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Microgravity Research Opportunities for the 1990s: Chapter 7 Microgravity Research Opportunities for the 1990s PART II—SCIENTIFIC ISSUES 7 Microgravity Physics INTRODUCTION The intersection between the special research possibilities offered by microgravity conditions and the general interest of the physics community is relatively small. This is understandable since gravitational forces play only the most minor role in the traditional areas of physics such as high-energy, nuclear, and condensed matter physics. Nevertheless, there are a number of problems of interest in which access to space and the microgravity environment may allow important research advances. REPORT MENU The subject of microgravity physics consists of many diverse topics united NOTICE by their common requirement for free-fall conditions. A similar situation holds for MEMBERSHIP low-temperature physics where the subject is united by the technical PREFACE EXECUTIVE SUMMARY requirements for low temperatures. The topics of microgravity physics may be PART I cast into three categories for the purposes of discussion. CHAPTER 1 CHAPTER 2 New instruments. The very low stress conditions possible in the PART II microgravity environment allow the construction of instruments of unique CHAPTER 3 sensitivity. Examples are the superconducting gyroscopes of the Gravity Probe-B CHAPTER 4 experiment and the mass balance of Everitt and Worden's equivalence principle CHAPTER 5 test.1 Access to space allows the construction of instruments on a length scale CHAPTER 6 CHAPTER 7 not limited by the size of the Earth, such as a long-baseline (106 km) gravitational PART III wave antenna. The freedom from the disturbance of microseisms that set the CHAPTER 8 noise floor for Earth-bound gravitational wave detectors is an important benefit of APPENDIX A the microgravity environment. APPENDIX B Unique samples. Under free-fall conditions, gravitationally induced compression effects are essentially absent and it is possible to create experimental samples with unprecedented uniformity. This possibility is file:///C|/SSB_old_web/mgoppch7.htm (1 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 particularly important in the area of critical phenomena. Relief from gravitational and vibrational stress will also be important for the preparation and study of delicate structures such as low-density granular materials near the percolation limit. New dynamics in fluids. Fluid systems will provide a rich area for the study of new dynamic phenomena that can be expected in the microgravity environment. The absence of buoyancy forces arising from nonuniform density distributions under free-fall conditions allows weaker forces to come into play and produce new dynamic behavior. Understanding the behavior of fluids in space is important not only from the point of view of fundamental science but also for the solution of practical engineering questions involved in spacecraft design. Fluids under microgravity conditions constitute an extensive subject that is discussed separately in this report. THE CURRENT MICROGRAVITY PHYSICS PROGRAM Two extensive reviews of the field of fundamental science in space2,3 discuss in considerable depth experiments currently being developed for flight in the near future and longer-term prospects for the field. It is useful to discuss a number of representative experiments to gain an appreciation of the motivations, promise, and difficulties involved in microgravity physics. Fundamental Physics Measurements Access to space and the microgravity environment will allow tests of several of the most fundamental aspects of physics. The most elemental of these is an experiment being developed at Stanford University4 to test the equivalence principle. This principle, which asserts the identity of inertial and gravitational mass or the impossibility of distinguishing within a system between acceleration and a gravitational field, lies at the heart of Einstein's theory of gravitation. The equivalence principle is so well imbedded in physics today that one tends to forget that tests of this principle have a long and honored history dating back to the experiments with falling bodies conducted by Galileo. In the 1960s, the equivalence principal was confirmed5 to 3 parts in 1011. The new experiment6 promises an eventual improvement on those results by some six or seven orders of magnitude. In the original version of the experiment, the acceleration due to Earth's motion in its orbit was compared to the gravitational attraction of the Sun, whereas the Stanford experiment will compare the gravitational attraction of Earth on an orbiting test mass to the acceleration associated with that orbit. The gravitational force on an object in low Earth orbit is three orders of magnitude greater than the attraction due to the Sun; hence, an immediate large gain in sensitivity is realized by going to space. A second advantage of the microgravity environment is the escape from microseismic disturbances that would limit an file:///C|/SSB_old_web/mgoppch7.htm (2 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 Earth-bound experiment. As currently developed, this experiment takes advantage of modern cryogenic technology with superconducting bearings for the confinement of the test mass and SQUID (Superconducting Quantum Interference Device) position detectors. A shuttle flight for this experiment would allow an important test of the operation of this apparatus and should allow an improvement in the test of the equivalence principle resolution to the level of 1 part in 1015. Given a successful shuttle flight, there would be a strong motivation to fly this experiment on an independent satellite where the microgravity environment could be substantially improved and the full promise of this experiment might be realized. A second experiment developed by the Stanford group is the very ambitious attempt to observe the "frame-dragging" or magnetogravitational effects predicted by Einstein's general theory of relativity. This experiment, which has been under development for more than 20 years, is known as Gravity Probe- B (GPB). It consists of four gyroscopes for sensing the frame-dragging effects that will appear as extremely small torques on the gyros. The spin axes of the gyros will then be compared with a fixed star. This experiment makes heavy use of advanced cryogenic technology with the utilization of superconducting gyros and superconducting readouts. The time scale envisioned for this experiment is on the order of a year. Although GPB can be given a shakedown test in a shuttle flight, an independent satellite will be required to satisfy the microgravity requirements of the experiment, which lie in the 10-10-g range. Critical Phenomena The study of critical phenomena has become prominent in condensed matter physics over the last 20 years. This period has seen remarkable advances on both the experimental and the theoretical fronts, in particular with the high- resolution experiments at the l-point transition of liquid 4He and with the development and successful application of renormalization group techniques to the calculation of the behavior of many systems in the neighborhood of their critical transitions. The subject of critical phenomena presents a very promising area for microgravity research. The essential advantage of microgravity for studies of critical phenomena lies in the possibility of achieving relief from the gravity- induced compression effects always present in the Earth-bound laboratory. These compression effects are particularly severe in liquid-vapor critical point studies because the compressibility of the system diverges as the critical point is approached. Near the transition, the gradient in the hydrostatic pressure leads to a large density gradient in the sample. Thus, only a diminishing portion of the sample will satisfy the condition for critical density as the transition is approached. The situation with the superfluid transition in liquid 4He is somewhat more favorable since the order parameter does not couple strongly to the density. Nevertheless, on Earth, gravity-induced hydrostatic pressure leads to a microkelvin shift in transition temperature for each centimeter of sample height. file:///C|/SSB_old_web/mgoppch7.htm (3 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 Gravity effects can be minimized, to some degree, by a reduction in the height of the sample, but this strategy soon runs into sensitivity problems related to sample size, and on a more fundamental level, problems associated with surface and finite-size effects set limits to sample thickness. The -point in liquid helium has been a paradigm for critical phenomena studies. Liquid helium offers a number of properties that make it an almost ideal subject for such studies.7,8 It is an exceptionally pure system. In the superfluid state, helium possesses a very rapid relaxation time, and compared to the liquid- vapor transition the superfluid transition is relatively insensitive to gravitational effects. The use of samples less than 1 mm thick has allowed useful data to be obtained at reduced temperatures, t = 1 - T/T , less than 10-7. Modern advances in low-temperature thermometry would allow at least a two-order-of- magnitude improvement in experimental resolution. Motivated by the central position of the superfluid -point transition in the critical phenomena field, Lipa and Chui at Stanford University have developed a microgravity version of the -point heat capacity experiment.9 In the development of this experiment, they have advanced the state of the art in high- resolution thermometry to the point that they can resolve temperature to 1 part in 1011. Microgravity conditions are required to fully exploit this superb resolution. In space, the heat capacity measurements can be extended two orders of magnitude to the 10-9 level in reduced temperature beyond the best results achieved in ground-based measurements. The Lambda Point Experiment (LPE) was flown on the shuttle in October 1992 (USMP-1). Analysis of the data indicates an improvement of nearly two orders of magnitude over previous data obtained on Earth. Unfortunately, the final analysis was delayed by several factors. The first was the failure of NASA to provide the investigators with the full data set on an expeditious time scale. At least four months passed after the touchdown of USMP-1 before a complete record of the raw experimental data became available for analysis. A second factor was the discovery, as is often the case during innovative research, of an unanticipated experimental problem. The cosmic-ray background encountered in orbit was found to produce stochastic thermal spikes in the response of the high- resolution thermometers, resulting in a somewhat degraded performance. The principal investigator has developed an algorithm for combing the data for the worst of these spikes. This was done in an unbiased way, and heat capacity data approaching within a few nanokelvin of the -point have been obtained. The LPE has been an outstanding success. This experiment gives a clear demonstration that highly sophisticated experiments involving the most sensitive and advanced instrumentation can be performed profitably in the microgravity environment. A second critical point experiment is also in an advanced state of preparation. This is the light scattering experiment developed for measurement of file:///C|/SSB_old_web/mgoppch7.htm (4 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 the correlation length at the liquid-vapor critical point of xenon.10 This experiment should be able to test the theory of density fluctuations in the nonhydrodynamic limit much closer to the transition than would be possible on Earth. It has been expected that the phenomenon of critical slowing (i.e., the divergence of the time scale for thermal equilibrium as the transition is approached) will be an important experimental constraint. This would be especially true for an experiment conducted on the shuttle, where there will be stringent time constraints. There are indications, however, that there may be an actual speeding up of equilibrium near the transition.11 If true, then the feasibility of liquid-vapor critical point experiments will be much enhanced. FUTURE EXPERIMENTS UNDER MICROGRAVITY CONDITIONS Critical phenomena, particularly studies at low temperature, have occupied a central position in the current program of microgravity research. This is due to the advanced level of experimental and theoretical sophistication achieved in this area of condensed matter physics as a consequence of the vigorous progress of the last two decades. It is expected that the future will see continued experimental advances from the marriage of modern high-resolution measurement techniques with the vastly improved sample uniformity possible under microgravity conditions. New directions are possible for the study of critical phenomena in space. Up to this time the emphasis has been on measurement of static properties, particularly the specific heat in the neighborhood of the critical transition. For the future, several new directions are possible that represent important extensions beyond the successful completion of LPE. Chief among these will be studies of systems far from equilibrium, nonequilibrium dynamic phenomena, and studies of finite size effects in well-characterized geometry. Again, liquid helium will be the system of choice, given the large body of experience and its experimental advantages. Enthusiasm for further work with liquid helium in space will be strongly enhanced by success with LPE. Although low-temperature critical phenomena experiments are the most advanced and tend to drive the field, there are many possibilities for studies of critical phenomena in other systems that can benefit from microgravity conditions. Wetting phenomena, nucleation and growth, and the critical behavior in microemulsions and polymer systems, as well as the behavior of fractal aggregate systems near their percolation threshold, hold considerable potential for microgravity research. Beyond critical phenomena, there are interesting studies to be made on the static and dynamical properties of low-density aggregate structures and suspensions. Freedom from sedimentation effects will be most important in such work. One interesting aspect of low-density aggregate structures is their fractal file:///C|/SSB_old_web/mgoppch7.htm (5 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 nature. On Earth, there is a minimum to the density of an aggregate structure that can be stable since such structures tend to collapse of their own weight, because their strength decreases more rapidly with decreasing density than does the gravitational stress. Atom trapping and laser cooling techniques have been advanced in recent years to the point where atomic transitions can be studied under conditions of free-fall. In "atomic fountain" experiments, groups of ultracold atoms are projected upward into a microwave cavity. The dwell time for these atoms, under free-fall conditions, is limited by the size of the cavity and Earth's gravitational field. Current experimental configurations allow observation times as long as 0.5 s and should allow the development of an atomic clock with the remarkable accuracy of 1 part in 1016. This type of experiment is a clear candidate for the microgravity environment, where the experimental observation time could be greatly extended with a further major increase in clock accuracy. The dynamics of crystal growth is another subject that can profit from microgravity conditions. To date, there has been considerable interest in the possibility of improved crystal size and quality with growth under microgravity conditions where convection is suppressed. Another aspect to the crystal growth problem is the influence of gravity on the facet size and ultimate equilibrium shape of the crystal. These questions could probably be best addressed through studies of growth of free-floating helium crystals under microgravity conditions.12- 14 Helium crystals are particularly good candidates for microgravity research because they can be rapidly grown, melted, and annealed. Surface relaxation is so rapid that melting-freezing waves are easily created. The roughening transition, readily observed in helium crystals, makes their shape strongly influenced by gravity.15 A large reduction in gravitational influences would facilitate studies of this unique type of phase transition. RECOMMENDATIONS AND CONCLUSIONS The experiments mentioned above, with the exception of the GPB, all represent extensions of work that is currently performed or can be performed in Earth-based laboratories. Much of the scientific value of the proposed space experiments will depend on the strength of the connection to Earth-bound research. Given the long time scale for the development through flight of a space experiment, there is a real danger that the scientific goals of the experiment may be bypassed by new developments or by major shifts in the value ascribed to the work. There is also the possibility that the principal investigator (PI) may lose contact with the field. This is particularly likely if the PI becomes heavily involved with the development of flight hardware for a long period of time. These considerations argue for considerable support of ground-based research in the same general area as the candidate flight experiments. Also, it is desirable to keep approved projects under periodic scientific review throughout the life of the experiment. file:///C|/SSB_old_web/mgoppch7.htm (6 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 The quality of the microgravity environment is a topic that must be examined critically in the context of each possible experiment. Minimum acceleration is the most obvious parameter of concern for many of the contemplated experiments; however, the time span over which a high-quality, low- gravity environment can be maintained may be of equal importance. For some experiments, accidental large accelerations, such as might result from the sudden movement of personnel or the firing of small thrustors, may destroy the object of study, for example, a low-density granular structure. When considering the quality of the microgravity environment available for experiment, one should remember that only the center of the orbiting spacecraft is in true free-fall and then only to the extent that orbital drag effects and other external influences such as solar wind and radiation are negligible. In a gravity gradient-stabilized spacecraft, there will be a steady rotation of any experiment about the center of mass of the entire spacecraft once each orbit. For a low Earth orbit, this will result in accelerations on the 10-7-g level at a distance 1 meter from the center of mass of the entire orbiting system. On both the space shuttle and the space station only a few experiments will be located close enough to the center of mass to ensure acceleration below the 10-6 level. It can be expected that a number of scientifically meritorious projects, such as the equivalence principle experiment and GPB, will require spaceflight independent of the crewed space facilities. In the future, one may anticipate a continued requirement for low- temperature facilities in space since low temperatures are important for the highest-resolution measurement techniques, particularly those based on SQUID technology. If the space station is to be a useful contributor to the area of fundamental science, access to liquid-helium facilities will be mandatory. REFERENCES 1. Hazelrigg, G.A., and J.M. Reynolds, eds. 1986. Opportunities for Academic Research in a Low-Gravity Environment. Progress in Astronautics and Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New York. 2. Hazelrigg, G.A., and J.M. Reynolds, eds. 1986. Opportunities for Academic Research in a Low-Gravity Environment. Progress in Astronautics and Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New York. 3. National Research Council. 1988. Space Science in the Twenty-First file:///C|/SSB_old_web/mgoppch7.htm (7 of 9) [6/18/2004 11:18:04 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 7 Century: Imperatives for the Decades 1995 to 2015. Fundamental Physics and Chemistry. National Academy Press, Washington, D.C. 4. Hazelrigg, G.A., and J. M. Reynolds, eds. 1986. Opportunities for Academic Research in a Low-Gravity Environment. Progress in Astronautics and Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New York. 5. Roll, P.G., R. Krotkov, and R.H. Dicke. 1964. Ann. Phys., 26:442. 6. Hazelrigg, G.A., and J.M. Reynolds, eds. 1986. Opportunities for Academic Research in a Low-Gravity Environment. Progress in Astronautics and Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New York. 7. Ahlers, G. 1976. Physics of Liquid and Solid Helium, Vol. I. K.H. Bennemann and J.B. Ketterson, eds. John Wiley & Sons, New York. 8. Wilson, K.G. 1971. Renormalization group and critical phenomena. I. Renormalization group and the Kadanoff scaling picture. Phys. Rev. B, 21:3174- 3183. 9. Hazelrigg, G.A., and J.M. Reynolds, eds. 1986. Opportunities for Academic Research in a Low-Gravity Environment. Progress in Astronautics and Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New York. 10. Bourkari, H., M.E. Briggs, J.N Shaumeyer, and R.W. Gammon. 1990. Phys. Rev. Lett., 65:2654. 11. Bourkari, H., M.E. Briggs, J.N. Shaumeyer, and R.W. Gammon. 1990. Phys. Rev. Lett., 65:2654. 12. Andreev, A.F., and A.Y. Parshin. 1978. Sov. Phys. JETP, 48:776. 13. Keshishev, K.O., A.Y. Parshin, and A. Babkin. 1979. Experimental detection of crystallization waves in HeSUP(4). Sov. Phys. JETP Lett., 30(1):56- 59. 14. Lipson, S.G., and E. Polturak. 1987. The surface of helium crystals. Pp. 127-188 in Progress in Low Temperature Physics, Vol. XI. D.F. Brewer, ed. North-Holland, Amsterdam. 15. Keshishev, K.O., and A.Y. Parshin. 1979. Sov. Phys. JETP Lett., 12:56. file:///C|/SSB_old_web/mgoppch7.htm (8 of 9) [6/18/2004 11:18:04 AM]