Why Doing Fundamental Physics on the ISS?—The Experimental Conditions

P1: GAD General Relativity and Gravitation (GERG) PP1066-gerg-477711 December 22, 2003 12:42 Style file version May 27, 2002 General Relativity and Gravitation, Vol. 36, No. 3, March 2004 (C 2004) Why Doing Fundamental Physics on the ISS?—The Experimental Conditions H. Dittus1 Received September 19, 2003 The International Space Station (ISS) already serves as a laboratory for experiments in fundamental physics. It could be used for a much wider range of experiments if the operational concept of the ISS will be changed. Operational constraints set limits for the precision level of experiments. Free-flying platforms with high precision attitude and orbit control (drag-free control) could co-orbit with the ISS and improve the experimental conditions. The paper summarizes the main constraints and discusses concepts for improvement. KEY WORDS: International Space Station; free fall; gravitation. 1. INTRODUCTION The International Space Station (ISS) developed by the international space agen- cies is still under construction. The completion of its assembly is expected within the next five years. It is now being offered as an opportunity for scientific experimentation under conditions of weightlessness, in a quite unique environment that cannot be attained in terrestrial laboratories. Although the range of 1st generation experimental facilities on board the ISS has nearly been fixed, it is worth emphasiz- ing the possibility of carrying out more high precision experiments in Fundamental Physics on board the ISS, in order to develop guidelines and requirements for the next generation of ISS facilities, and for future operation of the ISS. During the last few years experimental capabilities have increased hugely through advances in technology and improvements in our scientific understanding. These give us very good reasons for exploring standard physics with much higher precision than 1 ZARM, University of Bremen, Am Fallturm, 28359 Bremen, Germany; e-mail: [email protected] bremen.de 601 0001-7701/04/0300-0601/0 C 2004 Plenum Publishing Corporation P1: GAD General Relativity and Gravitation (GERG) PP1066-gerg-477711 December 22, 2003 12:42 Style file version May 27, 2002 602 Dittus hitherto. New experimental devices for performing greatly improved high precision tests of the basic tenets of physics have been developed. Laser-cooling, atomic interferometry, and atomic fountain clocks, are examples of new tools for exploring the interaction of quantum matter with gravitational and inertial fields. And these may even be improved further using Bose-Einstein condensates as coherent atomic sources. Very high precision frequency standards are now provided by ultrastable resonators, new devices for measuring tiny forces have been developed, and ma- chining techniques have been improved tremendously, so that sub-m accuracy can be achieved in the dimensions of metre-scale parts. Partly as a result of this technical progress, the domain of Fundamental Physics has become a burgeoning, dynamic, and hugely exciting area of science – driven by the potential for new discovery. On the theoretical side, the search for a theory combining Quantum Theory and Gravitation is still the most challenging task of modern physics, whilst on the experimental side new methods of carrying out better high precision measurements may in the future enable discriminating tests of the current physical theories to be made. A significant aspect of these high precision tests of the fundamental principles underlying physics is metrology. This encompasses the very important task of preparing, reproducing, and transporting from place to place the fundamental physical units like the second, the metre, the kilogram, and the Ampere. All efforts to redefine units in terms of uniquely reproducible quantum effects are only effective, if the underlying non–gravitational physical laws are locally valid everywhere. The procedure of basing the second on a certain atomic transition is possible only because time-keeping in a gravitational field is universal – although General Relativistic corrections must be made to account for different gravitational potentials; and that of defining the metre in terms of the second is only possible because of the constancy of the speed of light, a cornerstone of Special Relativity. Modern developments like the reduction of electrical units using the quantum Hall or Josephson effect, which have come about because of the close-to-ideal reproducibility of these units, are only possible if quantum theory and Maxwell’s theory are valid. Consequently, Fundamental Physics plays an important role, and it is clear that our ability to carry out high precision experiments in space becomes an increasingly pressing issue, in order to test the predictions of the current universal theories for all four physical interactions. 2. ADVANTAGES OF FREE FALL CONDITIONS In many cases the sensitivity of measuring devices and/or the accuracy of the measurement itself will increase if the experiments can be performed under conditions of free fall, that is, under conditions of weightlessness. The advantages of such conditions are: P1: GAD General Relativity and Gravitation (GERG) PP1066-gerg-477711 December 22, 2003 12:42 Style file version May 27, 2002 Why Doing Fundamental Physics on the ISS? 603 1. The infinitely long, and periodic, free-fall: As an example, long free fall conditions enable high precision tests of the Universality of Free Fall for all kinds of structureless (i.e. pointlike) matter. 2. Long interaction times: This is, for example, hugely advantageous in atomic or molecular interferometers, where the atoms or molecules may interact with other external fields for a long time and do not fall down. 3. High potential differences: In a large class of experiments (e.g. tests of the gravitational redshift), the search for signals depends on the difference in the gravitational potential. It is obvious that this can be achieved best in space. 4. Large velocity changes: For macroscopic devices (e.g. testing the depen- dence of light speed with respect to the laboratory velocity (Kennedy- Thorndike-tests) the maximum velocity on Earth might be of the order a few thousand km/h. In space this can be increased by about one order of magnitude. For example, the velocity variations along the orbit (e.g. in a high elliptical Earth orbit) are 30 times higher than one can attain using the Earth’s rotation. 5. Long distance measurements: In space, much longer distances are avail- able than in any laboratory on Earth, and this may be essential, e.g., for the study of low frequency (103 Hz) gravity waves using interferometric techniques, where the strain of spacetime is to be measured at or below the 1021 level. 6. A low noise/vibration environment: Seismic noise is a limiting factor for many experiments on Earth (e.g. for gravitational wave detectors and for torsion balances) in the frequency range below 10 Hz. It is clear that many, but not all, of these advantages are realized on board the ISS. Furthermore, there are some disadvantages due to the very existence and construction of the ISS: Due to the atmospheric drag, the true free-fall inside the ISS is rather short and, due to the circular orbit, the difference in the gravitational potential of the Earth is small. In addition, the large structure and movable parts on the ISS create a rather large vibrational noise, and the non-negligible Earth’s gravitational gradient as well as the gravitational field of the ISS itself gives a rela- tively high level of residual acceleration. Therefore, there are many high-precision experiments in Fundamental Physics that must be carried out on specially designed satellites, having highly precise attitude and orbital (drag-free) control. Equally, however, there are still important advantages so that there is a substantial number of experiments in the area of Fundamental Physics which may yield remarkable improvements compared to existing terrestrial laboratory results, if carried out aboard the ISS. Moreover, the ISS may be used as an important and very appro- priate test bed for certain dedicated Fundamental Physics mission satellites. On the other hand, the ISS environment enables experiments to be conducted in a P1: GAD General Relativity and Gravitation (GERG) PP1066-gerg-477711 December 22, 2003 12:42 Style file version May 27, 2002 604 Dittus way that would be quite impossible using satellites. Due to the regular servicing of the space station, exchange, repair, and improvements of experimental facilities on board the ISS are possible. Facilities also can be brought back to Earth for post-mission analysis of effects that may have been causing (e.g.) potential systematic errors, and, from a physical point of view, it is of prime importance to have the capacity to repeat experiments, and to test the reproducibility of results. Undeniably, one of the most powerful arguments for the utilization of the ISS for FP experimentation, notwithstanding the less than ideal environmental conditions on board, is the unrivalled opportunity for quicker and easier access to the experimental apparatus than is conceivably possible using dedicated satellites. In consequence, this facility must reduce considerably the time scales and costs involved in the realisation of such experiments. In what follows the present International Space Station’s design and operation is discussed. Although it becomes clear that the ISS environment currently circumscribes what may be achievable as regards very high precision experiments, it is interesting nevertheless to discuss proposals on how the Space Station’s operation and the facilities on board could be improved with respect to the feasibility of carrying out even higher precision experiments in the future. 3. ISS ENVIRONMENT AND OPERATIONAL MODES The ISS is a manned space platform. Therefore, its design, orbit, structure, and operation has been determined by safety and logistics consideration. It is a multi-purpose facility which requires compromises for any experimental activity. ISS is designed to be a laboratory, but has to serve as the home for astronauts also.

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