Einstein's Theory Put to the Test
Gravity Probe-B and laser-beam observatories will measure unproved relativity predictions
BOSTON — FAR off in the constellation Aquila, a rare object called pulsar 1913 +16 beams a radio signal that challenges physicists to answer the question: Was Einstein right? Is his theory of gravity - the general theory of relativity - right when it predicts the existence of ripples in the fabric of space and time called gravity waves?
Is the theory right when it predicts that a massive rotating object like Earth warps the space-time fabric in ways that change the orientation of gyroscopes?
These are two of the most important unproved aspects of Einstein's great theory. Now, after decades of experimentation and development, physicists have the technology to mount experiments to test these predictions.
With National Aeronautics and Space Administration (NASA) funding, a Stanford University team in Palo Alto, Calif., is constructing a satellite to test the gyroscope effect. Called Gravity Probe-B, it will carry the most precise gyroscopes ever made to perform what NASA has called ``the most challenging test we'll undertake in this millennium.'' The project's principal investigator, Francis Everitt, says it will make ``the most precise measurement ever made of an Einstein prediction.''
Meanwhile, the National Science Foundation is supporting a team of scientists at the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT) in designing an observatory that uses laser beams to detect gravity waves. The observatory is to be part of a global complex that, after several decades of failure with smaller detectors, should have the sensitivity to pick up these elusive phenomena. Then, says project director Rochus E. Vogt of Caltech, ``we can put Einstein's general relativity theory to the most stringent test ever.''
The pulsar in Aquila spurs on both projects, because here, for the first time, radio astronomers have seen signs of both untested effects. It is a system in which two dense, compact stars are in a close orbit. One star emits a radio beam that, like a lighthouse beam, sweeps past Earth where it is heard as a series of pulses.
Princeton University radio astronomer Joseph H. Taylor and various colleagues have studied this system since 1974. They have found its members spiraling closer together, just as expected if energy is radiating away in the form of gravity waves. Their observations agree with the relativity theory's prediction to within 1 percent. Because of this, Dr. Taylor says, ``you can say that the existence of gravity waves now has been accepted.''
Recently, Taylor and his colleagues have seen evidence of the gyroscope effect as well. Taylor calls this a ``qualitative confirmation'' of Einstein's prediction, because the scientists can't pin this down to the point of making a precise comparison with theory.
That kind of test has to wait for Gravity Probe-B. ``It's been a long time in coming and I hope it works,'' Taylor says. In fact, the late Stanford University physicist Leonard Schiff and his colleagues physicist William M. Fairbank and astronautical engineer Robert Cannon came up with the idea three decades ago.
IN relativity theory, space and time form a seamless four-dimensional fabric called space-time. Matter can warp this fabric. The theory predicts that the axis of a gyroscope moving through such warped or curved space-time will change direction, even though no force acts on it. It further predicts that a massive, spinning body will drag the space-time medium around with it, and this dragging will also move a gyro axis.
Dr. Schiff showed that our spinning Earth warps and drags space-time enough for a sensitive gyro in orbit to exhibit both effects. The directional change, called precession, due to space-time curvature would amount to 6.6 seconds of angular arc a year or a mere 6.5 degrees ``since the time of Moses.'' Precession due to space-time dragging would be much less - 0.04 degrees in that time. Measuring such minute changes would be tricky. But the three partners concluded it was feasible. Thus began one of our century's longest playing single scientific projects.
By early this year, Stanford announced it was working, with Lockheed Missile and Space Corporation as a subcontractor, to build the satellite for what is now a $108-million NASA mission program. The detector is first to be tested on a space-shuttle flight, probably in 1993, with the satellite going into orbit in about 1995. Then, after a year of data-taking, the project may have the answers it has so long sought.
The detector consists of four balls of pure quartz, each about 1.5 inches in diameter and round to within 40 atomic layers - about 0.3 millionths of an inch - of the roundness of a perfect sphere. They will be coated with niobium and cooled to the temperature of liquid helium. This will make the niobium superconducting so that it will carry an electric current without loss. That, in turn, will give the balls properties that allow them to be suspended electrically in a vacuum to provide four independent, stable gyroscopes.
Program manager Bradford W. Parkinson calls the precision needed in this construction ``challenging stuff.'' Yet, he adds, the prototype is ``working perfectly.'' He calls its performance ``absolutely dazzling.''
Professor Parkinson finds Taylor's pulsar work encouraging. He seems to be seeing a combined effect of space-time curvature and space-time dragging without being able to separate them, as Gravity Probe-B will do. Nevertheless, Parkinson says, ``It's nifty detective work'' that suggests the effects really do exist.
Meanwhile, gravity-wave hunters are also preparing. When such waves pass, they should cause slight changes in the separation of test masses. Several teams in Australia, Europe, and Japan, along with the Caltech-MIT project, are developing observatories in which masses are suspended from pendulums at either end of evacuated pipes several kilometers long. The pipes are connected to form ``L''-shaped structures. Laser beams reflected from the mirrors measure the separation of the test masses.
Dr. Vogt notes that, first, the detection of gravity waves will test relativity theory. If they exist and can be routinely observed, they may, he says, ``create a revolution in our view of the universe.''
The Bush administration requested $47 million in its fiscal 1991 budget to begin construction of the Caltech-MIT equipment that, eventually, is to be in two observatories at least 1,500 miles apart. However, the House currently has cut out those funds while the Senate has yet to act. At this point, the timing of the project is uncertain.
Meanwhile, one of the first observatories in the proposed global network may be built in Australia. Representatives from the Australian National University and the University of Western Australia have been working with representatives of Nagoya University and the University of Tokyo in Japan to design a joint facility to be located in Australia. They have applied to their governments for funding.
In one location or another, the international fraternity of gravity-wave researchers expects to have at least one of the laser-detector observatories operating somewhere in the world by the decade's end.