Gravity is the weakest force in the universe. Because of this weakness gravity waves, [See my column in the January-1988 issue of Analog.], the traveling waves made by disturbances in gravity, are below the present threshold of detectability and have never been directly observed. But in the year 2000 this should change.
Curiously, in some ways gravity is also the strongest force in the universe. It always adds, never subtracts, and can build up until it overwhelms all other forces.. In normal stars gravity is balanced by heat energy from fusion reactions in the star's core. Eventually, however, the hydrogen and heavier elements fueling these reactions are used up, gravity takes over, and the star collapses in on itself. The result is a supernova explosion, which converts a sizable fraction of the star's mass-energy to light, neutrinos, and gravity waves.
If the original star is similar to our sun, its supernova will leave behind a neutron star, a system in which gravity counterbalances the strong interaction and converts protons to neutrons, so that the entire star becomes one giant nucleus held together by gravity. If the star is about three times more massive than our sun, the gravitational forces will completely overwhelm the strong interaction that keeps the neutrons apart, so that the system will collapse to a black hole.
However, such stellar-scale catastrophes do not occur in isolation. Most star systems are binaries, and when the nuclear fuel of both members of a binary system is used up, the result can be a neutron star binary, a pair of neutron stars orbiting each other.
In 1974 Joseph H. Taylor of Princeton University and his graduate student Russell A. Hulse detected radio waves from a pair of orbiting neutron stars. The timing and Doppler shift of the radio waves from this system allowed them to precisely determine the rotation period (about 8 hours). They made measurements over several years and found that the rotation period was decreasing because the stars were slowly "spinning down", losing a tiny fraction of their rotational energy as they orbited. Taylor and Hulse showed that this loss of energy is precisely the amount predicted by Einstein's general theory of relativity due to the production of gravity waves by the two massive counter-rotating stars. Thus, in a rather indirect way gravity waves were "observed" though their removal of energy and angular momentum from the binary neutron star. In 1993 Taylor and Hulse received the Nobel Prize in Physics in recognition of this work.
What are gravity waves? A gravitational field, which can be viewed as a distortion of local space, surrounds every massive object. Gravity waves can be viewed as spreading ripples in this space distortion which arise when a massive object is moved and its gravitational fields disturbed. Like light, gravity waves travel at the speed of light and obey the inverse- square law [intensity is proportional to 1/(distance)2]. Gravity waves induce a kind of "kneading" distortion in the space through which they move, making local distances alternately larger and smaller. In one direction perpendicular to the wave's direction of travel space is stretched, while in the other direction space is compressed, with the stretch and compression exchanging places after half a period of the wave. The wave can be visualized a long sausage with its sides alternately pinched in side-to-side and top-to-bottom, with the pinches along the length of the sausage repeating with each wavelength and the entire sausage moving forward at the speed of light.
Gravity waves have two distinct states of polarization. Viewed head-on, gravity waves with the "+" polarization state alternately compress and expand space top-and-bottom and side-to-side (kneading space with a "+" PATTERN). Gravity waves having the "X" polarization state compress and expand space along lines 45 degrees to the right of vertical and to the left of vertical (kneading with an "X" pattern). These two polarization states makes gravity wave detection more difficult because a given detector is usually sensitive to only one of the two states and therefore detects only half of the possible signals.
Once fascinating aspect of the Taylor-Hulse work is its implication that after a few million years binary neutron star systems will lose energy and spin down faster and faster, until finally the stars collide and merge. This spin-down and collision releases a huge quantity of energy, appearing as an intense rising "chirp" of gravity waves. It is expected that other binary systems composed of normal stars, neutron stars, or black holes will also spin down and produce similar cosmic fireworks. These intense bursts of gravity wave energy are expected to occur every few years (or perhaps even every few months) and, given a suitable detector, should be detectable. The ability to detect gravity waves and determine their directions and points of origin would create a whole new science: gravity wave astronomy.
Constructing a suitably sensitive detector, however, is extremely difficult because gravity waves are the wave embodiment of the weakest force of the universe (gravity is 4.3 x 10-40 times weaker than electromagnetism), and the effects of gravity waves on matter are correspondingly small. Early detection attempts using large resonant cylinders as detectors, after some false-positive reports in the 1970's, were shown to be too insensitive to detect gravity waves at the intensities expected. At this writing (10/97) gravity waves have never been directly detected. However, this is likely to change soon.
Physicists at the California Institute of Technology (CalTech) and the Massachusetts Institute of Technology (MIT), with $356,000,000 in funding from the National Science Foundation, have joined forces to construct the Large Interferometer Gravitational Observatory or LIGO (pronounced LIE-go). LIGO consists of two widely separated detectors. The original plan was to construct one of these in California near CalTech and the other in Massachusetts near MIT. However, political and geological realities modified this plan, with the result that one detector is presently being constructed in a desert site near Hanford, Washington, and the other in Livingston Parish, Louisiana. Together they form a detection system with its elements separated by about 3000 kilometers (1900 miles), a distance considered optimum for geologic independence and good source position reconstruction. The target date for project completion and first operation is sometime in the year 2000.
The detector at each LIGO site is an L-shaped multi-pass optical interferometer. Light from one or more strong 5 watt stabilized lasers (eventually to be replaced with 100 watt lasers) is split into two beams. These are reflected many times with multiple mirrors and passed many times through two horizontal 1 meter diameter vacuum pipes, each 4.0 kilometers (2.5 miles) long and held at a vacuum of 10-9 torr. The two arms are oriented so as to form a giant right angle "L" on the ground. The laser beams emerging after the multiple passes through the two pipes of the interferometer are recombined at a low-noise optical detector, where the light beams interfere so as to reinforce or cancel. The light signal changes with any microscopic change in either path length along the two detector arms. The arms of the detectors at the Washington and Louisiana sites will be pointed in the same two spatial directions, maximizing the probability of coincident detection of gravity waves at the two sites. The "price" of maximizing this sensitivity is that LIGO will have zero sensitivity to one of the two possible polarization states of the signal (the polarization for which the compression- expansion axes are at 45 degrees to the LIGO arms).
LIGO is sensitive to any suitably polarized gravity wave arriving perpendicular to the plane of the detector, providing that the wave has a wavelength of more than about 16 km. This insures that one 4 km arm spans less than 1/4 wavelength of the wave, implicitly requiring a wave frequency of less than about 18 kHz. Such gravity waves cause one arm of the detector to shrink slightly and the other arm to lengthen slightly, but only by about one part in 1023 or so. This tiny change in length will shift the phase of the coherent light waves arriving at the detector from the two arms and will shift the detected light intensity, for example, from perfect cancellation to partial reinforcement. At the ends of the LIGO arms, mirrors reflecting the laser beams are mounted on "test masses" carefully suspended within vibration-suppressing feedback systems, but there are limits to the effectiveness of such systems in permitting the observation of a signal over vibration background. If the signal frequency from gravity waves is too low, it cannot be detected because it is washed out by system noise due to background vibrations.
LIGO is sensitive to gravity waves only in the frequency range from about 5 Hz to about 20,000 Hz. Fortunately, this range includes the gravity wave frequencies expected from type II supernovas and from the merger of a pair of neutron stars. LIGO has essentially no sensitivity in the frequency range 10-4 Hz to 1 Hz, where strong signals are expected from black hole formation or from the merger of a black hole binary system.
LIGO is not the only planned gravity wave detector, but it is the one which is likely to first detect gravity waves. There are several other ground- based projects: the Japanese TAMA project now being tested in a 300 meter prototype in Tokyo, the 600 meter GEO prototype now under construction near Hamburg, Germany, and the 3 kilometer French- Italian VIRGO project being constructed near Pisa, Italy. Some of these projects have target gravity-wave sensitivities similar to that of LIGO and all will have some operational capabilities in 2000. These should not be considered competitors of LIGO, however, but rather additional components of a greater overall system. Together the separated detectors will form vertices of an Earth-size polyhedron detector system that will permit more precise determination of the locations of gravity wave sources and will give improved polarization sensitivity.
All of these earth-bound detectors, however, are expected to "hit the wall" at frequencies below about 1 Hz because of background vibrations of the planet's surface due to human and seismic activity. Only space-mounted gravity wave detectors can overcome this low frequency limit. Fortunately one of the long term projects of the European Space Agency is LISA, a great triangle of interferometer satellites placed well away from the Earth, in their own stabilized orbits around the Sun. LISA, scheduled for around 2020, will be sensitive to the lower frequency 10-4 Hz to 1 Hz gravity waves from events involving black holes.
What will be discovered with this new array of gravity wave detectors coming on-line in the 21st century? That, of course, is impossible to say, but we can speculate. Like the new large neutrino detectors SNO (Canada) and Super- Kamiokande (Japan), LIGO should have sensitivity to supernova explosions in our galaxy, even when these cannot be observed optically. Correlating signals from LIGO with neutrino detectors should tell us more about the supernova process. Similarly, we expect fireworks in the optical, gamma ray, and perhaps neutrino domains when binary neutron stars merge. It will be very interesting to see if the unambiguous signal of neutron star merger detected by LIGO can be correlated with observations from other detectors. We will also learn if gravity waves accompany the mysterious gamma ray bursts, a cosmic phenomenon that still has no good explanation. [See my column in the October-1995 issue of Analog.] And of course, events near the center of our galaxy remain a deep mystery, recently heightened by the observation a giant fountain of antimatter positrons there. Such cataclysmic events may produce gravity waves, providing new information and insights.
But the main value of the project is that LIGO will allow us to look where none have looked before, giving good odds that we may see what none have seen before, and perhaps what no one even suspects. So be on the lookout for the new developments in gravity wave astrophysics in 2000 and beyond.
AV Columns On-line: Electronic versions of more than ninety past "The Alternate View" columns by John G. Cramer are available on-line on WorldWideWeb at the URL: http://www.npl.washington.edu/AV.
J. S. Higginbotham, "LIGO installation opens window to the universe", R&D Magazine. v39, #9, 15-16 (Aug. 1997).
K. S. Thorne, "LIGO, VIRGO, and the international network of laser-interferometer gravitational-wave detectors", in Quantum Physics, Chaos Theory, and Cosmology., Eds. M. Namiki, I. Ohba, K.I. Maeda, Y. Aizawa and M. Namiki, pp. 101-120, AIP Press (1996).