Review of Gravity Probe B
The experiment now known as Gravity Probe B (GP-B) was conceived more than 30 years ago. Bold and daring in concept, it has been under continuous development ever since. The aim of the experiment is to measure, rather precisely, an effect that is pre dicted by all viable relativistic theories of gravity but has not yet been observed. Just as Newton's law of gravity is paralleled by Coulomb's law of electricity, so also it is expected that the force between currents of electrical charge, described by Ampere's law, should be paralleled by a force between "currents" of flowing matter. It is this force that has never been directly observed.
A useful perspective on the GP-B experiment can be obtained from a historical profile of its funding. Until the late 1980s, the project was funded at a level of $1 M to $2 M per year to develop and demonstrate the necessary technology. Funding was th en increased to permit detailed engineering of the various subsystems and thorough ground testing. The funding level reached about $30 M/yr in FY 1992, when the project entered a "science mission" phase involving development of an appropriate spacecraft to carry the experiment. Since then the funding has been approximately $50 M/yr.
When the project was last reviewed for NASA 4 years ago, the Parker Committee, an ad hoc review committee convened by NASA Associate Administrator for Space Science and Applications Lennard A. Fisk and chaired by Eugene N. Parker of the University of C hicago, recommended that if GP-B were to go forward, it must be properly funded. That committee considered an appropriate funding level to be about $50 M/yr until the time of launch, which was anticipated to be late in the 1990s. Subsequent funding has in fact been at this level, and has allowed highly skilled teams to address thoroughly various technical details of the experiment and to start building the flight instrument package and integrating it into a spacecraft. By the end of FY 1995 about $240 M will have been spent on the project. NASA estimates that another $340 M will be needed for completion, including launch and subsequent data analysis.
1.2 Scientific Motivation
Like most other fields of science, Einstein's theory of gravity, the general theory of relativity or GR, has developed its own notation and jargon. Despite the simplicity and economy of its underlying assumptions, the theory in full glory leads to int ensely complicated nonlinear equations. Indeed, the equations have been fully solved only in a few special instances. However, much of the mathematical complication can be removed by assuming that all gravitational fields are weak. The equations then r educe to a form remarkably similar to those governing electromagnetism. Terms appear that are analogous to the electric field caused by charges (the gravitoelectric field, produced by masses), and to the magnetic field produced by the flow of char ge (the gravitomagnetic field, produced by the flow of matter). A spinning ball of electrical charge produces a well-prescribed static magnetic field, and correspondingly a spinning mass such as the Earth is expected to produce a static gravitomag netic field. Of course, general relativity has important differences from electromagnetism, as well: in particular, it represents gravitational forces as arising from geometric curvature in the structure of space and time.
Gravity Probe B aspires to detect and measure, at the 1 percent level, the gravitomagnetic field produced by the spinning Earth through a spin-spin interaction with an orbiting gyroscope. This effect of the gravitomagnetic field is often referred to a s "frame dragging," or the Lense-Thirring effect. In addition, GP-B will accurately measure the much larger "geodetic" precession, a combination of the effects of spin-orbit coupling and space-time curvature.
In the quarter century since inception of the GP-B project, many other tests of Einstein's theory of gravity have been made. The delay and deflection of light signals passing close to massive objects have been measured with increasing precision and fo und to agree with the predictions of GR at the 0.1 percent level. Geodetic precession has been detected and measured with 2 percent accuracy by laser ranging to the Moon. Gravitational radiation from accelerated masses in a binary pulsar system has been shown to be consistent with GR at the 0.4 percent level. Some of these tests involve gravitomagnetic effects related to the translational flow of matter, in combination with other relativistic gravitational effects, and therefore they provide indirect e vidence for the existence of gravitomagnetism. By contrast, GP-B proposes to provide a direct test of gravitomagnetism caused by rotation, in isolation from other relativistic gravitational effects.
The past quarter century has also seen the development of exquisitely sensitive new instruments based on developing technologies and located both on Earth and in space. Some of them have provided the means to probe more and more deeply into the nature and evolutionary history of the universe. Observations with such instruments have yielded one surprise after another, and they raise perplexing questions about missing mass, the age of the universe, and the circumstances giving rise to the large-scale d istribution of matter in space. In the past, laws of nature previously considered sacrosanct have sometimes been found deficient when subjected to much closer scrutiny or applied to new phenomena. As long as some discoveries defy understanding, it is im portant to continue testing nature's most fundamental laws.
1.3.1 Scientific Importance
The frame-dragging effect predicted by our principal theory of space and time, general relativity, has a deep conceptual significance involving the connections between rotation, distant matter, and absolute space. Frame dragging is a direct manifestat ion of gravitomagnetism. Its consequences have found important astrophysical applications in, for example, models of relativistic jets observed streaming from the cores of quasars and active galactic nuclei. A 1 percent measurement of the predicted fram e-dragging effect would be a significant and unique test of GR. Gravity Probe B is one of the few space missions NASA has conducted with relevance to fundamental physics. If successful, it would assuredly join the ranks of the classical experiments of p hysics. By the same token, a confirmed result in disagreement with GR would be revolutionary.
Since GP-B was conceived, significant progress has been made through experimental studies of gravity, both in improved precision and in performing qualitatively new tests. These tests are so constraining that there are now no examples of alternative t heories that are consistent with the experimental facts and predict a frame-dragging effect different from that predicted by GR at a level GP-B could detect. Yet the basic weakness of the gravitational force means that GR has been tested much less thorou ghly than the other fundamental theories of physics. Nevertheless, along with most physicists this task group believes that a deviation from GR's prediction for frame dragging is highly unlikely.
In addition to detecting the new gravitomagnetic effect of frame dragging, Gravity Probe B should be able to measure the geodetic precession of its gyroscopes to an unprecedented accuracy of about 75 parts per million (ppm). This result would provide a factor-of-20 improvement in the measurement of space curvature per unit mass (now known to about 2 parts in 1000) and would tightly constrain the deviations from GR predicted by other theories of gravity in the weak-field limit.
1.3.2 Technical Feasibility
The task group is highly impressed with the extraordinary talents and abilities of the technical team assembled to create Gravity Probe B. The group has consistently solved technical problems with great inventiveness and ingenuity. Moreover, in the c ourse of its design work on GP-B the team has made brilliant and original contributions to basic physics and technology. Its members were among the first to measure the London moment of a spinning superconductor, the first to exploit the superconducting bag method for excluding magnetic flux, and the first to use a "porous plug" for confining superfluid helium without pressure buildup. They invented and proved the concept of a drag-free satellite, and most recently some members of the group have pioneer ed differential use of the Global Positioning System (GPS) to create a highly reliable and precise aircraft landing system.
The task group finds progress in construction of the actual GP-B apparatus to be very impressive, as well. Working in concert with a team from the Lockheed Missiles and Space Company, the Stanford group is well on its way toward putting GP-B into spac e before the end of the decade, providing that the funding level is sustained. The task group has found no serious technical impediments to meeting the existing launch schedule. The spacecraft, experimental package, and projected methods of operation ar e well designed to meet the scientific requirements and prove the results valid. The team is well prepared to cope with a wide range of unanticipated phenomena. The task group considers the overall complexity of GP-B to be somewhat greater than that of the Cosmic Background Explorer (COBE) but much less than that of the Hubble Space Telescope (HST). An ordinary hardware failure is no more likely than in other comparable space missions. Furthermore, GP-B has been designed with extensive in-flight testi ng of all parts, four independent sensor gyros to provide immediate confirmation of results, and in-flight calibration using observations of the aberration of light caused by the motion of the satellite.
Nevertheless, the extraordinary experimental requirements and the impossibility of ground tests of some critical systems at the necessary level of accuracy introduce significant risks. Despite an extensive list of detailed questions put to the GP-B te am by the task group, no specific weakness or likely points of failure have been identified. A majority of the task group believes that GP-B has a reasonably high probability of achieving its design goals and completing the planned measurements. However , based on their experience with complex scientific experiments on the ground, several members remain skeptical about the large extrapolations required from ground testing to performance in space. This minority believes it likely that some as yet unknown disturbance may prevent GP-B from performing as required. The task group notes that in any event, should the GP-B experiment be completed successfully but yield results different from those predicted by general relativity, the scientific world would alm ost certainly not be prepared to accept them until confirmed by a repeat mission using GP-B backup hardware, or by a new mission using different technology.
1.3.3 Comparison with Other Proposed Programs
The scientific objectives of GP-B involve testing one of the fundamental laws of nature. The goals are therefore quite different from the objectives of a common situation in which natural laws, as inferred theoretically and tested in terrestrial labor atories, are used to interpret observations of astrophysical phenomena. In particular, the ambitions of GP-B are qualitatively different from those underlying most astronomical work, including NASA projects such as the HST, the Stratospheric Observatory for Infrared Astronomy (SOFIA), the Space Infrared Telescope Facility (SIRTF), and the Advanced X-ray Astrophysics Facility (AXAF). Tests of nature's laws are the ultimate foundations of physical science and are the only rational basis for belief that th ese laws are, at least in part, "understood." Despite its omnipresence, gravity remains the least well tested of all the fundamental forces.
NASA's highly successful COBE satellite was designed primarily to answer certain astrophysical and cosmological questions. Nevertheless, its results have implications in fundamental physics as well, particularly for questions concerning the origin of the universe. The task group's considered judgment is that the most likely of successful outcomes of the GP-B experiment(the measurement and confirmation of two specific effects predicted by general relativity(will be an important milestone, but will hav e less impact on the scientific world than the cumulative results of COBE. The reason is simple: there is no serious alternative to the general theory of relativity that predicts effects differing from those of general relativity by amounts that GP-B co uld detect. The GP-B experiment has been exciting for many scientists because of the need for confirmation of gravitomagnetism and the possibility of a great surprise, but the latter chance now seems more remote than before.
Other proposed satellite tests of frame dragging or spatial curvature, such as LAGEOS III, are intrinsically an order of magnitude less precise than GP-B. Another proposal claiming to offer higher accuracy is now in the conceptual stage and might even tually become a worthy successor to GP-B. It is discussed briefly in Section 2.4.
NASA estimates that $340 M will be required to complete the construction, launch, and data analysis phases of GP-B. If the experiment delivers as promised, so that the frame-dragging effect is measured to 1 percent accuracy and the geodetic term to 75 ppm, is it worth the cost? This question must be viewed in the context of other NASA projects of comparable magnitude, and necessarily its answer involves subjective scientific judgments. The task group was not able to achieve a clear consensus on the question of competitive value, even after extensive discussion and deliberation. Its members agree unanimously that all scientists would find it appealing to see a clean and direct demonstration of the frame-dragging effect, and that a confirmed discrepa ncy between the result of the GP-B experiment and the prediction of general relativity would fully justify the mission's cost, including the additional expense of a confirming experiment. However, in light of existing tests of gravitation theories such a discrepancy is considered highly unlikely.
Consequently, the task group's members hold a range of opinions on the relative cost-effectiveness of GP-B. A significant minority judge that the purpose of the mission is too narrow in comparison with missions that explore wide-open scientific issues and have a high probability of making new discoveries. This minority assigns high weight to the fact that essentially all experts believe that gravitomagnetism must exist, and consequently it does not appear likely that unexpected new knowledge will be gained.
In contrast, the task group's majority judgment gives higher weight to the importance of experimental verification in GP-B's unique and direct test of general relativity. Considering also the possibility of a revolutionary discovery, however remote, t he majority judges the GP-B project well worth its remaining cost to completion.
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