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4 Fundamental Physics Research Program INTRODUCTION Through its Microgravity Research Division recently renamed the Physical Sciences Division (PSD) NASA has supported research in fundamental physics for over two decades. Initially the program focused on low-temperature and condensed-matter physics, particularly on critical phenomena and phase transition studies, as these fields stood to benefit from access to the weightlessness of space. Over the last decade, the program was broadened to include research in the fields of laser cooling and atomic physics, gravitational and relativistic physics, and biological physics. The Fundamental Physics Disciplinary Working Groups recently published a report outlining and advocating the current and future programs of fundamental physics, Fundamental Physics in Space, a Roadmap to Unlock Myster- ies of the Universe by Exploring the Frontiers of Physics in Space (Bigelow, 2001~. This brochure refers to the two goals that have guided funding selections by the fundamental physics program: To discover and explore fundamental physical laws governing matter, space and time; and to discover and understand organizing principles of nature from which structure and complexity emerge. The funda- mental physics branch of the PSD now stands as a key funding agency for scientists whose research would be clarified in the absence of Earth's gravity, many of whom work in the areas of low-tempera- ture and phase-transition physics, cold atom physics, and gravitational physics. Thus the unique portfo- lio of NASA complements the broader physics research programs that exist within NSF, DOE, and other federal agencies. Review of the Current Program Because of the complexity of the experiments, the long lead time for flight preparation, and the limited number of space shuttle flights, only four flight experiments, all in the area of low-temperature Discipline working groups are internal advisory committees to NASA; they are composed of academic and industrial scientists who represent the interests of the research communities that utilize NASA's microgravity research platforms. 40
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FUNDAMENTAL PHYSICS RESEARCH PROGRAM 41 and condensed-matter physics, have been completed. For this reason, the program is reviewed here by experiment rather than by subdiscipline, as is done in the other chapters. Currently about half a dozen flight experiments, in low-temperature and condensed-matter physics, in laser cooling and atomic physics, and in gravitational and relativistic physics, are being prepared for flight onboard the ISS. In addition, the fundamental physics program also provides support for ground-based projects that comple- ment flight projects or have the potential to become flight projects. The duration of the ground-based projects is typically 4 years. Currently approximately 42 ground-based projects are supported by NAsAlfundamental physics. The committee first identifies the flight experiments that have taken place, with consideration of their scientific and technical impact being deferred to the next section. Since there are approximately 40 scientifically distinct ground-based experiments, it would be impractical to discuss each one. The committee therefore comments on just a representative few. While some of the impacts of these projects extend well beyond NASA, others set the stage for the flight experiments that should go onboard the ISS. Completed Flight Experiments Four flight experiments sponsored by the fundamental physics program have flown on space shuttles: the lambda point experiment (LPE), the critical fluid light scattering experiment (ZENO), the critical viscosity experiment (CVX), and the confined helium experiment (CHeX). These experiments aimed to provide stringent tests for the current understanding of the nature of a continuous phase transition. The smearing effect due to gravitationally induced density stratification is greatly reduced by carrying out these measurements in space rather than on Earth, allowing the critical point or the precise transition temperature to be approached much more closely. For example, because of Earth's gravity, the singular heat capacity related to the superfluid transition becomes rounded when the temperature of the labora- tory sample cell approaches to within 1 microkelvin of the transition temperature, the Lambda point. The results of these highly successful experiments are presented in the section below entitled "Low- Temperature and Critical-Point Physics." The ZENO and CVX experiments were concerned with how a fluid system (especially xenon) extremely close to its critical point responds to perturbations. By removing the smearing effect of gravity, CVX was able to cleanly demonstrate viscoelasticity in Xenon near its critical point, where such effects are magnified by collective behavior. CHeX made use of much of the hardware recovered from LPE and studied the effect of two-dimensional planar boundaries on the singular heat capacity near the Lambda point to confirm the prediction of the effect of finite sample size. Flight Experiments Being Prepared for the ISS While the completed flight experiments in fundamental ohvsics are exclusively in the area of low- 1 ~ 1 1 ~ ~ temperature and condensed-matter physics, fundamental physics projects designated for flight onboard the ISS also include experiments in laser cooling and atomic physics and in gravitational and relativistic physics. Approximately 35 experiments are scheduled for flight on the ISS or are being designed and prepared for future flight on the ISS or another carrier. Among the flight-approved experiments are the following: · Critical dynamics in microgravity (DYNAMX), · Heat capacity at constant heat current (CQj,
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42 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Boundary effect near the superfluid transition (BEST), · Microgravity scaling theory experiment (MISTE), · Superconducting microwave oscillator (SUMO), · Rubidium atomic clock experiment (RACE), · Primary atomic reference clock in space (PARCS), · Gravity Probe B. and · Satellite test of equivalence principle (STEP) (tentative). The first four of these experiments have been chosen and already passed flight definition through rigorous and multilayer peer review processes. SUMO has passed science concept review anct Is an approved ISS experiment. RACE and PARCS have been designated as future flight experiments and are currently supported as preflight ground-based research. The first five of these experiments will make use of the low-temperature microgravity physics facility on the ISS. Having passed flight definition review, the first four are basically ready for launch in the next 2 years. SUMO is scheduled for requirements definition review in early 2004. DYNAMX and BEST measure thermal conductivity very near the superfluid transition point to understand the dynamical response near a critical point both for a bulk sample (DYNAMX) and for a finite size sample (BEST). MISTE seeks to critically examine current understanding of the renormalization group theory as it is applied to the liquid-vapor critical point. This will nicely complement the findings of the LPE. Interestingly, DYNAMX and CQ can be accomplished with the same apparatus. Results of the first four experiments, taken together with those of the LPE and CheX experiments, are expected to provide a full picture of the equilibrium behavior of systems near critical points, including the role of boundaries and the dynamical response to perturba- tions. Ground-Based Experiments Ground-based experiments are chosen to complement the flight projects, and some may evolve as potential flight projects in the future. As noted earlier, NASA/fundamental physics currently supports about 42 ground-based experiments on very diverse topics. This chapter comments on just a few of them: · The measurement of Casimir-like effects near the superfluid transition and the 3He/4He tri- critical point showed a direct analogy between forces due to the cutting off of long-wavelength excita- tion modes associated with thermal rather than quantum electrodynamic fluctuations. · A second experiment demonstrated the possibility of operating in space a superfluid gyroscope that makes use of superfluid helium's ability to detect absolute rotational motion. · Nucleation and growth of helium crystals from superfluid helium: Since superfluid helium carries no entropy and is a perfect heat conductor, the dynamics of crystal growth in such a system are not hindered by latent heat effects. In fact melting/freezing waves have been seen. Using microgravity to suppress gravitationally induced gradients, such a system is ideal for addressing a number of funda- mental questions about crystal growth. · A paramagnetic material can be levitated in an inhomogeneous magnetic field (i.e., with a magnetic field gradient). At least two such facilities are supported by the fundamental physics program, one at NASA's Jet Propulsion Laboratory and the other at Brown University, and are being built to simulate a zero-gravity environment. Because of the small region that is available and the presence of a strong magnetic field and field gradient, such an experimental configuration will not replace the
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FUNDAMENTAL PHYSICS RESEARCH PROGRAM 43 microgravity environment that is available on the ISS or other spacecraft. However, such facilities should be useful for testing components of some of the flight projects or the ideas behind them. · Currently the low-temperature microgravity physics facility, to be installed on the ISS, can achieve a temperature of 1.5 K. This facility makes it possible and convenient to carry out a number of the scheduled flight experiments such as DYNAMX, BEST, CQ, MISTE, and SUMO. However, many other experiments, for example astrophysical experiments that require sensitive microwave and far- infrared detectors and bolometers, will require lower temperatures. One ground-based project is de- voted to the building of a He3 and He4 dilution refrigerator that can reach a temperature below 0.1 K. · Another ground-based program focuses on laser frequency stabilization and development of active antivibration techniques that will ensure optimum performance in the ISS environment. Unex- pected advances have broadened this program to include absolute optical frequency measurement via a new and powerful method, as described in the section below entitled "Optical Frequency Measure- ment." Some of the other notable ground-based projects that may lead to future flight studies include the search for an electron electric dipole moment with laser-cooled atoms in space, the demonstration of atom interferometry for detecting acceleration and rotation, study of the feasibility of forming and using Bose-Einstein condensates in space, study of the feasibility of space-based lunar laser ranging, and development of ultrastable laser sources for space science experiments and deep-space communication. Biological Physics Initiatives Fundamental physics has taken a leading role in initiating a biological physics effort within the Physical Sciences Division. In the 2000 NRA exercise, six ground-based projects in biological physics were selected for support. Projects in this area include a microscale mixer for protein folding, biomimetic self-assembly of mesostructures in microgravity, and microfabrication of a cell-based estrogen sensor. Support of these projects seems likely to increase the awareness on the part of the biological physics community of NASA's interest in biological physics. IMPACT OF NASA'S RESEARCH IN FUNDAMENTAL PHYSICS One way to discuss the significance of NASA's funding in fundamental physics is to consider the field's publication record. In 1998, NASA/fundamental physics is credited partly or exclusively with sponsoring work resulting in the publication of 4 papers in Nature, 2 papers in Science, and 12 papers in Physical Review Letters. In 1999, the numbers were 2 in Science, 3 in Nature, and 18 in Physical Review Letters. In 2000, the numbers were 1 in Nature, 2 in Science, and 16 in Physical Review Letters. It should be noted that Nature and Science are generally acknowledged as the two premier journals that cover all areas of science, while Physical Review Letters is one of the leading journals in physics. This record of publications in these premier journals speaks well for the impact and quality of science supported by NASA/fundamental physics. Another indication of the success of the fundamental physics program's competitive selection process is the scientific stature of the people involved. Currently six Nobel laureates are supported by and actively involved in the research activities of the fundamental physics program in both flight and ground-based projects. At least nine principal investigators are members of the National Academy of Sciences, and 25 are fellows of the American Physical Society. For the purpose of comparison, the entire fundamental physics program at the time of its expansion consisted of 64 principal investigators.
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44 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA To give an indication of the significant advances in ground-based fundamental science supported (usually only in part) by NASA's fundamental physics program, the committee now looks at just a few examples of the remarkable success evident over the last half-dozen years or so. Low-Temperature and Critical-Point Physics Near a critical point, a homogeneous fluid sample is predicted to show an infinitely sharp peak in a plot of some physical quantity such as specific heat as a function of temperature. In the laboratory, because of Earth's gravity, a fluid sample is inhomogeneously loaded by the weight of the overhead material. This obscures the specific heat or density fluctuation measurement and produces an important broadening when the temperature of the laboratory sample cell closely approaches the transition tem- perature to within 1 microkelvin. Four experiments utilizing a microgravity environment were spon- sored by the fundamental physics program and flown on space shuttles: the LPE, ZENO, CVX, and CHeX. These experiments aimed to provide stringent tests of the current understanding of the nature of a continuous phase transition. In orbit, the heat capacity measured in the LPE was found to remain sharp to within 1 nanokelvin (1 billionth of 1 K) of the lambda point. The shape of the peak was found to be in exact agreement with the prediction of the renormalization group theory of the critical point for , , · , ~ r ~ TO , ~ ~ · ~ ~ · , ~ r ~ TO ~ , ~ ·, · ~ · , AT · , ~ ~ ~~ ~N temperature Intervals from 1 K to 1 billionth of 1 K around the critical point (Papa et al., limo). Renormalization group theory, as elucidated by Nobel laureate Kenneth Wilson, is applicable to all physical systems undergoing a continuous phase transformation. The LPE succeeded in making the renormalization group theory one of the most stringently tested theories in physics and brings confi- dence in our approach and in our ability to understand the organizing principles of seemingly complex systems. A valuable additional result of the LPE and CHeX was the development and refinement of very- high-resolution magnetic thermometry, which now reaches a resolution near 10-~ K in a 1-Hz band- width. This result has led to the adoption of this technology for a number of other experiments, including both flights and ground-based NASA projects. In addition to refining and deepening our understanding of phase transition phenomena, this set of flight experiments also demonstrated that it is possible to build highly sophisticated experiments that can survive launch and operate for extended intervals at cryogenic temperatures in space. The success in recycling much of the hardware of the LPE for CHeX provided the impetus for and confidence in establishing the low-temperature microgravity physics facility on the ISS as a platform for accommodating future flight experiments requiring cryo- genic temperatures. Optical Frequency Measurement One of the unanticipated successes achieved under NASA sponsorship was the recent and rapid development of revolutionary techniques for the measurement of optical frequencies. As recently as 7 years ago a top-rated German team had made the most accurate measurement of an optical frequency, that of a laser stabilized to a particularly suitable resonance line in atomic calcium vapor (Schnatz et al., 1996~. Their method was the traditional frequency-multiplier method, which employs repeating multi- plier stages to span the huge factor of ~5,000,000 between the microwave standard frequency and the 2The flight experiments, as planned for the ISS, are the DYNAMX, MISTE, BEST, and SUMO.
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FUNDAMENTAL PHYSICS RESEARCH PROGRAM 45 optical frequency system of interest. Altogether some 17 stabilized frequency sources were used as intermediaries, with more than a dozen frequency-stabilizing and frequency-counting systems. This tour de force produced the most accurate measurement of an optical frequency up to that time, with an uncertainty of less than 1 x 10-~2. (It should perhaps be mentioned that the five authors of this report [Schnatz et al., 19961 were adding onto a facility that had been under development and evolving since the early 1970s.) Considering that such national frequency measurement teams are maintained by the United States, Canada, the United Kingdom, Germany, France, Japan, and Russia, one sees that this standards work actually represents a serious scale of effort. However, in 2000 and due in part to NASA funding, it became possible to measure optical frequencies with even greater accuracy using a transpar- ent and simple new approach based on femtosecond lasers, disciplined with high-precision optical phase control systems (these were the subject of the NASA funding). It seems a very unlikely linkup of technologies from the physics domains of the most slowly varying and the most rapidly varying: ultrastable lasers and ultrafast pulse lasers. This unexpected convergence and mutual relevance now has dramatically reduced the physical scale required for successful optical frequency measurement. An- other important attribute is that the time between the decision to measure a completely fresh laser and the first results is now measured not in years but in hours, and there is a total generality for the new laser's frequency, across a full octave of wavelengths. This new capability comes from the joint efforts of contributors around the world (Reichert et al., 1999~. Indeed, the announcement paper (Diddams et al., 2000) carried the names of authors at several contributing laboratories, but in fact this published work came from a U.S. laboratory where NASA was sponsoring the development of laser frequency stabilization and measurement techniques for continuous lasers. The applicability of these techniques to pulsed lasers was completely unanticipated. In thinking about the impact of NASA' s research, it is important to recognize that this revolutionary development came about in the United States mainly because of the NASA connection, and built on the support for and on the intellectual excitement about the vastly higher sensitivities that would be avail- able for future missions if stable lasers and frequency metrology tools could be developed. Indeed, a number of proposed new space experiments in the fundamental physics area are based on these develop- ments and related techniques; they are discussed in the second half of the next section. Several future missions will use space-based interferometers. For example, LISA the multina- tional Laser Interferometer (gravitational wave) Space Antenna will use 5 million-km baselines. The Space Interferometry Mission (SIM) will use various specialized interferometers designed to detect any possible local curvature of our space. Space Technology 3 (ST3) and Terrestrial Planet Finder (TPF) are missions to be carried out with separated-spacecraft interferometry, which will require improved control and stability. The needed laser frequency stability (better than 1 x 1o-~3 at 1 second) has become available due to current NASA-funded ground-based research. NASA's research is also enabling better distance metrology in space. For example, in the gravity recovery and climate experiment (GRACE), the along-track separation between two spacecraft is being measured using microwave techniques to map Earth's gravity field. A follow-on program, EX-5, will use a drag-free spacecraft and laser-based distance tracking to improve by 10-fold the sensitivity to changes in Earth's gravity field (compared with GRACE). Even people outside the scientific and technical community have heard about the breakthrough known as the Bose-Einstein condensation (BEC). While BEC surely was not one of NASA's highest priorities, the selection process for deciding on w. hich investigators w. ill be supported is a Rood one: It 1 1 0 0 1 1 0 Is competitive, unbiased, and open to a certain degree of scenic r~sk-tak~ng. lnvest~gators In BL;(: are now contributing to the microgravity program, having received partial NASA funding. Each has a unique vision for important low-temperature experiments, which in the case of BEC-related research
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46 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA may take place in the temperature range near 10 nanokelvins. One NASA investigator is using cold atom physics to produce molecules (Wynar et al., 2000) that are born basically at rest, without thermal motion. It is expected that the revolutionary insights and capabilities provided by BEC for the larger scientific community will lead to very important follow-on breakthroughs. FUTURE DIRECTIONS IN FUNDAMENTAL PHYSICS Any snapshot of a program likely to last a decade or more will show some overlap of future and present. Before it presents completely new directions, the committee repeats its list of flight experi- ments being prepared for the ISS or another carrier i.e., approved flight experiments: · Critical dynamics in microgravity (DYNAMX), · Heat capacity at constant heat current (CQj, · Boundary effect near the superfluid transition (BEST), · Microgravity scaling theory experiment (MISTE), · Superconducting microwave oscillator (SUMO), · Rubidium atomic clock experiment (RACE), · Primary atomic reference clock in space (PARCS), · Gravity Probe B. and · Satellite Test of Equivalence Principle (STEP) (tentative). These choices have been repeatedly evaluated and are heartily endorsed by the committee. The first four, which have already passed the flight definition peer review processes, are discussed earlier in this chapter. SUMO, RACE, and PARC S have been designated as flight experiments. Gravity Probe B may be the most difficult of these experiments, and ground research has been under way for many years. While STEP has been funded by NASA/fundamental physics, the Medium-Size Explorer (MIDEX) competition will soon decide whether it will be approved for flight. The first four experiments will make use of the low-temperature microgravity physics facility on the ISS and, having passed flight definition review, are basically ready for launch in the next 2 to 3 years. DYNAMX and BEST measure the thermal conductivity very near the superfluid transition point to understand the dynamical response near a critical point for a bulk sample (DYNAMX) and a finite-size sample (BEST). MISTE seeks to critically examine the renormalization group theory as it is applied to the liquid-vapor critical point. These will complement nicely the findings of the LPE. Interestingly, the DYNAMX and CQ can be accomplished with the same apparatus. The completion of these four experiments, taken together with the LPE and CheX experiments, is expected to provide a full picture of the equilibrium behavior of systems near critical points, including the role of boundaries and the dynamical response to perturba- tions. The committee believes the series of scheduled flight experiments will elucidate (1) the nature of the dynamical response and fluctuation for bulk and finite-size samples (DYNAMX, BEST, CQ) and (2) the concept of universality classes (MISTE) near a critical point. One can make a strong case that with the successful completion of these flight projects, critical phenomena in the microgravity environment will be a mature subject. Thus, barring unexpected developments, if these follow-on experiments continue the great success of the already-flown program, they paradoxically may form a natural conclu- sion to this highly successful line of spaceborne measurements. The frequency of SUMO is fixed by means of a resonance of an electromagnetic wave in a low-loss microwave superconducting cavity. The frequency of this system, thus depending on material dimen- sions and the speed of light, has its stability enhanced in a low-gravity, low-vibration environment. It
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FUNDAMENTAL PHYSICS RESEARCH PROGRAM 47 will make use of the low-temperature microgravity physics facility, as did the previous four experi- ments. Special bidirectional links to ground stations can provide accurate Doppler cancellation and thus enable high-precision frequency measurements relative to ground-based atomic frequency standards. Making measurements also with the onboard atomic clock ensemble will lead to an enhanced test of the general relativistic red shift (see below). By contrast, RACE and PARCS are two flight experiments that will make use of space to substan- tially improve the precision and stability of atomic clocks relative to those on Earth. These clocks are based on hyperfine transitions of electrons in atoms such as cesium and rubidium and have been developed to a high degree of performance over the past 35 years. In recent years the performance limitations set by the atoms' thermal motion have been greatly reduced by the use of laser atom-cooling techniques. Now the limit for such an atomic fountain clock becomes the finite time available before the atoms fall back down because of Earth's gravitational pull. These cold-atom fountain clocks, the most precise atomic clocks on Earth, have an accuracy of 1 part in 10~5 (or an error of 30 billionths of a second per year). However in space PARCS and RACE systems are poised to improve the performance of cold- atom clocks by a factor of 10 or even 100 (RACE). Such improvement is possible because in space the laser-cooled atoms will not be subjected to the influence of gravity and will not "fall" relative to the measuring apparatus. The increased atom interaction time and thus improved precision available on the ISS will make it possible to address a number of fundamental and interesting questions about the nature of our physical world. The impacts of these improved space clocks on science and technology will probably be wide and deep but difficult to fully anticipate and appreciate at this time. The key to maximizing the scientific gain from the space clock experiments PARC S and RACE is to fly simultaneously another type of high-performance clock that can serve as a comparison local oscillator. An excellent candidate for this second type of clock is SUMO, discussed above. By comparing its microwave cavity frequency (based on its cavity geometry) with that of the local atomic clocks, it will be possible to measure as a function of position and gravitational potential the gravi- tational redshift. These measurements will be a powerful test of Einstein's weak equivalence principle, which states that the rates of clocks are independent of their composition and have the same change with gravitational potential. A European atomic clock experiment on the ISS, called PHARO, will enhance and be enhanced by the three U.S. clock experiments, PARCS, RACE, and SUMO. These very precise atomic clocks will enable sensitive searches for the possible time dependence in the basic numbers in physics. As an illustration of the latter issue, consider the fine structure constant e2/ (2Ło he), usually denoted by or, which shows up in atomic spectra and hence in atomic clocks. (Here e is the electron charge, h is Planck's constant, c is the speed of light, and So is called the permittivity of the vacuum and is a constant from the theory of electromagnetism.) The present value of or is 1/137.035 999 77~61) and is one of the most precisely determined values in all of physics. This number expresses the strength ratio of atomic interactions arising from electrostatic and velocity-dependent magnetic origins, and so it should be obtainable from a basic theory, but such a theory does not yet exist. Lacking reliable predictions, it has been suggested that the value of or might not be constant (Webb et al., 2001) but rather might have an exceedingly small but not zero time rate of change. The projected perfor- mance of the space clocks makes this a target of opportunity to ascertain if the hypothesized changes are indeed real. The recent progress in atomic clocks has been so powerful that clock tests at the level of parts in 10~4 per year have already been reported (Udem et al., 2001~. These experiments will get another great resolution increase with the use of space. In the not-so-distant future, optical atomic clocks based on different atoms, or even different physics (Coulomb versus vibrational versus hyperfine energies) will be measured on a common basis, even though they are operating at different frequencies.
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48 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA This will be enabled only by using the femtosecond-laser comb-based approach to comparing atomic clocks, which has recently come from programs funded in part by NASA, as discussed above. Concerning the BEC effect, of most significance for future NASA missions will probably be the prospect of interferometers that use not light waves but matter waves, coherently obtained from a BEC sample. Such an "atom laser" can produce atom waves with discipline and coherence, just as a conventional laser produces well-organized light waves. These matter wave interferometers promise inertial sensing at a precision scale unimaginable with mechanical or conventional laser gyros. They will give us new insight into general relativity and cosmological questions. It is no surprise that many of these future dream space experiments are being pursued as ground-based research with NASA support: It is because the full experiment will require access to space. The physics community's excitement at having access to microgravity has generated a large number of ideas for experiments. Most are in the discussion stage, and some have a modest level of NASA funding as ground-based research, sometimes from outside the fundamental physics program. Several of these new space experiments make use of some of the NASA-supported developments and related techniques noted above. The list of future missions includes long-baseline, space-based interferometers such as LISA, deep-space coherent communications, and specialized interferometers designed to detect any possible local curvature of our space. SIM needs subnanometer control and stabilization of its optical element positions to measure the relative positions and distances of stars: 1 rim is required to carry out successful experiments of starlight pulling, 200 pm for wide-angle astrometry, and 50 pm for narrow-angle astrometry. The ST3 and TPF are missions to be carried out with separated-spacecraft interferometry, leading to an elevated requirement on control and stability. Laser frequency stability of about 1 x 1 o-~4 at 1 second will make these tasks realistic. The need for a more stable atomic clock will always be there as researchers try to depict the entire universe on a finer and finer map. Laser metrology between satellites will improve the precision of enhanced formation flying. For example, in the GRACE program, Earth's gravity field is being mapped by measuring the along-track separation between two spacecraft using microwave techniques. A follow-on program, EX-5, calls for a drag-free spacecraft and laser-based distance tracking to achieve a >100-fold precision improvement (to ~ 1 nary). This enhanced capability will improve the sensitivity to changes in Earth's gravity field by between one and two orders of magnitude compared with GRACE. Several other ongoing flight experiments are not fully supported within the fundamental physics program. These include LISA, Gravity Probe B. and STEP. One other the committee has so far not mentioned is the alpha magnetic spectrometer. These experiments complement and extend the broad nature of NASA's fundamental investigations. More generally, several interesting future fundamental physics (particle astrophysics to be more exact) science experiments can be done in space, possibly on the ISS: · Antimatter search and measurements. The alpha magnetic spectrometer experiment slated to fly on the ISS is aimed mostly at searching for heavy antimatter, such as anticarbon nuclei, but also will search for positrons and antiprotons. While there is no theoretical basis to expect success, a positive finding would be highly significant for astrophysics and cosmology. · Elemental composition survey. Measuring the cosmic-ray elemental composition up to and beyond the "knee" in the cosmic-ray spectrum should provide the best clues about the origins of cosmic rays. Moreover, such a survey perhaps could also provide useful input into the larger "origins" question. In the most recent NRA (2000), two additional experiments were chosen by the Physical Sciences Division as possible future flight experiments:
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FUNDAMENTAL PHYSICS RESEARCH PROGRAM 49 · The condensation laboratory aboard the space station (CLASS), which aims to create and study Bose-Einstein condensation in the gravity-free environment onboard the ISS, and · Quantum interference tests of the equivalence principle (QUITE), which aims to test Einstein's equivalence principle using freely falling cesium and rubidium atoms. Other future experiments will compare clocks moving at different relative velocities. In these ways, the ISS will allow a number of important questions in gravitational physics and general relativity to be addressed with improved clarity and resolution. These questions concern our "reasonable" assumptions about physics, including the Einstein equivalence principle, spatial isotropy in the speed of light (the Kennedy-Thorndike and Michelson Morley experiments), the local Lorentz invariance, the exact uni- versality of gravity, and a host of other interesting issues. REFERENCES Bigelow, N. 2001. Fundamental Physics in Space: A Roadmap to Unlock Mysteries of the Universe by Exploring the Frontiers of Physics in Space. Fundamental Physics Discipline Working Group, National Aeronautics and Space Administration, Washington, D.C. Diddams, S.A., Jones, D.J., Ye, J., Cundiff, S.T., Hall, J.L., Ranka, J.K., Windeler, R.S., Holzwarth, R., Udem, T., and Hansch, T.W. 2000. Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb. Phys. Rev. Lett. 84: 5102-5105. Lipa, J.A., Swanson, D.R., Nissen, J.A., Chui, T.C.P., and Israelsson, U.K. 1996. Heat capacity and thermal relaxation of bulk helium very near the lambda point. Phys. Rev. Lett. 76: 944-947. Reichert, J., Holzwarth, R., Udem, T., and Hansch, T.W. 1999. Measuring the frequency of light with mode-locked lasers. Opt. Commun. 172: 59-68. Schnatz, H., Lipphardt, B., Helmcke, J., Riehle, F., and Zinner, G. 1996. First phase-coherent frequency measurement of visible radiation. Phys. Rev. Lett. 76: 18-21. Udem, T., Diddams, S.A., Vogel, K.R., Oates, C.W., Curtis, E.A., Lee, W.D., Itano, W.M., Drullinger, R.E., Bergquist, J.C., and Hollberg, L. 2001. Absolute frequency measurements of the Hg+ and Ca optical clock transitions with a femtosecond laser. Phys. Rev. Lett. 86: 4996-4999. Webb, J.K., Murphy, M.T., Flambaum, V.V., Dzuba, V.A., Barrow, J.D., Churchill, C.W., Prochaska, J.X., and Wolfe, A.M. 2001. Further evidence for cosmological evolution of the fine structure constant. Phys. Rev. Lett. 87: 091301. Wynar, R., Freeland, R.S., Han, D.J., Ryu, C., and Heinzen, D.J. 2000. Molecules in a Bose-Einstein condensate. Science 287: 1016-1019.
Representative terms from entire chapter: