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Introduction, Overview, and Recommendations I~ GRAVITATIONS ~ ~wo~ Of the four fundamental forces of nature, gravity has been studied the long- est, yet gravitational physics is one of the most rapidly changing areas of science today. Gravity is an immediate fact of everyday experience, yet presents us with some of the deepest theoretical and experimental challenges of contemporary physics. Gravitational physics has given us some of the most accurately tested principles in the history of science, yet gravitational waves one of its most basic predictions have never been detected by a receiver on Earth. Gravitational physics is concerned with some of the most exotic phenomena in the universe- black holes, pulsars, quasars, the big bang, the final destiny of stars, gravitational waves, the microscopic structure of space and time, and the unification of all forces challenges to understanding that have captured the imaginations of physi- cists and lay persons alike. Yet gravitational physics is also concerned with the minute departures of the motion of the planets from the laws laid down by Newton, and is a necessary ingredient in the operation of the Global Positioning System used every day. The challenges of gravitational physics have been the central concerns of some of the most famous 20th-century scientists Albert Einstein, S. Chandrasekhar, Robert Dicke, Stephen Hawking, and Roger Penrose to mention just a few examples. As the Committee on Gravitational Physics (COP) outlines below, the past decade has seen major achievements in gravita- tional physics. The next decade promises to be even more exciting, yielding revolutionary insights. This report reviews past accomplishments in the emerg 7

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8 GRAVITATIONAL PHYSICS: EXPLORING THE STRUCTURE OF SPACE AND TIME ing field of gravitational physics, describes opportunities for future research, and recommends priorities for the most promising of these. Gravity is the weakest of the four fundamental forces. The gravitational force between the proton and electron is 104 (1 followed by 40 zeros) times smaller than the electric force that binds these particles together in atoms. How- ever, gravity is a universal force. All forms of matter and energy attract each other gravitationally, and that interaction is unscreened there is no negative "gravitational charge" to cancel the attraction. It is therefore gravity that governs the structure of matter on the largest scales of space and time and thus the structure of the universe itself. Gravity is also central to the quest for a unified theory of all forces whose simplicity would emerge at very high energies or very small distances. Gravity is the last force to be included in contemporary unified theories, yet many of the ideas for these "final theories" come from gravitational physics. Indeed, it would not be an exaggeration to say that many frontier problems in elementary-particle physics originate in gravitational physics. Gravitational physics is thus a two-frontier science. Its important applica- tions lie on both the very largest and the very smallest distance scales that are considered in today's physics. (See Figure 1.1.) On the largest scales, gravity is linked to astrophysics and cosmology. On the smallest scales, it is tied to elemen- tary-particle and quantum physics. These frontiers are not disjoint; they become one in the early universe at the time of the big bang where the whole of today's observable universe was compressed into a minuscule volume. The theory of gravity proposed by Isaac Newton more than 300 years ago provided a unified explanation of how objects fall and how planets orbit the Sun. But Newton's theory is not consistent with Einstein's 1905 principle of special relativity. In 1915, Einstein proposed a new, relativistic theory of gravity- general relativity. When gravity is weak for example, on Earth or elsewhere in the solar system general relativity's corrections to Newton's theory are tiny. But general relativity also predicts new strong-gravity phenomena such as gravi- tational waves, black holes, and the big bang that are quantitatively and qualita- tively different from those accounted for in Newtonian gravity. Modern gravita- tional physics focuses on these new phenomena and on high-precision tests of general relativity. The basic formulation of general relativity was complete in 1915 and was almost immediately confirmed by tests in the solar system the precession of the orbit of Mercury and the bending of light by the Sun. Over the ensuing decades theoretical analyses deepened the understanding of the theory and exhibited the richness and variety of its predictions. But, except perhaps for cosmology, the theory had little observational impact until the middle 1960s. Then, develop- ments on several different fronts led to a renaissance in gravitational physics that

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 9 `,, 1 o20 1 1o-1o 1 o-2o 1 o-3o Milky Way Universe at helium fusion Sun GPS orbit Primordial black hole evaporating today Universe at end of inflation The quantum gravity scale Probed by best accelerators Hydrogen atom Human Measurement of Newton's G Strand of DNA 10-3 1 o-2o 1o-1o 1 101 1o2o Distance in Meters FIGURE 1.1 Gravitational physics deals with phenomena on scales of distance and mass ranging from the microscopic to the cosmic the largest range of scales considered in contemporary physics. There are phenomena for which relativistic gravity is important over this whole range of scales. Representative ones are indicated by filled circles; other illustrative phenomena in which gravitation plays little role are shown by filled squares and italics. Phenomena above the diagonal line are unobservable, because they take place inside black holes. Phenomena close to the diagonal line are in the strong-gravity regime. The largest scales are the frontier of astrophysics; the smallest, of elementary-particle physics. Scales referring to the universe at various moments in its history denote the size of the volume light could travel across since the big bang, and the mass inside that volume, if the universe always had the expansion rate it had at that moment.

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10 GRAVITATIONAL PHYSICS: EXPLORING THE STRUCTURE OF SPACE AND TIME continues today. First, the discoveries of pulsars, quasars, and galactic x-ray sources revealed for the first time astrophysical phenomena for whose under- standing relativistic gravity was essential. At the same time, the theory was subjected to increasingly varied, accurate, detailed, and systematic tests of its predictions for the weak gravitational field of the solar system. General relativity emerged from these tests confirmed in a wide domain. Today it is the only serious contender for a classical relativistic theory of gravity. Indeed, in certain areas of physics, the curvature of spacetime has become a realistic concern or a tool to be exploited. Examples include accounting for the effects of spacetime curvature in the operation of the Global Positioning System, correcting for the bending by the Sun of the light from quasars used to precisely monitor the rotation of Earth, the use of gravitational lenses to measure the properties of galaxies and cosmological parameters, and the use of general relativity to mea- sure the masses of binary neutron stars. While these astrophysical and experimental developments were taking place on large length scales, progress toward relativistic gravity was being made at the smallest distances considered by physics. The concerns of elementary-particle physics were moving to higher and higher energies, or equivalently to shorter and shorter distances another regime where relativistic gravity is important. Progress was made toward a unified theory of the strong, electromagnetic, and weak forces. Gravitational physics became the next frontier of particle physics, and the unification of gravity with quantum mechanics and the other forces of nature is today a major challenge of theoretical physics. The past decade saw many achievements in gravitational physics. Any short list of highlights would include the following: . The confirmation of the existence of gravitational waves by the detailed analysis of the shortening of the orbital period of the Hulse-Taylor binary pulsar, showing that the radiated power in gravitational waves agrees with the prediction of general relativity to within a third of a percent. The 1993 Nobel Prize in physics was awarded to Russell Hulse and Joseph Taylor for discovering this pulsar system. The accurate measurements of the cosmic background radiation the om- nipresent light from the hot big bang that has cooled to a little under 3 degrees above absolute zero in the subsequent expansion of the universe. The observa- tions verified detailed predictions of the character of the radiation from the hot big bang. They also revealed for the first time the tiny fluctuations that arose from minute early irregularities that grew under the attractive force of gravity to become the galaxies, stars, and planets of today. These measurements have given scientists the most detailed picture of the early universe yet available. . The development of a new generation of high-precision tests (to parts in a thousand billion) of the equivalence principle that underlies general relativity, and the verification of general relativity's weak-field predictions to better than

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 11 parts in a thousand. The new techniques provide high sensitivity to interactions that violate the equivalence principle with ranges from infinity down to a centi- meter, and sharply constrain speculations in particle and cosmological physics. The identification of candidate black holes in two major classes of astro- nomical objects: double stars called x-ray binaries, where black hole candidates of a few solar masses have been found, and the centers of galaxies, where com- pact objects with masses up to a billion solar masses or more have been discov- ered. Black holes are no longer a theorist's dream; they are central to the expla- nation of many of astronomy's most dramatic phenomena. The use of gravitational tensing as a practical astronomical tool to inves- tigate the structure of galaxies and galactic clusters, and to search for dark matter in the universe. Thus, one of the first experimental verifications of general relativity the deflection of light by mass was put to practical use. The increasing use of large-scale numerical simulations to solve Einstein's difficult nonlinear equations. These simulations can predict the effects of strong gravity that will be seen in the next generation of experiments. The use of numerical simulations of gravitational collapse to discover "critical phenomena" associated with the onset of black hole formation. These critical phenomena are analogous to those that occur in transitions between dif- ferent states of matter. The development of string theory and the quantum theory of geometry as promising candidates for a finite, workable theory that unifies quantum mechan- ics and general relativity. The first descriptions, in the above theories, of the quantum states of black holes. The demonstration within string theory that the topology of space can change. The analysis, without recourse to weak-field approximations, of quantum gravity effects in the context of the quantum theory of geometry. The development of powerful mathematical tools to study the physical regimes where Einstein's theory can break down. Under special assumptions, it was shown that this can occur only at an initial big bang, inside a black hole, or at a final "big crunch," thus supporting the cosmic censorship conjecture that these are the only places where the theory breaks down. In addition to these scientific achievements, the past decade saw the start or continuation of experimental projects whose results will shape the field in the next decade. Notable were the final preparation of the Gravity Probe B mission to measure the minute twisting of the spacetime geometry ("dragging of inertial frames" effect) caused by Earth's rotation, and the start of construction for the Laser Interferometer Gravitational-Wave Observatory (LIGO) and other large- scale gravitational wave detectors. These gravitational wave receivers will open a new window on the universe by being sensitive enough to see the gravitational waves expected to be produced by astrophysical sources. . . .

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12 GRAVITATIONAL PHYSICS: EXPLORING THE STRUCTURE OF SPACE AND TIME The transformation of the science of gravitational physics will accelerate in the next decade, driven by new experimental, observational, and theoretical op- portunities. It would therefore be most accurate to think of gravitational physics as an emerging new area of physics despite its long history. In subsequent sections the COP discusses many exciting opportunities, but a single theme runs through most of them: the exploration of strong gravitationalfields. Until now our direct evidence of general relativity has been through weak-field effects in the solar system and ground-based experiments. To be sure, physicists have convincing evidence for strong gravitational effects such as black holes and the big bang, but in nothing like the detail expected in the next decade. In the following the CGP lists opportunities that could be realized in the next decade. Whether these opportunities will be realized depends largely on the availability of funding, and on the fortunes of observational and theoretical dis- covery. . The first direct detection of gravitational waves by the worldwide net work of gravitational wave detectors now under construction. . The first direct observation of black holes by the characteristic gravita- tional radiation they emit in the last stages of their formation. The use of gravitational waves to probe the universe of complex astro- nomical phenomena by the decoding of the details of the gravitational wave signals from particular sources. The continuing transformation of cosmology into a data-driven science by the wealth of measurements expected from new cosmic background radiation satellites, new telescopes in space and on the ground, and new systematic surveys of the large-scale arrangements of the galaxies. . The first unambiguous determination of the basic parameters that charac- terize our universe, its age and fate, the matter of which it is made, how much of that matter there is, and the curvature of space on large scales. The unambiguous measurement of the value of the cosmological con- stant, with profound implications for our understanding of the fate of the uni- verse, and also for particle physics and quantum gravity. The use of gamma-ray, x-ray, optical, infrared, and radio telescopes on Earth and in space to detect new black holes in orbit about companion stars and to explore the extraordinary properties of the geometry of space in the vicinity of black holes that are predicted by general relativity. The measurement of the dragging of inertial frames due to the rotation of Earth at the 1 percent level by the Gravity Probe B mission scheduled for launch in 2000. Dramatically improved tests of the equivalence principle that underlies general relativity.

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 13 The understanding of the predictions of Einstein's theory in dynamical, strong-field, realistic situations through the implementation of powerful numeri- cal simulations and sophisticated mathematical techniques untrammeled by weak- field assumptions, special symmetries, or other approximations. The development of current ideas in string theory and the quantum theory of geometry to achieve a finite, workable union of quantum mechanics, gravity, and the other forces of nature, potentially resulting in a fundamentally new view of space and time. The application of this new theory to predict the outcome of black hole evaporation and the nature of the big bang singularity. The continued development within quantum gravity of a theory of the quantum initial condition of the universe capable of making testable predictions of cosmological observations today. If these opportunities are realized, the CGP expects the next decade of re- search in gravitational physics to be characterized by the following features: A much closer integration of gravitational physics with other areas of science. On the frontier of the largest scales the CGP expects gravitational physics to become increasingly integrated with astrophysics and cosmology as more phenomena for which relativistic gravity is important become accessible to detailed observation and theoretical analysis. This will be ensured by the new data from the worldwide network of gravitational wave detectors now under construction, from the cosmic background radiation satellites now planned, and from new gamma-ray, x-ray, optical, infrared, and radio telescopes on Earth and in space. The CGP expects these phenomena to yield increasingly accurate tests and demonstrations of strong-field gravitational theory. On the frontier of the smallest scales the committee expects the integration of quantum gravity with elementary-particle physics to continue. Gravity is a key ingredient in any uni- fied theory of all forces, and conversely that unified theory is one source of a manageable theory of quantum gravitational phenomena. Much larger experiments yielding much more data. Again the ground- based gravitational wave detectors now under construction are enough to ensure this. Gravitational wave detectors and other experiments in space will only accelerate the trend. International collaborations are likely to be required to realize the full potential of these experimental possibilities. A much closer relationship between theory and experiment. The experi- ments now under way require theoretical analysis at a level of detail, depth, and coordination only now being appreciated. The CGP expects that the next decade will see the emergence of a new cadre of gravitational phenomenologists focused on using fundamental theory to analyze data from experiment. A much wider, more important role for computation in gravitational phys- ics. Understanding actual phenomena requires realistic solutions to Einstein's equation incorporating realistic properties of the matter (fluid, gas) sources. This

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14 GRAVITATIONS PHYSICS: E~LOHNG THE STRUDEL OF SPACE^D TIME means large-scale numerical simulations carried out by teams of theorists em- ploying state-of-the-art computers. Chapter 2 of this report contains a brief description of general relativity and key phenomena in gravitational physics. In Chapter 3 the CGP analyzes the achievements of the past and opportunities for the future in gravitational waves, black holes, cosmology, testing general relativity, and quantum gravity. The COP's recommendations arising from this analysis of the most promising scien- tific opportunities to pursue are described immediately below. The scientific opportunities summarized above and described in detail in Chapter 3 are many and varied. In this section, the CGP sets out what it believes are the highest-priority goals for gravitational physics in the next decade and makes recommendations on how to achieve these goals. Basis for the Goals and Priorities The CGP based its goals and priorities for gravitational physics on its as- sessment of the scientific impact on the field that would follow from realizing these goals in the next decade. The committee has not shrunk from the challenge of making these assessments across the entire subject of gravitational physics. Thus expensive efforts (e.g., gravitational wave detectors) are prioritized along with inexpensive ones (e.g., theoretical research in quantum gravity). The reader wishing to construct subsists, of expensive projects for example, should have no difficulty doing so. In this discussion the CGP assumes that the scientific objectives of a number of projects now under way will be achieved. These are the Gravity Probe B experiment (now with a definite launch window in 2000), construction of the LIGO gravitational wave detector (now nearing completion in time for an initial data run starting in 2002), the Chandra X-ray satellite, which was launched in July 1999, and the MAP cosmic background satellite currently under construc- tion, to be launched late in 2000. Although fully endorsed by the CGP, these projects do not appear in its recommendations. The CGP focused on assessing the scientific opportunities that will be pre- sented by the next decade. It did not attempt a detailed assessment of the techni- cal readiness of any of the large projects proposed. That task should be under- taken by appropriate committees at appropriate junctures. The CGP does not therefore mention by name specific unapproved projects that have been proposed for realizing these scientific opportunities. Rather, it describes the important scientific goals and measurement objectives.

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 15 As can be expected in a science as cross-disciplinary as gravitational phys- ics, many of the projects entering into the COP's priorities have strong arguments for support from related areas of physics and astronomy. These arguments were taken into account, but the COP's list of priorities reflects its view of the projects' potential impact on gravitational physics. Goals The COP believes that the most important goals for gravitational physics are those on the following unordered list. These goals constitute the COP's long- term vision for the field. The list is ambitious. Some of these goals could take longer than a decade to realize depending on the availability of funding, the adequacy of technology development, and the fortunes of observational and theo- retical discovery. Receive gravitational waves and use them to study regions of strong gravity. Study of the Hulse-Taylor binary pulsar 1913+16 proved that gravitational waves exist, but the discovery is still incomplete, in the same way that neutrinos needed to be detected even after their existence was proved from the study of beta decay. Reception of gravitational waves will allow the precise comparison of their properties with those predicted by general relativity. However, beyond these tests, gravitational waves provide a window into regions of strong and rapidly varying gravity in the universe that are largely invisible using electromag- netic signals. The strongest waves come from the most extreme and catastrophic events in the universe and can provide important clues to the nature of those events. Supernova explosions, stellar and black hole collisions, and the big bang are all examples. Gravitational waves can provide unique signatures for the existence of black holes. They can also be used to test the validity of general relativity. A worldwide network of gravitational wave observatories is poised to begin exploiting this new astronomical window. . Explore the extreme conditions near the surface of black holes. Astronomers have discovered black hole candidates with masses several times that of the Sun in binary star systems, and up to a billion times larger in the centers of galaxies. Radio, optical, x-ray, and gamma-ray observations of the candidates imply dense concentrations of matter in very small regions of space. Einstein's equations of relativity, together with our understanding of the proper- ties of matter, do not allow any viable interpretation of the observations other than that the objects are black holes. However, there is not yet direct confirma- tion of the black hole nature of the candidates. Much can be learned from the

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16 GRAVITATIONS PHYSICS: E~LOHNG THE STRUCTURE OF SPACE^D TIME detailed study of the environment near the black hole surface, using electromag- netic observations. Gravitational waves from the damped vibrations of newborn or disturbed black holes can supply even better probes. Measure the geometry of the universe and test relativistic gravity on cosmological scales; explore the beginning of the universe. General relativity, together with the observation that the universe is expand- ing, implies that the universe began in a big bang. Yet the overall geometry, material content, and ultimate fate of the universe are still open questions. A number of astronomical tools, including observations of the microwave back- ground radiation, studies of distant supernovae, measurements of the large-scale distribution of galaxies, and studies of gravitational lenses, provide ways to mea- sure the geometry of the universe and the value of the cosmological constant. Extending our understanding of physics to enormous densities, temperatures, and curvatures in the earliest moments of the universe is one of the great challenges of theoretical physics. Not only is a quantum theory of gravity needed, but also a theory of the universe's quantum initial state. . Test the limits of Einstein's general relativity and explore for new physics. General relativity has passed all experimental tests performed to date. Yet there is now a growing expectation that deviations from the predictions of pure general relativity will occur at some level, mediated by interactions of hitherto unseen elementary particles. High-precision experiments in terrestrial laborato- ries or in space can place constraints on such interactions, and possibly detect them. Unify gravity and quantum theory. Despite the outstanding success of general relativity, this theory is not able to describe the strongest gravitational fields in the universe, such as the earliest moments of the big bang, or the ultimate fate of a star that collapses to form a black hole. To describe these situations, a quantum extension of Einstein's theory is needed. The significant progress over the past decade has given hope that this long-sought theory may soon be completed. Its implications from cosmology and black hole physics, to a new understanding of space and time, to a possible unification of all of the known forces and particles in nature are enormous.

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 17 Recommendations Listed below are the COP's specific recommendations for research in the next decade to reach these goals. This list of four is ordered with the highest- priority area of recommended actions given first. The recommendations within each of the four categories have equal weight. 1. Gravitational Waves As is described in more detail in Chapter 3, the search for gravitational waves divides naturally into the high-frequency gravitational wave window (above a few hertz) accessible by experiments on Earth, and the low-frequency gravitational wave window (below a few hertz) accessible only from space. The COP did not attempt to prioritize one of these windows over the other. Both are important. While there are perhaps more currently known sources accessible from the low-frequency window, the high-frequency window is the one that most clearly will open up in the next decade. The highest priority is to pursue both of these sources of information. The High-Frequency Gravitational Wave Window Carry out the first phase of LIGO scientific operations. Enhance the capability of LIGO beyond the first phase of operations, with the goal of detecting the coalescence of neutron star binaries. Support technology development that will provide the foundation for fu- ture improvements in LIGO's sensitivity. The main U.S. opportunity for the direct detection of gravitational waves in the next decade lies in the Laser Interferometer Gravitational-Wave Observatory. The LIGO detectors are sensitive to waves with frequencies of several kilohertz down to 50 Hz in their initial data run, extending downward toward 10 Hz after they are upgraded. In the high-frequency window are several candidate sources of gravitational waves whose detection would contribute important new astro- nomical and physical information. These include inspiraling and merging binary systems of black holes or neutron stars, gravitational collapse of stellar cores in supernova events, unstable oscillations of newly formed neutron stars, and a random background of waves, possibly from processes in the early universe. The discovery of waves from binary neutron star inspirals can reveal information about the nature of matter at supernuclear densities and could shed light on the origin of gamma-ray bursts, while waves from merging double black holes could show how event horizons coalesce and provide proof of their existence. Detec- tion of gravitational waves from pulsars would reveal whether or not their sur- faces are distorted and provide key clues as to their internal structure.

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18 GRAVITATIONS PHYSICS: E~LOHNG THE STRUDEL OF SPACE^D TIME The CGP recommends support for the initial operation of LIGO. It recom- mends support for sustained development of the technology necessary to upgrade LIGO to a sensitivity necessary to detect neutron star binary coalescences. In particular the CGP supports the mid-decade enhancement of LIGO' s sensitivity by reasonable extrapolations of existing technology. (See Table 1.1.) If further improvements by deployment of new technology involve a large increase in costs, the CGP recommends that the project be reviewed when the funding step is required. The review should consider developments in detector sensitivity, de- tected sources, and current astrophysical understanding. The Low-Frequency Gravitational Wave Window Develop a space-based laser interferometer facility able to detect the gravitational waves produced by merging supermassive black holes. Gravitational waves below a few hertz provide a window on the universe that is different from that studied by LIGO, much as the universe seen in radio waves differs from that seen in visible light. Seismic noise makes this low-frequency window inaccessible to ground-based observations; observations from space are required. In this window we should be able to detect gravitational waves from known binary stars and from the merging and formation of supermassive black holes, and to search for waves from the earliest moments following the big bang. The detection of any of these gravitational waves will meet important goals in both physics and astronomy. In astronomy the low-frequency gravitational wave window offers the possibility of detecting objects that can be seen in no other way, such as supermassive black holes; probing the interiors of some of the most energetic events in the universe, such as those occurring in quasars and active galactic nuclei; and investigating the collisions of galaxies in epochs close to the time of their formation. In physics, observations in this window would allow precision tests of the properties of gravitational waves, tests of strong-field theo- ries of the production of these waves, detailed confirmations of the predicted properties of black holes in general relativity, and observational tests of the theory of gravitational collapse. Limits on the gravitational waves from the big bang would constrain the physics of the fundamental interactions at the ultrahigh energies realized in the early universe. For these reasons the CGP supports the development of key technologies aiming at the deployment of such an interfer- ometer late in the first decade of the 21st century.

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20 GRAVITATIONS PHYSICS: E~LOHNG THE STRUT OF SPACE ^D TIME 2. Classical and Quantum Theory of Strong Gravitational Fields Support the continued development of analytic and numerical tools to obtain and interpret strong-field solutions of Einstein's equations. Full exploitation of the opportunities of the coming decade in strong-field gravitational physics will require a deeper grasp of the underlying theory than we currently possess. A detailed understanding of solutions to Einstein's equations will be necessary to understand regions of strong gravity, including black holes. In addition to analytic techniques, computational approaches will be essential, because Einstein's equations are too complicated to yield entirely to analytic methods. The investigation of gravitational theory should be two-pronged. On the one hand, gravitational physicists must aim at achieving a fundamental understanding of general relativity. On the other, there are important astrophysically relevant questions that need reliable answers if the goals outlined above are to be fully met. The most urgent of these is to understand quantitatively the outcome of black hole and neutron star collisions. If these calculations are to supply predic- tions of the gravitational waves produced by such events by the time LIGO is on line, an expanded effort is required, including adequate human resources and increased access to supercomputer facilities. Support research in quantum gravity, to build on the exciting recent progress in this area. The most fundamental questions about space, time, the nature of the big bang, and the interior of black holes cannot be answered within classical general relativity. They require a quantum theory of gravity. Recent developments in string theory and the quantum theory of geometry have brought us closer to constructing this new fundamental physical theory. It is already possible to answer some long-standing questions about the quantum properties of black holes. In the next decade, the COP recommends a concerted effort to complete the construction of this new theory, possibly by combining some of the ideas in these two approaches. Applications of this theory to the nature of the very early universe should be explored, possibly resulting in modifications of our current ideas about a very early epoch of ultrarapid expansion of the universe (inflation) and shedding light on its initial state. The potential implications of such a theory are extraordinary. At the micro- scopic level, it may be necessary to abandon such basic notions as the spacetime continuum, causality, and locality. The universe may have extra "hidden" spatial dimensions. Fundamental entities may be extended objects like strings rather than point particles.

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 2 Much of the recent progress in quantum gravity has occurred through a confluence of ideas from gravitational physics, elementary-particle physics, and mathematics. Fostering close contacts between these communities (for example through joint research, conferences, and schools) is vital for continued progress. 3. Precision Measurements Dramatically improve tests of the equivalence principle and of the gravi- tational inverse square law. The equivalence principle is one of the foundations of general relativity, and any violation requires new physical interactions that could also modify the in- verse square law, which is satisfied by general relativity in its Newtonian limit. Quantum theories of gravity, as well as some cosmological theories, could pro- duce apparent violations of the principle at some level. New experiments carried out in terrestrial laboratories and in space can improve the precision, explore much shorter length scales, and test the effects of exotic forms of matter. . Continue to improve experimental testing of general relativity, making use of available technology, astronomical capabilities, and space opportunities. Modest investments in promising laboratory techniques or space missions could yield important improvements in experimental tests that could probe the limits of general relativity. Examples include continued lunar laser ranging, placing high-precision clocks on satellites, tracking of Earth-orbiting and inter- nlanetary spacecraft, and binary pulsar observations. 4. Astronomical Observations The astronomical observations recommended below have strong arguments for support from astronomy and astrophysics. The ones listed are those that the COP expects will have the greatest impact on gravitational physics in the next decade. . Use gamma-ray, x-ray, optical, infrared, and radio telescopes on Earth and in space to study the environment near black holes. Such observations provide important insights into the extreme environments from which a broad spectrum of radiation is emitted and can potentially pin down basic properties of black hole candidates, such as their masses and spins. The observations may also lead to definitive proof of the black hole nature of the objects.

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22 GRAVITATIONS PHYSICS: E~LOHNG THE STRUT OF SPACE ED TIME Measure the temperature and polarization;fluctuations of the cosmic back- ground radiation from arcminute scales to scales of tens of degrees. Microwave background observations measure variations in spacetime, from the scale of galaxies up to the scale of the visible universe. These ripples can be due either to fluctuations in the density of the universe or to gravitational waves with wavelength comparable to the size of the universe. The COBE satellite detected these temperature fluctuations at the largest angular scales. In the fu- ture, the MAP and Planck satellites, and ground-based and balloon-based experi- ments, will map these fluctuations at finer scales. Gravitational waves and den- sity fluctuations also generate polarization fluctuations whose amplitude is expected to be a few percent of the temperature fluctuations. Observations of these polarization fluctuations could lead to the detection of a stochastic back- ground of gravitational waves from the early universe. Search for additional relativistic binary systems. Astronomers have detected only perhaps a percent of the pulsars in our Galaxy. Future surveys may detect a pulsar orbiting a black hole. A black hole- pulsar binary system would be a powerful laboratory for gravitational physics, testing with high precision whether the orbital motion and gravitational wave generation of black holes conform to the general relativistic predictions. . Launch all-sky gamma-ray and x-ray burst detectors capable of detecting the electromagnetic counterparts to LIGO events. Cross-correlation of electromagnetic and gravitational signals will help to establish the reality of gravitational wave detections, and may immediately yield crucial clues to the nature of the emitting objects. For example, the enigmatic gamma-ray bursts might be explained if a gravitational wave burst is detected in coincidence. Similarly, supernova searches from ground-based telescopes and neutrino detectors could play a mutually reinforcing role with gravitational wave detectors. Since the first gravitational waves to be detected may well come from transient events, it is urgent to continue the development of the space-based electromagnetic observational capabilities that have already revealed a rich range of astronomical phenomena. Use astronomical observations of supernovae and gravitational lenses to infer the distribution of dark matter and to measure the cosmological constant. Certain supernovae appear to be "standard candles"; their intrinsic bright- ness seems to be the same from case to case, and thus their distance from Earth can be determined from their apparent brightness. Observations of these super

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INTRODUCTION, OVERVIEW, AND RECOMMENDATIONS 23 novae at different locations measure the relationship between distance and red- shift. Current observations of this kind suggest that the expansion rate of the universe is accelerating. This surprising result suggests the existence of a cosmo- logical constant whose value is of fundamental importance for physics. Future observations can bet ? reduce both statistical and systematic errors in these re- sults. Observations of gravitational lenses can map the distribution of dark matter. By observing lenses at different redshifts, astronomers can determine the evolu- tion of density fluctuations with redshift. Since the evolution depends on the composition of the universe, gravitational lens observations are an independent tool for determining cosmological parameters.