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, the department's strong commitment to its educational program was demonstrated in classroom instruction, curricular innovation, and research mentorship.
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 for Research and Dean for Graduate Education, respectively. In 1995-96 the total number of faculty was 76. Samir Mathur and Bolek Wyslouch were promoted to Associate Professor without tenure; Lisa Randall was promoted to Associate Professor with tenure, and Wolfgang Ketterle, Leonid Levitov, and Paraskevas Sphicas were promoted to Professor. Faculty on leaves or sabbaticals during this year included: Aron Bernstein, Bruno Coppi, Edward Farhi, Jeffrey Goldstone, Roman Jackiw, John Joannopoulos, Henry Kendall, Richard Milner, Ernest Moniz, John Tonry, Rai Weiss, and Barton Zwiebach. Xiangdong Ji and Pawan Kumar left MIT for positions elsewhere.
The physics faculty garnered numerous awards and honors. Professor Jerome
Friedman, who stepped down as Interim Head of the Department at the end of
January, was elected Vice President of the American Physical Society (APS) and
will become President in 1999. Professor Robert Birgeneau received the 1997
IUPAP International Magnetism Award. Professor John Joannopoulos was awarded
the APS David Adler Lectureship. Professor Henry Kendall has been named winner
of the APS 1997 Nicholson Medal for Humanitarian Service. Professor Wolfgang
Ketterle received the APS I. I. Rabi Prize. Professor Daniel Kleppner was
awarded the Oersted Medal. Professor Toyo Tanaka was recognized with a 1996 R
& D 100 award. Professor Samuel Ting received the China International
Science and Technology Award from the Chinese government. Professors
Joannopoulos and Craig Ogilvie shared the Department's 1996 Buechner Prize for
excellence in teaching, while Professors Tomas Arias and Edward Farhi won
School of Science Teaching Prizes for undergraduate and graduate education,
Professors Arias and Takashi Imai received Alfred P. Sloan Foundation awards. Several faculty were awarded chairs in recognition of outstanding contributions to the Department and Institute: Wit Busza (Friedman Chair), Claude Canizares (Rossi Chair), John Joannopoulos (Davis Chair), Paul Schechter (Burden Chair), and Toyo Tanaka (Morningstar Chair).
The Department hosted two major events: the National Conference of Black Physics Students and the United States Particle Accelerator School.
The Department continues to maintain a large number of undergraduate and graduate students and credit units per faculty member. This year the number of undergraduate majors was 155, the number of minors was 2, and the number of graduate students was 259. The number of degrees awarded totaled 50 S.B., 5 S.M., and 45 Ph.D.
The introductory physics subject 8.01 was taught in its new format, in which the primary instruction takes place in small classes, for a third year. A departmental committee was appointed to evaluate the course, based on the three years' experience with the new format. By and large, the participating faculty are enthusiastic about this format, but the approach does place significant demands on our staffing of other courses. A decision will be made in January, 1998 as to whether the new format will be continued in future years.
To sustain the high quality of physics teaching at all levels with fewer faculty, we are slightly reducing our course offerings in ways that do not reduce the breadth of opportunity but do reduce student flexibility. For example, the subjects Quantum Theory I and II will each be offered in only one of the two semesters; Junior Laboratory will be reduced from four to three sections; small increases in recitation class sizes will be implemented selectively.
The planned increase in the number of graduate teaching assistants over the next years will allow the Department to address these problems.
8.01L, the "long" version of 8.01 intended for students with less preparation in physics and mathematics, continues to be well-received. Individual tutoring for each student is an important component. The enrollment has grown steadily over the five years in which the course has been offered, from about 60 to over 150 this past year.
As part of the new curriculum for majors, the course Quantum Theory III (8.059) was taught for the first time. The expanded quantum theory sequence incorporates numerous applications of quantum mechanics to physical systems. A new IAP course, Advanced Project Laboratory (8.122), was oversubscribed, an issue to be addressed in the upcoming year.
Our recently introduced interdisciplinary courses continue to be successful: Biological Physics (8.515J), in collaboration with Health Sciences and Technology, and Fluid Physics (8.292J) in collaboration with EAPS. A new course entitled "Entropy, Information, and the Brain" will be offered in IAP 1998.
Many students entering MIT with Advanced Placement for 8.01 have been found to be disadvantaged in 8.02 with respect to peers who have completed 8.01. Accordingly, the Department has raised the standard for "placing out" of 8.01.
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 planets around nearby stars to accreting black holes in the Milky Way to quasars and clusters of galaxies 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 similarly wide range of topics that often complement the observational program.
X-ray astronomy continues to be a major area of research. The Rossi X-Ray Timing Explorer satellite, containing an all-sky monitor developed at MIT, has been operating since December 1995. Of particular interest has been the discovery of quasi-periodic oscillations at frequencies in excess of 1000 cycles per second. These are interpreted as the orbital frequencies of material in Keplerian orbits close to the surfaces of neutron stars. With the launch of the Advanced X-ray Astronomy Facility (AXAF) scheduled for summer 1998, considerable effort has gone into planning the observations.
The search for and exploitation of gravitational lenses are major activities at both radio and optical wavelengths. The difference in light travel time as measured from optical flux variations of the images in one lens system has been used to measure the distance to the lensing galaxy, yielding a one step measurement of the Hubble constant that skips the many rungs of the cosmological distance "ladder." In a parallel effort, several new lenses have been discovered.
Other optical programs include followup work on X-ray sources, in particular candidate black holes. Programs such as these will benefit greatly from the Magellan project. Construction of the enclosure and structure for the first telescope have been completed. First light is expected in early 1999 with the completion of the first mirror.
Work continues on LIGO (laser interferometer gravitational wave observatory). Construction is well underway, and it is anticipated that the LIGO observatory will be operational by the year 2000. Development work is underway for a second generation detector.
The MIT Plasma Science Experiment on board Voyager 2 will be the first spacecraft to escape the heliosphere and directly measure plasma conditions in the 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.
Numerical calculations of the dynamical evolution of systems of planets are being carried out, with the goal of explaining newly observed patterns of planet masses and distances which are very different from those observed in our solar system. 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.
MIT has one of the three groups world wide that has been able to observe Bose-Einstein condensation in a gas of ultra-cold alkali atoms. The MIT group has been amazingly productive, and in the last year has achieved record densities of condensed atoms, observed the normal modes of oscillation of the condensate, developed a non-destructive technique for imaging the condensed and uncondensed atoms in the trap, and studied the interactions between two separate clouds of condensed atoms. Their latest work demonstrates the coherent nature of groups of atoms released from the trap, the first realization of an "atom laser."
Biological and Medical Physics
The objective of this program is to apply the intellectual approaches and experimental techniques of modern physics to real problems in medicine and biology. Two examples illustrate how this can be done.
Alzheimer's disease is associated with the precipitation of the amyloid [beta]-protein in plaques which interfere with the transmission of signals in the nervous system. A method of light scattering spectroscopy has been developed which allows a detailed quantitative study of the growth of amyloid [beta] -protein aggregates. Experiments in different environments have led to a specific model for the kinetics of the growth mechanism.
On the theoretical side, advanced techniques in statistical mechanics have been used to model some simple but fundamental aspects of visual perception, specifically those associated with pattern and orientation recognition.
Condensed Matter Physics
Research in this area is aimed at understanding the new physical phenomena which manifest themselves in the bulk states of matter. The following are some highlights from the past year.
Studies of the two-dimensional electron gas, in particular in the quantum Hall and fractional quantum Hall regimes, are still at the forefront of condensed matter research. An experimental study of tunneling into the gas at its boundaries has revealed an unexpectedly rich structure in the edge states of the gas. Subsequent theoretical work has been able to explain many of the features in terms of Bragg scattering of the incoming electrons from quasi-periodic modulations in the two-dimensional electron density perpendicular to the boundary.
Work on "Bucky Balls," the soccer ball shaped C60 molecule, has advanced to the fabrication of "Bucky Tubes," long hollow cylinders of ordered carbon atoms only about 40 angstroms in diameter. The exact winding pattern of the carbons about the cylinder has profound effects on the electron transport down the tubes, effectively changing them from conductors to insulators. The details of the relationship between nanostructure and electrical properties are being studied both experimentally and theoretically. Practical applications of these unique materials are being investigated.
Nano-technology is advancing at such a rate that it may soon be possible to make structures such as beams with cross sections only tens of atoms across. Under these circumstances the properties of the beam, such as strength and elasticity, will depend not only on the bulk crystal structure, but on the particular surface structure (surface reconstruction) that might occur for these small dimensions. A study is under way to use ab-initio theoretical structure calculations to determine what surface reconstructions might occur for specific materials such as silicon, and to predict the resulting mechanical properties.
One of the unique features of the new high temperature superconducting materials is the presence of isolated layers of copper oxide in which the magnetic moments on the copper ions form a nearly perfect two dimensional Heisenberg antiferromagnet. In a new series of experiments involving a variant of these materials (containing extra Cu 2+ ions) the layers are used as a "test bed" for studying novel magnetic effects. A specific magnetic geometry is set up in which the leading terms in the magnetic interactions cancel, exposing higher order terms which lead to subtle magnetic behavior. This behavior is similar to that involved in "spin ladder" geometries but allows the determination of important parameters unmeasured in those geometries.
Modern ab initio wavefunction calculations carried out on large computers have had a great deal of success in explaining the structures of molecules and solids. These techniques have now been applied to clusters of molecules in a study of how a single excess electron forms metastable complexes with groups of water molecules. Specific structures are identified that correlate well with the observed behavior of electrons immersed in liquid water. More over, the study has given insight into the generic electrofilic nature of the dangling H atoms in water clusters. This insight can add to our understanding of electron transport in complex biomolecular systems.
The goal of research in nuclear physics is to understand at a fundamental level the structure and behavior of strongly interacting matter, ranging from terrestrially observed hadrons and nuclei to new forms of matter that may be created experimentally or observed astrophysically.
Several experimental initiatives have been undertaken to discover new states of matter by colliding relativistic heavy ions. A systematic study of nucleus-nucleus collisions using gold beams at the BNL AGS is examining the energy dependence of collision products for signatures of new phenomena, and another BNL experiment has produced upper limits on the production of fragments rich in strange quarks. Measurements of pion production at CERN's heavy ion facility have now been completed and place a significant upper limit on the production of a novel excited state of the vacuum known as a disoriented chiral condensate. The PHOBOS detector, presently being constructed under MIT leadership, will be one of the first detectors to explore the new high energy regime at RHIC and will provide a unique window on novel collective phenomena. To fully exploit these emerging opportunities, a theorist recognized for seminal ideas in relativistic heavy ion physics has recently been recruited.
Electron scattering research includes measurements of the neutron charge and magnetization distributions, studies of quasi-elastic scattering from nuclei at high momentum transfer, and the use of parity violation as a novel probe of proton structure. The MIT led Hermes experiment at DESY, using polarized electrons on a polarized Helium-3 target, is producing data revealing the spin structure of the neutron. The recent DOE approval of the BLAST spectrometer opens the way for unique experiments on polarized gas targets to explore spin physics at Bates. Extensive theoretical calculations of the effects of relativistic corrections and final state interactions in the scattering of polarized electrons from polarized nuclei enable the quantitative analysis of current experiments at Bates and TJNAL and provide valuable guidance in planning future experiments.
A major focus of theoretical research is the numerical solution of QCD on a discrete space-time lattice, which provides the only known way to calcuate the structure of hadrons and the thermodynamic properties of the quark-gluon plasma from first principles. Recent developments include a powerful new formulation of the theory analogous to quantum spin systems and a renormalization group method for dramatically reducing artifacts associated with the finite lattice. Lattice calculations have provided insight into the essential role that topological excitations of the gluon field play in hadron structure, and current calculations include the distribution of quarks in the nucleon and the properties of exotic six-quark states. The recent donation of a 24 Gigaflops cluster of symmetric multiprocessors will greatly enhance lattice calculations and the ongoing collaboration with the Laboratory for Computer Science to advance high performance scientific computing.
Researchers in particle physics seek to extend and unify our understanding of the fundamental constituents of matter and the theory that governs them.
The very successful Standard Model of Particle Physics has been tested by detailed studies of the Z particle using the L3 detector at LEP. Meanwhile data acquisition has begun at the upgraded LEP2 collider, which will provide collisions above the W+W- threshold. An exciting new result is the preliminary determination of the non-Abelian weak boson self-coupling gZWW.
Collisions of protons and antiprotons at the highest energies have been studied with the CDF at Fermilab. After obtaining the first direct evidence for the existence of the elusive top quark, the CDF team is now carefully studying its mass and decay modes. CDF is also providing important new information on the production and decay of bottom quark-antiquark (b-bbar) pairs. Analysis techniques developed in this experiment offer the possibility of the first measurements of CP violation in the b-bbar system. In addition, the MIT group is involved in both the ATLAS and CMS detector initiatives for the Large Hadron Collider (LHC) program under development at CERN.
Using polarized electrons and the SLD detector at SLAC, experimenters have taken a first look at the left-right cross-section asymmetry for Z production, and have checked the predictions of the Standard Model for heavy quark production and decay.
An experiment led by MIT scientists to study the flux of antimatter in cosmic radiation using a permanent magnet spectrometer on a space vehicle (space shuttle/station) is now under construction, for a first launch in 1998.
A novel search for the axion, a particle that has been posited to solve the so-called 'strong CP problem' of Quantum Chromodynamics and serendipitously is a candidate for the solution of the dark matter problem of astrophysics, has been mounted in a collaboration with Livermore and others. After a year of data collection this experiment has yielded its first publication of an upper limit on the coupling of axions to photons.
Recent research in particle physics theory has led to a number of advances, including important progress on the problem of black hole radiation. Although Hawking developed a semiclassical theory of black hole radiation over 20 years ago, the absence of a true theory of quantum gravity has prevented physicists from going beyond this approximation. Fundamental questions, such as the ultimate fate of the information that falls into a black hole, have remained unanswered. Theorists are now hopeful, however, that the riddle of quantum gravity has been solved by a new approach called string theory, which offers a complete description of both matter and gravity with no free parameters. Recently an MIT theorist (with collaborators) showed that string theory leads to a successful microscopic description of low-energy Hawking radiation, reproducing the semiclassical results both for the emission rate and the angular momentum distribution.
Since string theory reduces to traditional quantum field theory at low energies, it can be used to analyze the properties of field theory. An MIT researcher (in collaboration) has uncovered strong evidence that some of the exciting dualities (i.e., exact equivalences between seemingly disparate theories) recently found in quantum field theories can be understood as low-energy consequences of string theory symmetries.
String theory implies that the behavior of bosons (particles of integer spin) is connected to the behavior of fermions (particles of half-integer spin) by a relationship called supersymmetry, a relationship which is also motivated by other lines of reasoning. Since the masses of bosons do not match the masses of fermions, however, this symmetry must be dynamically broken. MIT researchers have been active in formulating detailed models of how this can happen.
MIT theorists have also been pursuing the use of the early universe as a complement to accelerator experiments in testing particle theory ideas. Recently they have developed, and are pursuing, a new version of inflationary cosmology based on the underlying particle physics of supersymmetry breaking.
The restructuring of the U.S. fusion program will leave MIT's new plasma fusion machine, Alcator C-Mod, as one of only two large research tokamaks in the program. In the few years since it was commissioned, C-Mod has proven to be an extremely versatile research tool. Most recently, it was used to demonstrate a novel new method for heating plasmas. When there are two ion species in a magnetized plasma, a resonance exists between the two ion cyclotron frequencies due to collective effects in the plasma. Having a resonance in the plasma offers a unique opportunity for efficient absorption of incident electromagnetic wave power, with a concomitant heating of the plasma. The predicted behavior was verified in C-Mod for two different ion species and magnetic fields. Since the exact location of the absorption in the plasma can be controlled, this technique will ultimately allow control of the plasma pressure and current profile.
Ernest J. Moniz
MIT Reports to the President 1996-97