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OCR for page 90
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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(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:
doppler tracking
