ELISABETH PATE-CORNELL, Stanford University
NICHOLAS P. SAMIOS, Brookhaven National Laboratory
CHANG-LIN TIEN, University of California at Berkeley
NORMAN METZGER, Executive Director
v
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vim
Preface
The Committee on Gravitational Physics (COP) was organized by the Na-
tional Research Council's (NRC's) Board on Physics and Astronomy (B PA) as
part of the decadal survey Physics in a New Era. The committee's main 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 the next
decade and describe the resources necessary to realize those opportunities. This
report fulfills those charges.
As is made clear in the report, the field of gravitational physics has signifi-
cant overlaps with astrophysics, elementary-particle physics, and cosmology,
areas that have been or will be assessed by the NRC. Elementary-particle physics
is the subject of a separate volume of the current physics survey, Elementary-
Particle Physics Revealing the Secrets of Energy and Matter (National Acad-
emy Press, Washington, D.C., 1998~. Cosmology is discussed in Cosmology: A
Research Briefing (National Academy Press, Washington, D.C., 1995~. Astro-
physical phenomena in which gravitation plays a key role were considered in the
NRC study A New Science Strategy for Space Astronomy and Astrophysics (Na-
tional Academy Press, Washington, D.C., 1997) and will be a part of the NRC's
Astronomy and Astrophysics Survey now under way. Reports with overlapping
content and emphases are to be expected because of emerging interdisciplinary
areas of physics. Naturally, each of these reports makes its recommendations
from the perspective of the subfield of physics involved. This report sets priori-
ties and makes recommendations based on the committee's assessment of the
impact of opportunities for research in gravitational physics.
. .
vat
. . .
vile
PREFACE
As part of its task, the COP reevaluated the estimates of the event rate for a
number of sources of gravitational waves that might be received by the LIGO
gravitational wave detector in the next decade in the light of current theoretical
and observational understanding. These estimates are reported in the addendum
to Section I of Chapter 3. The discussion given there should be regarded as the
output of the entire committee, but we would be remiss if we did not also ac-
knowledge that the detailed analysis is the work of three of us Ramesh Narayan,
Joseph Taylor, and David Spergel.
The COP was helped in its tasks by input from many sources, some orga-
nized by the committee and some submitted by members of the gravitational
physics community in response to various requests for input. The COP's activi-
ties, in which the BPA staff headed by Don Shapero and Roc Riemer assisted
greatly, are described in Appendix A.
The committee's work was supported by grants from the National Aeronau-
tics and Space Administration, the National Science Foundation, and the U.S.
Department of Energy. We thank them for this support.
James B. Hartle, Chair
Committee on Gravitational Physics
Acknowledgment of Reviewers
This report has been reviewed by individuals chosen for their diverse per-
spectives and technical expertise, in accordance with procedures approved by the
National Research Council's (NRC's) Report Review Committee. The purpose
of this independent review is to provide candid and critical comments that will
assist the authors and the NRC in making the published report as sound as
possible and to ensure that the report meets institutional standards for objectivity,
evidence, and responsiveness to the study charge. The contents of the review
comments and the draft manuscript remain confidential to protect the integrity of
the deliberative process. We wish to thank the following individuals for their
participation in the review of this report:
Mitchell C. Begelman, University of Colorado,
James E. Failer, University of Colorado,
J. Ross Macdonald, University of North Carolina at Chapel Hill,
Riley D. Newman, University of California at Irvine.
Kenneth Nordtvedt, Northwest Analysis,
Andrew Eben Strominger, Harvard University,
J. Anthony Tyson, Lucent Technologies,
Robert M. Wald, University of Chicago, and
Edward Witten, Princeton University.
Although the individuals listed above have provided many constructive com-
ments and suggestions, the responsibility for the final content of this report rests
solely with the authoring committee and the NRC.
Six
Contents
EXECUTIVE SUMMARY
INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 7
~ ~ < ~ ~ ~ ~,~ ~ `~ ~ c~ ~ ~ ~ `: ~ 7
It Ac~s of the Past Decade, 8
:~:[q Opportun~;~s for the Nc:~t D`~' 12
~ ~ K :0 ~ ~ ~ ~ ~ ~ ~t ~ ~ ~ ~ ~ ~ ~ ~ ~ ~1 4
2 IDEAS AND PHENOMENA OF GENERAL RELATIVITY
Id ~ C-~rLe:~ Relat~, 24
Key Phenom`~a ~n Grav~ Phy$~, 27
3 ACHIEVEMENTS AND OPPORTUNITIES IN
GRAVITATIONAL PHYSICS
I ~G:~;~:! Waves' 32
:~ Black :~--~, 52
~ ~ K ~] ~ ~ ~ t~ ~ ~\ 0~5 ~ ~6 6
~ ~ ~ ~ ~ 8 9
APPENDIXES
A Activities of the Committee on Gravitational Physics
B Glossary
x~
24
32
101
104