Space Studies Board

Fundamental Physics and Chemistry: Relativistic Gravitation and Microgravity Science

A common link among traditional space, sciences such as space astronomy and astrophysics, planetary exploration, and solar plasma physics is their use of spacecraft for their observations. One of the objectives of this study was to determine whether there is likewise a potential to use space vehicles as laboratories in which fundamental physical and chemical laws might be investigated. The answer is decidedly positive. Spacecraft can provide a unique environment for at least two kinds of studies: those that would further our knowledge of relativistic gravitation and those exploring fundamental processes that require very small gravitational forces or very small gravitational gradients. The implications of using space vehicles for the study of general relativity have been understood for some time, and a specific strategy for investigations of relativistic gravitation from spacecraft after 1995 has been set forth here. On the other hand, the implications of exploiting the nearly gravity-free environment of space to study basic properties of matter have not been well delineated before, and the identification of opportunities in this realm is an important new achievement-one of the most exciting to emerge from this study.

General relativity relates the geometry of space and time to the distribution of matter in the universe. Gravitation is the consequence of the way this space-time geometry affects the movement of matter in space. As a theory, general relativity is well developed; it has important consequences that can be tested. There are three classical tests of general relativity in weak fields-such as those near the Sun or Earth. The first involves the precession of the perihelion of a solar system object such as the planet Mercury. The second utilizes the deflection of light passing close to the Sun. The third involves the gravitational red shift of spectral lines, which attests to the effect of a gravitational field on the rate of clocks. All of these effects can be measured with much greater precision in space than on the surface of the Earth, permitting more accurate predictions of the gravitational field strength.

Currently, we can verify the predicted deflection of a light ray grazing the limit of the Sun with about 2 percent uncertainty. But we could improve this by 2 orders of magnitude if we could make the measurement with an optical interferometer flown on the Shuttle. This instrument would consist of an articulated pair of stellar interferometers, having their viewing axes approximately 90 degrees apart. It would have two pairs of mirrors 25 cm in diameter and an interferometer length of 2 m. A free-flying spacecraft could improve even on this precision by providing longer exposure and more stable pointing.

The gravitational red shift is a consequence of the difference in the rate at which identical clocks measure time at different depths in a gravitational well. This effect has already been found to agree with the prediction of the theory of general relativity to within 1 part in 10⁴. The experiment consisted of measuring the rate of a hydrogen maser clock as it was carried to a height of 10,000 km on a rocket. However, a qualitatively different test of general relativity theory could be performed by carrying an improved hydrogen maser close to the Sun, where the red shift will be more pronounced since the clock will be deeper in the gravity well. Significant, variance of the measurements made there from the predictions of general relativity would cause a major rethinking of the theory.

There is another prediction of the general theory that has never been tested. This is a nonstatic effect, and it states that rotating bodies drag nearby inertial frames. Although the effect is exceedingly small in weak fields near solar system bodies, it might be enormous and astrophysically important near a rotating black hole. The relativity gyroscope experiment called Gravity Probe B has been devised to search specifically for the frame-dragging effect produced by the rotating Earth. It will use the most precise gyroscopes yet devised. This mission has been likened in importance to the classical Michelson-Morley ether drift experiment of 1887. The proof that there was no ether drift buttresses Einstein's special theory of relativity and has changed fundamental concepts of space and time., Although many times more sophisticated than any experiment yet attempted in space, there is considerable confidence that the Gravity Probe B mission will be successful. Gravity Probe B should be flown before 1995 unless the consequences of the Challenger accident delay it.

General relativity is based on a fundamental principle called the principle of equivalence. The principle asserts that the gravitational mass of an object, that is, the quantity that measures the gravitational force it produces, is identical to the mass that responds inertially to any force. In short, it states that gravitational and inertial masses are equal. The validity of the principle has been demonstrated to a level of one part in 10¹¹ in the famous Eotvos experiment. Shuttle flight of an experiment to test this equivalence at the level of one part in 10¹⁴ is proposed for the near term (before 1995). During the period covered by this study, a similar experiment flown in a free-flying spacecraft would provide a test to the level of one part in 10¹⁷.

Another important physical principle is called Mach's principle. It suggests that the expansion of the universe might cause the effective local value of the gravitational constant G to decrease with time as a consequence of the effect of distant mass on the inertial properties of local matter. Microwave ranging to a Mercury orbiter could improve our knowledge of the time rate of change of G by 3 orders of magnitude. A by-product of this experiment would establish the extent to which gravity is itself a source of gravitation.

In Newtonian theory gravitation propagates instantaneously over infinite space. The concept of waves is not applicable. In contrast, Einstein's general relativity requires gravitation to propagate with the speed of light, just as does electromagnetic radiation. Electromagnetic waves jiggle charged particles; gravitational waves accelerate mass. When traversing a large object, a gravitational wave will deform it. In the language of relativity, a gravitational wave ripples the curvature of space-time, deforming any mass that sits in space.

The detection of gravitational waves is one of the most challenging problems in experimental gravitation today. Observation of gravitational waves would open new astronomical windows. It would provide information about exotic sources of gravitational radiation: collapsing stellar cores, colliding neutron stars or black holes, decaying binary star systems, and rotating or vibrating neutron stars. In the meantime, the discovery of a radio pulsar in a binary system containing, most likely, another neutron star has provided very convincing evidence that gravitational waves do indeed exist. The orbit of this system is decaying almost exactly as expected if such waves were being emitted.

If the explosive events in quasars and other active galaxies are generated by black holes or supermassive black holes, each explosion must generate a great gravitational wave that rattles everything in the universe. On the other hand, the radiation produced by many astronomical interactions, such as that of a black hole with neighboring matter, is of a very low frequency—below 10 Hz. Its detection requires an observatory in space, free from interference by seismic noise. A gravitational wave detector consisting of three spacecraft orbiting the Sun, each one a million kilometers from the next and possessing a precise system for monitoring their separation by laser ranging, would allow a detection of gravitational waves from astronomical sources in the range of periods from 0.3 s to 10 days. Gravitational waves would cause the distance between these spacecraft to oscillate. The estimated sensitivity achievable with such a system is one part in 10²² for narrow-band periodic signals and as much as one part in 10²⁰ for transient pulses at megahertz frequencies. Such a detection system offers us our best chance of directly observing the radiation produced by distant matter accelerating in strong gravitational fields such as those produced by black holes.

Pulsars spinning with periods close to a millisecond approach relativistic instability; their surfaces move at close to the speed of light. The discovery of such objects could provide the frequency key to ground-based gravitational wave detectors in their search for gravitational wave radiation. The steering group recommends building a very large proportional-counter x-ray detector with a receiving area of about 100 m² that could be attached to the Space Station or orbit as a free flyer. This very large detector would search the sky for very fast x-ray pulsars.

In summary, the steering group anticipates that several space experiments prior to 1995 will advance our understanding of general relativity in weak fields and offer a possibility of detecting gravitational radiation. These are:

2. Microwave ranging to the Galileo-Jupiter mission to search for low-frequency gravitational waves;

3. Microwave ranging to the Mars Observer spacecraft to improve the accuracy of measurements of the gravitational red shift, and variation of G with time;

4. Shuttle flight of a cryogenic experiment to test the weak principle of equivalence to one part in 10¹⁴.

1. Laser Gravitational-wave Observatory in Space (LAGOS). This mission will attempt to detect gravitational radiation at frequencies below 10 Hz from space. The mission, as proposed, consists of an optical heterodyne interferometer system accurately measuring the separation of three spacecraft in orbit.

2. Mercury Relativity Satellite. An improved measurement of the time rate of change of the gravitational coupling constant such as could be obtained by microwave ranging to a spacecraft orbiting Mercury.

3. Precision Optical Interferometer in Space (POINTS). This instrument will provide a second-order test of the effect of the Sun on electromagnetic radiation.

4. STARPROBE. This experiment involves the flight of an accurate clock (hydrogen maser) on a spacecraft close to the Sun, allowing the measurement of the gravitational red shift to the second order.

5.Principle of Equivalence Experiment. This experiment will be mounted on a free-flying spacecraft and will test this principle to one part in 10¹⁷.

6. Large-Area X-ray Detector. The flight of such a detector with microsecond timing capability will allow detection of x-ray pulsars.

The successful implementation of this strategy should leave us with a very good understanding of the validity of the general theory of relativity in weak fields. It would also advance our knowledge of the behavior of matter in the neighborhood of objects such as black holes, where gravitational effects occur in fields far stronger than those hitherto observed.

The microgravity environment of a space platform may provide a useful arena for testing basic theories of matter and observing new processes and new states in matter. Gravitational fields cause nonuniformities in the distribution of matter in a given sample and can cause fragile structures to collapse. The spacecraft environment can provide a very low effective gravitational field that might provide protection from these effects. Under conditions of low gravity, we may enhance our understanding of nonequilibrium phenomena in fluid flow, and in condensation, combustion, and similar dynamic processes. Low-gravity conditions may also allow the development of static or dynamic states of matter that cannot exist in normal gravitational fields.

Three categories of investigations have been considered in these studies of states of equilibrium. The first deals with the case in which gravitational effects induce nonuniformity in the equilibrium state of a system, and thus prevent the observation of particular states of equilibrium, such as phase transitions near critical points. These states involve correlation lengths that are long compared with the distance over which uniformity in a system can be maintained in normal gravitational fields. The most well-known example is the continuous phase transition in liquid helium at its lambda point. Plans are well advanced to carry out an experiment investigating this phenomenon in space, where gravitational effects will be small enough to allow uniform temperature in an extended sample of liquid helium. This experiment should be completed before 1995.

Another category of investigation involves the study of stationary states of matter that gravity destroys rather than distorts. For example, there is the possibility that in low gravity, objects can develop so-called fractal aggregates. These structures may be so fragile that they can exist only in a microgravity environment. In another case, gravitational effects can interfere with the evolution of a precipitate because of flows induced by buoyancy or because of sedimentation. In a microgravity environment these effects could be avoided, and precipitation solely under the control of diffusion could be observed.

A third class of phenomena that can be observed only under conditions of low gravity are those that exhibit complex dynamical behavior as they are driven far from equilibrium. Plans have been formulated for studying examples of this sort of behavior on the Space Shuttle, including the combustion of clouds of particulates, or surface-tension-driven hydrodynamical flows. But the steering group believes that the possibilities for research in this field are greater than we now realize and that they may have important implications for biology. Many questions beckon for answers: Will a given process will be chaotic or not? Will spatial patterns formed be stable? What is the role of the gravitationally induced breaking of underlying symmetries, such as the front-to-back symmetry in flames?

In its treatment of microgravity science here, the steering group has concerned itself solely with basic scientific questions. Until these are answered, there does not seem to be any way to structure a rational program of materials processing in space. A basic research program of this sort is a necessary precondition to the development of an applied program. As a branch of space science, microgravity science is in its infancy. Thus, before we can gauge the prospects of the field over the next 20 years, we must know the results of preliminary experiments now being developed. If the nation hopes to attract expert scientific talent into the field, flight of these experiments merits very high priority.

In scheduling experiments for flight, NASA should make every effort to fly the best of the microgravity physics and chemistry experiments as soon as possible, and see to it that the results are rapidly published.
Spacecraft gravity levels and vibration spectra should be precisely measured, characterized, and displayed on those spacecraft carrying chemistry and physics experiments.
Strategies for producing the lowest possible gravity conditions should be considered at this time, since experiments dealing with long-range order are open-ended in their low-gravity needs.