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go and potential scientific benefit of a giant Cerenkov neutrino detector in the form of a cubic kilometer of sea- water with a three-dimensional lattice of photomulti- pliers. It has been estimated that such an installation might detect neutrinos with energies above 1012 eV from the brightest sources. However, it has not yet been demonstrated that the astronomical benefits will be suf- ficient in themselves to warrant construction of a full DUMAND detector. Further studies may substantially increase the ratio of potential benefits to cost. X. GRAVITATIONAL-WAVE ASTRONOMY A. Introduction Gravitational-wave astronomy has the following two broad scientific goals that might be achieved in the 1980's but that are more likely to require a sustained effort into the 1990's. 1. Verify the Existence of Gravitational Waves and Use Them to Test the General Theory of Relativity Einstein's General Theory of Relativity implies the exis- tence of gravitational waves, but none have been detected as yet. The discovery of gravitational waves that propo- gate with the speed of light and have polarization proper- ties of a spin-2 field would prove that gravity is a field phenomenon and would disprove every theory of gravity that has been invented, except general relativity and theories that differ from it by small corrections for quantum-mechanical or torsion effects. Measurements of the propagation speed and polarization are likely to be achieved within a few years after the discovery of gravitational waves. 2. Harness Gravitational Waves for Observational Astronomy It is likely that gravitational waves will reveal proper- ties of their sources that one can never learn by elec- tromagnetic, cosmic-ray, or neutrino studies. This is due to the following features of gravitational waves, as predicted by general relativity:
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91 (a) They are generated by coherent bulk motions of matter and carry information about those bulk motions. In contrast, cosmic electromagnetic and neutrino radia- tions are usually incoherent superpositions of emission from individual atoms and charged particles. (b) They are emitted most strongly in regions of space where gravity is relativistic and where the ve- locities of bulk motion are near the speed of light as in the cores of supernovae and near the horizons of black holes. (c) They pass through surrounding matter with impunity, in contrast with both electromagnetic waves, which are easily absorbed and scattered, and neutrinos, which easily traverse a normal star but scatter many times in leaving the ultradense core of a supernova. Among the phenomena that one might study by observing gravitational waves are the dynamics of the collapsing cores of supernovae; the dynamical evolution of newborn, rapidly rotating neutron stars; quakes in neutron stars; the dynamics of the formation of black holes by stellar collapse; collisions between compact objects such as black holes and neutron stars in the nuclei of distant galaxies; the internal structures of common-envelope binary stars; and white-dwarf oscillations produced by nova outbursts. In addition there may exist a stochastic background of gravitational waves, pregalactic or · . . . . primordial In origin. The strength of a gravitational wave is measured by the dimensionless strain h produced in a detector. Theo- retical estimates of the values of h for gravitational waves from various sources are highly uncertain. However, the estimates support the following conclusions: (a) The search for gravitational waves should extend over the frequency from 10 4 Hz to 10+4 Hz. (bi Wave bursts with h as large as 10-16 (~/1 kHz) /2 could arrive at Earth once per month without violating conventional ideas about the nature of gravity or the structure of the Universe, though the strongest waves are probably much weaker than this. (c) Supernovae in our Galaxy, which occur once per 10 to 30 years may produce wave bursts with f in the range from 30 to 3000 Hz and h from 10 17 to 10 20. (d) It would not be surprising to find extragalactic wave bursts with h near 10- 0 (~/1 kHz) 6/7 every few months.
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92 (e) It would be surprising if gravitational waves with h on the order of 10 1 were not seen in several portions of the frequency range from 10 4 Hz to 10+4 Hz. B. Progress during the 1970's 1. Ground-Based Detectors By the beginning of the 1970's, investigators at the University of Maryland had constructed the world's first ground-based gravitational-wave detectors: 1-ton alumi- num bars suspended in vacuum and instrumented with piezo- electric strain transducers. They observed coincident excitations of these detectors that they regarded as evidence either for a positive detection or for the presence of some unknown kind of "background." Between 1970 and 1975 a number of other laboratories constructed and operated detectors similar, but not identical, to those at Maryland. When some of these showed no evidence of coincident excitations and others showed only marginal evidence, most investigators con- cluded that gravitational waves had not yet been detected However, it was clear that various design changes and the use of new technology could improve the detector's strain sensitivities a thousandfold or more. This motivated the "second-generation" detectors, the first of which went into operation in 1980. m ey are of two types: (a) bars made from aluminum, niobium, sapphire, or silicon, cooled to about 4 K, and instrumented with "active" strain trans- ducers (SQUID magnetometers, microwave cavities, mechani- cally modulated impedances in rf circuits) and (b) multi- reflection laser interferometers with arm lengths of 1 to 40 m. A useful measure of the sensitivity of such detectors is the dimensionless amplitude h of a gravity wave of frequency f, which would produce unity signal-to-noise ratio after acting for a time 1/f. It is estimated that . the detectors of the early 1970's had values of h near 10 16 and that the best of the subsequent first- generation bars in 1975 improved on this by a factor of about 3--both at f near 1 kHz. In 1980 the first of the second-generation detectors was operating with values of h of about 3 X 10~18--a thirtyfold improvement in the minimum detectable strain or 1000-fold energy improve- ment in the minimum detectable energy flux since the -
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93 early 1970's. This sensitivity is sufficient to detect gravity-wave bursts of the maximum strength consistent with conventional theories at a rate of one per month. In principle, it could detect a wave burst from a very nonspherical supernova anywhere in our Galaxy. Further substantial improvements in sensitivity in variants of second-generation detectors are expected. The effort to detect gravitational waves is now an important stimulus for development of ultra-high- precision measurement techniques. Its high-technology spin-offs during the 1970's have included the following: (a) Mechanical resonators with Q values of the order of 4 X 109, which may have application in stable oscillator technology; (b) New designs for sensitive accelerometers, which have led to significant advances in gravity gradiometers; (c) A new technique for locking lasers to cavities and thereby achieving far better short-term (<10-2 see) laser frequency stabilities than heretofore--a technique that will find application in laser spectroscopy; (d) Improvements in low-noise amplifiers, displacement transducers, stable microwave cavities, and microwave frequency sources; (e) Understanding of how to circumvent what were previously believed to be quantum limits in high- precision measurements. 2. The Earth as a Detector Effective use of ground-based detectors is limited to frequencies much larger than 1 Hz by seismic noise and gravity-gradient noise. The sole exception is the use of the Earth's quadrupole oscillations as a detector at specific resonant frequencies of 3.1 X 10 4 Hz, 6.8 X 10 4 Hz, 1.1 X 10 3 Hz, etc. Gravimetric monitoring of the 3.1 X 10-4 Hz mode has achieved a sensitivity, limited by seismic noise, corresponding to h near 2 X 10-14, which is sufficient to detect wave bursts of the maximum strength compatible with conventional theories. None have been detected. 3. Doppler Tracking of Spacecraft A promising approach to the detection of gravitational waves with frequencies in the range from 10-2 to 10-4
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94 Hz is Doppler tracking of interplanetary spacecraft. The theory of the interaction of gravity waves with the Doppler tracking system was developed in the mid-1970's. In the late 1970's the Deep Space Network was upgraded to include a hydrogen-maser clock as the master oscillator. Soon thereafter Doppler data were recorded with the Viking spacecraft. The dominant noise source in these data were fluctuations in plasma dispersion, and this limited the sensitivity at S-band (radio signals of 13-cm wavelength) to values of _ near 6 X 10-14 when the spacecraft was within about 10 deg of the antisolar direction, and to h near 6 X 10-13 far from opposition. gravitational waves was found. 4. The Binary Pulsar No evidence of m e binary pulsar, discovered in 1975, has an orbital period that decreases at a rate that agrees within about 20 percent with the value expected from gravitational radiation reaction according to Einstein's theory. This is the strongest indirect evidence so far for the exis- tence of gravitational radiation. 5. Gravitational-Wave Theorv Progress in gravitational-wave theory during the 1970's included elucidation of how the search for gravitational waves can test theories of gravity, estimates of the characteristics of the waves emitted by astrophysical sources, and the development of new mathematical tech- niques for computing the forms of gravitational waves produced by various hypothetical sources. These results have led to a clear formulation of the scientific goals of the field and now guide the design of experiments. Inventory of Present or Approved Resources Currently under development are second-generation bar detectors at Louisiana State University, the University of Maryland, Rochester University, and Stanford University and second-generation laser-interferometer detectors at the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT). The use of the Earth as a detector is not now being pursued anywhere.
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9s Experiments based on Doppler tracking of spacecraft are under development or consideration for two major interplanetary missions. On Galileo the approved experi- ment has an S-band uplink and X-band downlink; a change to X band on both links would greatly improve the quality of the experiment. An experiment with X band on both links is under consideration for the International Solar Polar Mission (ISPM). Both the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA) support work on the theory of the generation of gravita- tional waves, on estimates of the characteristics of waves from astrophysical systems, and on new ideas for detection technology that are pertinent to the detection programs that each of the agencies has funded. m e effort to detect gravitational waves is worldwide and benefits from cooperation that transcends national boundaries. Second-generation bar detectors are being developed in Peking, Canton, Moscow, Perth, Rome-CERN, and Tokyo. The Moscow and Rome-CERN efforts are larger than any in the United States. Laser-interferometer systems with arm lengths of 10 m or greater are being developed in Munich and in Glasgow and are closer to operation than the comparable U.S. systems. In 1980 the Soviet Union initiated development of a very large laser system. Doppler tracking of spacecraft for the purpose of detecting gravitational waves is not currently being pursued outside the United States, except for European cooperation on the ISPM. Theoretical work outside the United States has concentrated on the mathematical theory of gravitational radiation. D. Recommendations for the 1980's 1. Ground-Based Detector Program Each of the several groups developing second-generation ground-based detectors under the sponsorship of the NSF is pursuing a different strategy and design. This variety enhances the likelihood of success since the number of plausible approaches is large, and different technologies will be optimal for different frequency bands and wave- forms. m us the various second-generation efforts should continue to be supported at a healthy level. Once second-generation bar or laser systems are fully operational, at least three of them, widely separated, should be kept in continuous operation to watch for rare
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96 events such as a wave burst from a supernova in our Galaxy. Cross correlation between two (and preferably three) detectors, with comparable sensitivity and fre- quency response, will be essential for the elimination of · . spur lOUS S. lgna. S . Whether or not the second-generation efforts are successful in detecting gravitational waves, a third- generation effort should be undertaken with the aim of achieving sensitivities corresponding to values of h less than 10- 0 by 1990. Promising ideas have been put forward for bar detectors that would achieve such sensi- tivities but probably with responses confined to a narrow band inside the frequency range from 10 to 10 Hz. They might be able to detect cosmic signals but would probably miss most of the details of any waves except those from periodic sources. Laser systems appear at present to be the most promis- ing means for making broadband observations. They may well be the first to detect any waves at all in the frequency range from 10 to 1000 Hz. A sensitivity of h better than 10-2° in this frequency range might be achieved by 1990 with a kilometer-scale laser interferom- eter. The Caltech and MIT second-generation laser inter- ferometers are 10-m-scale prototypes of larger systems. If they achieve sensitivities near their photon-counting limits, then construction of a kilometer-scale or larger interferometer system should be undertaken. Ideally, the U.S. program should have two such interferometers so that cross correlation can be carried out to eliminate spurious events. It is still not certain that bar and laser systems will be the only effective detectors at frequencies above 1 Hz or that they can ever be brought to the level of sensitivity required for the detection of gravitational waves. Thus it is essential that appropriate support be given to the exploration of alternative approaches such as the detection of frequency variations in resonant superconducting microwave cavities. 2. Space-Based Detectors The only viable detection scheme for frequencies f much less than 1 Hz in the 1980's is Doppler tracking of space- craft under the purview of NASA. An optimal program would include simultaneous gravimetric monitoring of the Earth's normal modes. It is crucial for the success of the track-
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97 ing program that the Galileo and Solar Polar spacecraft be configured for X-band tracking on both the uplink and the downlink. X-band tracking may well achieve a sensi- tivity of h near 10-15 in the frequency range from 10-3 to 10-4 Hz, which would be sufficient to detect plausible, but not certain, disturbances involving super- massive black holes in galactic nuclei at the Hubble distance. Without X-band capability the sensitivity will not exceed a value of h equal to 10 14. planet missions should be configured in the initial planning stages for optimal collection of gravitational wave data. Future outer- - Sensitivities much better than h = 10-15 are required for detection of most of the likely low-frequency sources. To achieve such sensitivities, a new Research and Analysis program should be initiated, independent of specific mis- sions, to develop the technology for Gravitational wave experiments in space. ,, _ _ , _ Such a program might include the development of improved transponders, Doppler readout systems, and clocks; development of multifrequency tracking systems and of a multilink tracking system with an ultra-stable clock aboard the spacecraft; and develop- ment of laser-tracking and drag-free satellites. If appropriate multilink and onboard-clock devices are ready in time, they should be flown on the ISPM. Other- wise they might be carried by a dedicated gravity mission, which would perform several tests of general relativity as well as search for gravitational waves. A sensitivity of h in the range from 10-16 to 10-17 is a reasonable goal for such a mission. Laser tracking of one spacecraft by another could possibly achieve a sensitivity of h better than 10-21 at all frequencies in the range from 30 Hz to 10-4 Hz. Such a system might be flown in the 1990's and would likely permit broadband detection of gravitational waves from a variety of astrophysical sources. Detailed feasi- bility studies for such a system should be carried out in the early 1980's. 3. Theoretical Studies . . . Continued theoretical work is essential to the design of better detection systems and to the interpretation of data it and when gravity waves are detected. Especially important is numerical solution of the Einstein field equations to determine the gravitational waveforms in(t) produced by astrophysical sources.
Representative terms from entire chapter: