During the past year, the Physics Department maintained its position as a leader across the frontiers of physics research. A flavor of these activities is given in the brief accounts later in this report and in the more detailed reports from laboratories with significant physics participation. On the academic side, continuing thrusts speak to the department's commitment to its educational program.
The members of the Physics Department continue to provide leadership for major MIT interdepartmental laboratories. Currently, the Directors of the Laboratory for Nuclear Science (LNS), Bates Linear Accelerator Center, Center for Space Research (CSR), Center for Materials Science and Engineering (CMSE), Plasma Fusion Center (PFC) and Harrison Spectroscopy Laboratory are members of the Physics Department, as well as the Associate Director of the Research Laboratory of Electronics (RLE). In addition, Professors Robert J. Birgeneau and J. David Litster serve as Dean of the School of Science and Vice President and Dean for Research and Dean of the Graduate School, respectively. In 1995-96 the total number of faculty was 81. Leonid Levitov was promoted to Associate Professor without tenure, Jackie Hewitt and Xiao-Gang Wen were promoted to Associate Professor with Tenure, John Tonry, Mehran Kardar and Edmund Bertschinger were promoted to Professor. One Assistant Professors joined our faculty: Frederic Rasio. Faculty on leaves or sabbaticals during this year included: Boris Altshuler, Robert Jaffe, Patrick Lee, Min Chen, Richard Milner, Irwin Pless. Professors Louis Osborne, John King and Michel Baranger retired from the Physics faculty. Professors Bernard Burke, Ali Javan, and George Clark took the early retirement incentive with half-time appointment for several additional years. Professor Boris Altshuler is leaving MIT for another position.
The physics faculty garnered their share of awards and honors. Department Head Ernest J. Moniz was confirmed by the U.S. Senate for the position of Associate Director for Science in the Office of Science and Technology Policy. Professor George Benedek received the 1997 Proctor Medal, sponosored by the Association for Research in Vision and Opthalmology, for outstanding contributions to visual science and opthalmology and also received the 1995 Vinci d'Excellence Award in Science by the LVMH Moët Hennessy-Louis Vuitton Company. Institute Professor Mildred Dresselhaus was elected president of the American Association for the Advancement of Science. Professor Lee Grodzins received a 1995 R&D 100 award for most technogically significant new products and processes for his lead detector device. Professor Emeritus Vera Kistiaskowsky became a Fellow of the Association for Women in Science (AWIS) for "demonstrated exemplary commitment to the achievement of equity for women in science and technology". Professor Pawan Kumar was awarded the Vainu Bappu Gold Medal by the Astronomical Society of India for exceptional contributions in the field of astronomy and astrophysics. Professor Walter Lewin was elected to the Royal Dutch Academy of Sciences. Professor Paraskevas Sphicas was awarded the 1995 Buechner Prize for excellence in Teaching. Professor Toyoichi Tanaka was awarded the 1996 Editors' Choice Award of Discover Magazine for the development of smart hydrogels. Two junior faculty, Fred Rasio and Uwe-Jens Wiese, were awarded Alfred P. Sloan Foundation Awards. Senior Research Scientist Richard J. Temkin was awarded the 1995 Kenneth J. Button Prize in Far Infrared Physics by the Institute of Physics, London, in recognition of outstanding contributions to the field of infrared and millimeter waves for his research on high-frequency gyrotrons.
The Department continues to maintain a steady number of graduate, undergraduate students and number of credit units per faculty member. This year the number of undergraduate majors was 196 (including double majors), the number of minors was 6 and the number of graduate students was 256. The number of degrees awarded totaled 47 S.B., 7 S.M., 34 Ph.D.
A number of changes took place in the educational program this year or are in the planning stages for implementation over the next several years. The new 8.01 format continues this year with some adjustments in format based on experience gained in the inaugural year. The emphasis of the course continues to be active student participation in the teaching/learning process through self-motivating study guides. With feedback from the first year's experience, some changes have been made to the basic format. The Department has made a concentrated effort to put its better teachers into the class instructor positions. In addition, an additional "problem solving/tutorial" one-hour session (staffed by graduate teaching assistants) has been introduced, so that the class instructors can concentrate more on introducing material and less on problem solving. Also, graded problems sets have been introduced, as well as a weekly MIT TV problem solving session, which runs continuously 24 hours a day.
8.01L , the extended version of 8.01 is in its fourth year. The students who participated in 8.01L are enthusiastic both about its instructors and about the extended time schedule. The acceptance of the course is reflected in part in its increased enrollment, which reached 125 students in the Fall of 1995, as compared to 80 or so in previous years. Part of the academic success of the program is the intensive use of course tutors; each student must spend a half hour with a tutor each week. From studies of the distribution of Math Diagnostic scores in 8.01L as compared to 8.01, it is clear that the Department is successfully targeting students with weaker backgrounds in mathematics.
The new physics curriculum was fully implemented in Fall, 1995. As previously reported, the new curriculum adds an intermediate mechanics course in the second year and a third quantum mechanics course to allow the infusion of contemporary applications and examples into the three-term sequence. In addition, students will be required to take one of two IAP courses: advanced mechanics or advanced project laboratory.
The Advanced Project Laboratory started during the past IAP. This course was designed for physics majors who have taken at least the first three courses in the physics sequence. Students are encouraged to use computers on-line for data acquisition whenever possible. Normally, students work in pairs as a research team, proposing a project based on physics phenomena they have learned and would like to investigate. The teaching staff monitors the feasibility of the projects based on time limitations, degree of difficulty of the project, availability of equipment, and background in technical experience and laboratory work of the students. Approximately 20 students enrolled for this first official year of the project lab.
Changes in the graduate curriculum are fully implemented. Core requirements have been developed in each subunit (condensed matter, atomic, astrophysics, nuclear and particle experiment, nuclear and particle theory and plasma) that present students with a coherent set of courses in these areas.
We have encouraged the development of courses offered in conjunction with other departments at the Institute, and this is the first year for two of those efforts: 8.515J, Biological Physics, joint with Health Sciences and Technology, and 8.292J, Fluid Physics, joint with Earth, Atmospheric and Planetary Sciences. The Biological Physics course was given in Fall, 1995, with an extremely successful enrollment of about 50 students, most of whom were physics graduate students. The Fluid Physics course was offered in Spring, 1996. A course in neural nets, joint with Brain and Cognitive Sciences, is in the initial planning stages.
The range of high quality forefront basic research activities pursued by the MIT physics faculty is unmatched at any other physics department. This is reflected in the large number of Institute laboratories and centers which support substantial research programs of the Physics faculty. The reports from the Laboratory for Nuclear Science, including the Bates Linear Accelerator Center and the Center for Theoretical Physics, the Center for Materials Science and Engineering, the Research Laboratory of Electronics, the Center for Space Research, the Plasma Fusion Center, the Harrison Spectroscopy Laboratory, and the Haystack Observatory should be consulted for a more complete description of some of these research programs. We can provide only a brief overview in the space made available here.
Research in Astrophysics deals with our attempts to understand the universe on the largest scales. Phenomena ranging from stellar oscillations, to accreting black holes in the galaxy, to quasars at cosmological distances are studied. Observational programs involve the collection, analysis, and interpretation of data from a wide variety of ground-based and space-based observatories. Theoretical research is carried out on a wide range of topics that are both complementary to, and independent of, the observational program.
X-ray astronomy continues to be a major area of research. The X-Ray Timing Explorer satellite, containing an all-sky monitor developed at MIT, was launched in December 1995. Among the most interesting objects studied during its first months was a newly discovered transient source, thought to be a neutron star undergoing unsteady accretion from a normal companion star. Other observational programs utilize the Japanese satellite ASCA, which features a CCD imaging X-ray detector developed at MIT and the German satellite ROSAT. Binary systems with accreting compact objects (including black holes), hot plasma in supernova remnants, clusters of galaxies containing dark matter, distant quasars, and the cosmic X-ray background are being actively studied.
The search for and exploitation of gravitational lenses are major activities of the radio astronomy group. Gravitational lenses serve as probes of the lensed object and help map the dark matter within the lensing galaxy. Several such systems are being monitored for flux variations. These yield one-step determinations of the Hubble constant which skip the many rungs on the cosmological distance "ladder." The VLA radio interferometer in New Mexico is used in many of these studies.
MIT optical astronomers carry out most of their observations at the Michigan-Dartmouth-MIT observatory in Arizona, although other telescopes, especially the Hubble Space Telescope, are also used. Several programs aimed at identifying extragalactic objects found in the radio and X-ray bands are underway. Large scale flows in the universe are being investigated with the fluctuation-distance-indicator technique, developed at MIT, which yields relative distances accurate to ~5%. Gravitational lenses are being monitored for optical flux variations to measure time delays, in a program which complements parallel efforts at radio wavelengths. Programs such as these will benefit greatly from the Magellan project, a consortium of 5 institutions including MIT which is building two 6.5 meter diameter telescopes in Northern Chile. Completion of the first telescope is expected in late 1998.
Development work continues on LIGO, a collaborative project of MIT and Caltech to construct a laser interferometer gravitational wave observatory with two 4-km baseline facilities capable of detecting gravity waves from astrophysical sources. Construction is well underway, and it is anticipated that the LIGO observatory will be operational by the year 2000.
The MIT Plasma Science Experiment on board Voyager 2 continues to measure the properties of the solar wind in the distant heliosphere, and will be the first spacecraft to directly measure plasma conditions in the very local interstellar medium. A plasma experiment on the WIND satellite is part of the International Solar Terrestrial Physics program designed to study the nature of solar-terrestrial interactions.
Theoretical research on the normal modes of oscillation of stars like the Sun is leading to a better understanding of the tidal interactions and evolution of close binaries. Theoretical studies continue on the formation and evolution of binary systems containing collapsed stars, especially the newly discovered class of "supersoft" X-ray sources. Hydrodynamic calculations of stellar collisions and mergers are also being carried out. Collisions explain anomalous stars seen in dense star clusters while mergers are potentially detectable sources of gravitational waves.
Numerical simulations of cosmic structure formation, including the use of large N-body simulations, and high precision calculations of microwave background fluctuations are being extensively investigated.
Nuclear and particle physicists are working to uncover the fundamental particles and forces and to understand how these yield the properties of the strongly interacting matter which makes up nearly the entire mass of the visible universe. These studies are intimately related with cosmological studies of the early universe.
In Intermediate Energy Nuclear Physics, electron scattering research programs included measurements of the neutron charge and magnetization distribution, studies of quasi elastic electron-proton scattering in nuclei at high momentum transfer and high missing energies, study of the phenomenon known as "color-transparency" and use of parity non-conservation as a novel probe of proton structure. Complementary studies of pion-induced reactions are being carried out at Los Alamos and at PSI (Switzerland). During the current year the Hermes experiment , under the co-leadership of MIT Prof. Richard Milner, has begun data taking at the HERA collider at DESY. This experiment uses novel polarized He3 gas targets in conjunction with the polarized electron beam of HERA to study the spin structure of the neutron.
At Bates, work continued on commissioning the South Hall Ring and on improving the reliability of the accelerator. Important future initiatives are: use of the new Bates Out-Of-Plane Spectrometer and Focal Plane Spectrometer; and the completion of the high resolution spectrometer system for CEBAF.
In Heavy Ion Physics, with the installation of Au beams at the Brookhaven AGS, the systematic study of nucleus-nucleus collisions was extended into the region of higher matter density. A new initiative was started at CERN's heavy ion facility by one of the junior faculty members, Bolek Wyslouch, to search for evidence of the creation of different states of the vacuum. This program is now in the data taking phase.
Following the approval from BNL, the design and construction of the PHOBOS detector began under MIT leadership. It will exploit the opportunities offered by the new collider, RHIC, at BNL. Also at BNL, collaboration in an experiment has been undertaken in the search for strange matter produced in heavy ion collisions.
In Particle Physics, the current research included:
A new proposal led by MIT scientists to study the flux of anti-matter in the cosmic radiation using a permanent magnet spectrometer on a space vehicle(space shuttle/station) has been favorably received by the scientific community and the funding agencies.
Significant effort was spent in exploring the research opportunities at the future accelerator LHC planned at CERN. Involvement in the construction of the world's largest underground cosmic ray laboratory at Gran Sasso, the development of novel nuclear and particle detectors.
Finally, a novel axion search has been mounted in a collaboration with Livermore and others under the leadership of Prof. Leslie Rosenberg, a junior faculty member. This experiment is now being commissioned and data taking should start in the near future.
Research at the Center for Theoretical Physics seeks to extend and unify our understanding of the fundamental constituents of matter and the theory that governs them. In addition, our present knowledge of this theory is used to advance our understanding of a variety of subjects, including the structure and interactions of hadrons and nuclei, new forms of matter which may be created experimentally or observed astrophysically, and the behavior of the early universe. While the Standard Model of Particle Physics is consistent with all reliable experiments, most physicists are nonetheless convinced that it is only the low-energy approximation to a fundamentally simpler theory. Theorists at the CTP are working both to enhance our understanding of the Standard Model, and to explore the theoretical possibilities for physics beyond the Standard Model.
A bold approach to physics beyond the Standard Model is string theory, which aims to unite all the known interactions of nature and to explain the observed hierarchy of particles. An important contribution at MIT has been the development of a general field theory of closed strings. This theory has now been shown to be independent of the background field that is used in its construction, and progress has been made toward developing a more general formulation which is not tied to backgrounds representing classical solutions. Ideas from string theory are also being used in studies in less than four space-time dimensions to understand fundamental issues in the quantization of gravity, black hole entropy, and the paradoxical question of whether information is lost in the formation and evaporation of black holes.
Topological terms in field theories, which were introduced by this group several years ago, are now widely studied in problems both within and beyond the Standard Model, ranging from gravity to high temperature superconductivity. Recently, these terms were used to examine the high temperature behavior of quantum chromodynamics (QCD), including the response function of the quark-gluon plasma.
The role of underlying quark and gluon degrees of freedom in hadrons, hadronic interactions, and nuclear structure is of fundamental interest. New formulations of Yang-Mills gauge theory have been developed in terms of gauge-invariant and geometric variables, and the possibility of observing new multiquark resonances in hadron scattering has been studied.
A major recent thrust has been in the area of lattice gauge theory, which provides a unique tool to solve, rather than model, QCD. In this approach continuous space is approximated by a lattice of discrete points, so the description becomes arbitrarily accurate as the lattice spacing is decreased to zero. Recent work has used renormalization group techniques to dramatically reduce the errors caused by the size of the lattice spacing, resulting in practical QCD calculations of greatly improved accuracy. Lattice calculations have provided strong evidence that the structure of nucleons, pions and other light hadrons is dominated by topological excitations of the gluon field. Currently, moments of the structure functions characterizing the distribution of quarks in the nucleon are also being calculated. To exploit the opportunities in this field and to advance the national effort in high performance computing, a prototype for a Teraflops-scale cluster of symmetric multiprocessors is presently under construction in collaboration with an industrial partner and the Laboratory for Computer Science.
MIT has played a pioneering role in the use of high energy scattering to determine the quark and gluon structure of nucleons and nuclei. A major development this year was a QCD calculation showing that, at high momentum transfer, only half of a proton's spin is contributed by the quarks, while the remainder comes from gluons. This provides an understanding of the experimental observation that only a fraction of the proton spin arises from valence quarks, which appeared paradoxical in the context of simple quark models.
An attractive proposal for physics beyond the Standard Model is a set of relationships between integer-spin and half-integer-spin particles known as supersymmetry. Supersymmetry is implied by string theory, but could be valid even if string theory is not the correct description of nature. Supersymmetric grand unified theories provide a unified description of strong and electroweak interactions, but such theories make it difficult to explain the existence of a light doublet of Higgs bosons, as is needed in the Standard Model. MIT researchers recently proposed a mechanism that allows for a light doublet of Higgs bosons, while raising the mass of related particles by many orders of magnitude, thereby avoiding unphysical proton decay rates.
To supplement the knowledge that can be gained from accelerators on Earth, particle theorists have turned to the early universe as a testing ground for ideas about high energy interactions. MIT physicists have found that supersymmetric particle theories provide an attractive context for inflationary cosmology. Supersymmetric theories contain many fields with very small potential energies associated with them, which is exactly what inflationary models require. Viable inflationary models can be constructed in which all the requisite parameters appear naturally in terms of mass scales already present in the particle theory, without the ad hoc introduction of small parameters. Furthermore, these new models make testable predictions.
A subtle feature of the Standard Model is the violation of baryon number conservation, a process which is very improbable under normal circumstances but which becomes common at high temperatures (above 1016) and presumably in high energy collisions. To understand such collision processes, CTP researchers have studied a modified electroweak theory in which the Higgs sector is described by a field theory admitting soliton solutions. Under suitable conditions, an incoming W-boson field is shown to destroy a soliton and thereby induce baryon nonconservation.
Electroweak nuclear interactions are a continuing focus of research. The unique opportunities provided by the new ring at the Bates accelerator have motivated studies of reaction mechanisms, of new ways to use nuclei to test fundamental symmetries, and of spin and polarization observables. The use of neutral current probes to study the strange quark content of the nucleon has also been studied.
Efforts have continued to understand the nature of periodic solutions in multi-dimensional classical systems and their implications for quantum chaos. Nuclear reaction theory has been used to study parity violation and time reversal symmetry violation in low energy neutron scattering, compound nucleus enhancements in photon production by proton-nucleus scattering, and multi-step compound reactions.
As an example of the impact on other fields, ideas and techniques from lattice theory have led to significant developments in understanding quantum spin systems. Combining chiral perturbation theory with powerful cluster algorithms, CTP researchers have calculated the low energy parameters of the two dimensional Heisenberg model, which describes the precursor insulators of high temperature superconductors. They have calculated the behavior of two dimensional spin ladders, and the differences between half-integer and integer one-dimensional spin systems.
Research in this Division is aimed at understanding the new physical phenomena which manifest themselves in the bulk states of matter and at studying novel situations which can arise when a radiation field interacts with matter.
Physics made headline news this year when a group at JILA in Colorado observed Bose-Einstein condensation in an ultra-cold vapor of Rubidium atoms. That discovery used two techniques developed earlier at MIT: evaporational cooling and the dark spot trap. This fall an MIT group observed Bose-Einstein condensation in sodium vapor. The MIT experiment achieved a density exceeding 1014 cm -3 and had more than 105 atoms in the condensate, about two orders of magnitude more than in the JILA experiment. The density was sufficiently high that effects of the interaction between the atoms in the condensate were evident.
"Atom" interferometry has advanced to the stage where molecules can be made to produce sharp interference fringes. Using micro-fabricated gratings, a beam of sodium molecules Na 2 is split into two spatially separated beams and then recombined. Changes in the conditions in one of the two paths, such as the presence of a gas of other atoms or an electric field, causes a shift in the phase of the interference pattern.
Micro-lasers have been pushed to a new extreme with the demonstration of oscillation in the visible region with less than one atom at a time in the cavity. Correlation of the number of photons emitted per second with the mean number of atoms in the cavity reveals significant differences with predictions of the best current theoretical models.
Scientists at MIT are world leaders in understanding the physics of gels. This basic understanding has produced yet another practical application. A gel has been used in a reversible cycle to extract a specific targeted ion from a dilute solution and release it on command in another solution, which could have a high concentration of the ion. It is anticipated that this cycle can be applied to a number of industrial and environmental problems.
The aqueous protein solutions present in mammalian eyes are being studied in connection with diseases such as glaucoma and cataracts. These solutions also present model systems for studying critical phenomena in binary solutions composed of large macromolecules and water. A detailed study of the dynamic critical behavior near the phase separation point has shown significant deviation from the behavior characteristic of binary solutions composed only of small molecules.
MIT physicists, collaborating with a group from Stanford University, have used ultra-violet photoemission spectroscopy the map out the dispersion relation for the electrons in a proto-typical high temperature superconductor. The results demonstrate that conventional band theory calculations are inadequate to explain the dynamics of the electrons in these strongly correlated systems.
One of the forefront areas in mesoscopic physics concerns "artificial atoms", carefully shaped two dimensional wells in semiconductor hosts. Experimenters and theorists have combined efforts to study an "atom" with about 30 electrons. They find that Hartree-Fock theory provides a quantitative description of the spin dynamics of the electrons whereas a semiclassical electrostatic model does not. In particular, it explains a divergent spin susceptibility observed at particular values of the magnetic field.
The technique of photon correlation spectroscopy has been extended into the x-ray region of the spectrum. The dynamics of an order-disorder transition in the alloy Fe3 Al were probed using x-rays from an undulator-based synchrotron source. The experiments were able to resolve correlation times as long as 1000 seconds for the critrical fluctuations, corresponding to a frequency resolution of 3 x 1021 relative to the x-ray frequency of 3 x1018 Hz.
In the phenomenon of electronic tunneling into metals there is an anomaly near zero bias (associated with the Coulomb interaction between the electrons) which causes a suppression of the tunneling current. A general theory of tunneling has been developed which is valid even when the Coulomb suppression is strong. This allows the theory to be applied to a number of physical situations which have appeared at the forefront of condensed matter research where the Coulomb effects dominate the behavior, for example disordered metals and semiconductors near the metal-insulator transition and tunneling into two-dimensional electron gases.
The discovery of photonic band-gap materials has given rise to many new and ingenious techniques for controlling the propagation of light. MIT's pioneering work in this area has given rise to the first textbook on the subject: Photonic Crystals, published by Princeton University Press.
Jerome I. Friedman