MIT Department of Physics: Spring 2006 Colloquia

Colloquia

Department of Physics Colloquia Schedule

SPRING 2006

> > FALL 2005

Thursday, February 9, 2006

URS ACHIM WIEDEMANN, Assistant Profesor
SUNY Stony Brook, NY

"Heavy Ion Collisions at RHIC and at the LHC"

Thirty years after its formulation, Quantum Chromodynamics has developed into a mature theory, whose precision frontier continues to be of great practical importance for disentangling the QCD background of new physics searches. As for any mature fundamental theory, the question arises how collective phenomena and macroscopic properties of matter emerge from the fundamental interactions. Heavy ion collisions provide a unique tool to address this question in the region of extreme matter densities. In this talk, I shall present my view on why and how the much wider kinematical range accessible in nucleus-nucleus collisions at RHIC and at the LHC opens qualitatively novel opportunities for studying the properties of dense matter, and which central questions of QCD equilibrium and non-equilibrium dynamics become accessible in this way.

Time: 4:15 pm
Place: Room 10-250 / MIT

Refreshments @ 3:45 pm in 8-329 (Physics Common Room)

Thursday, February 16, 2006

RAYMOND E. GOLDSTEIN, Professor of Physics and Applied Mathematics
University of Arizona

"How the Stalactite Got Its Shape"

As far back in recorded history as the writings of the Elder Pliny in the first century A.D. are found references to the fascinating structures found in limestone caves, particularly stalactites. Although the subject of continuing inquiry since that time, the chemical mechanisms responsible for their growth have only been well-established since the 19th century, and there has been no quantitative understanding of the morphological evolution of these strange and beautiful forms. In this talk I will describe a synthesis of calcium carbonate chemistry, diffusion, thin-film fluid dynamics, and nonlinear dynamics which shows that stalactites evolve according to a novel geometric growth law which exhibits extreme amplification at the tip. Studies of this model show that a broad class of initial conditions is attracted to an ideal parameter-free shape, not previously known in science, which is strikingly close to a statistical average of natural stalactites. Similar hydrodynamic and geometric considerations lead to a quantitative theory for the shapes of icicles, and an understanding of why stalactites and icicles look so similar, despite the vastly different physics underlying their growth.

Time: 4:15 pm
Place: Room 10-250 / MIT

Refreshments @ 3:45 pm in 8-329 (Physics Common Room).

Thursday, February 23, 2006

ADAM G. RIESS, Associate Astronomer
Space Telescope Science Institute

"Dark Energy and Kinematics from SNe Ia at z>1"

Type Ia supernovae (SNe~Ia) provide the only direct evidence for an accelerating universe and the existence of dark energy. We are in the third year of the first space-based search and follow-up program to find SNe Ia at z>1 using the Hubble Space Telescope to further explore the kinematics of the expanding universe and to characterize the nature of dark energy.

Time: 4:15 pm
Place: Room 10-250 / MIT

Refreshments @ 3:45 pm in 8-329 (Physics Common Room).

Thursday, March 2, 2006

MEHRAN KARDAR, Professor of Physics
MIT

"The Shape Dependence of Fluctuation-Induced Forces"

The Casimir force is an attraction between parallel conducting plates due to quantum fluctuations of the electromagnetic (EM) field. Thermal fluctuations of correlated fluids (such as critical mixtures or superfluids) are also modified by boundaries, resulting in similar interactions. A nice demonstration is provided by the thinning of a wetting film of helium at and below the superfluid transition. Quantitative understanding of the latter requires inclusion of surface undulations. The EM Casimir force is also modified for corrugated surfaces in non-trivial fashion. I shall also discuss other non-trivial geometries, in particular addressing the possibility of a repulsive force for a piston, and the force between a plate and a cylinder.

Recent torsional oscillator measurements of solid helium confined in porous media [1,2] and in bulk form [3] found evidence of non-classical rotational inertia indicating superfluid behavior below 0.2K. Measurements of solid samples at different pressure (and hence different density) allow us to map out the boundary of this supersolid phase. The low temperature supersolid fraction shows a non-monotonic dependence on pressure. It first increases with pressure reaching a maximum of 1.5% near 55 bars and then decreases with further increase with pressure. We have also obtained preliminary results indicating solid molecular hydrogen also exhibit a similar transition, at a much lower temperature and with a much smaller supersolid fraction. This work is done in collaboration with Eunseong Kim, Tony Clark, Xi Lin and Josh West, and is supported by the National Science Foundation.

EDWARD C. STONE, David Morrisroe Professor of Physics
California Institute of Technology

In December 2004 at 94 AU, Voyager 1 crossed the termination shock marking the abrupt slowing of the supersonic solar wind and began exploring the region where the solar plasma presses outward against the local interstellar medium. The radial flow in this region is much slower than expected and the turbulence differs from than in the supersonic solar wind. In contradiction to many predictions that the shock was the source of medium energy anomalous cosmic rays, their intensity did not peak at the shock, indicating their origin remains to be discovered. However, the shock is the source of low energy ions that reveal new aspects of the acceleration process. Recent results from Voyager 2 at southern solar latitudes suggest that the shock may be up to 10 AU closer than at Voyager 1 in the north, consistent with an asymmetric distortion of the heliosphere by a local interstellar magnetic field. The Voyagers will provide more insight into this outermost region of the heliosphere and what lies beyond as they continue their journeys to interstellar space.

A new paradigm for the origin of the laws of physics may (or may not) be emerging out of observational cosmology and theorists efforts to understand string theory. The ordinary 15 billion light-year universe is being replaced by a vastly bigger "megaverse" consisting of a huge number of what Guth calls "pocket universes." If this is true then many of the Laws of Physics that we normally think of as "written in stone" will be local environmenal facts. I will explain the evidence for this controversial view, its implications, and the various views of leading physicists and cosmologists.

The fundamental incompatability of quantum mechanics with general relativity together with our well-quantified ignorance of large-scale gravity (dark energy, dark matter) strongly suggests that we intensify our tests of gravity. APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) is a new project that will bring about order-of-magnitude improvements in testing several fundamental aspects of gravity. Using a 3.5 meter telescope to bounce laser pulses off of the retroreflector arrays left on the moon by the Apollo astronauts, APOLLO will be capable of millimeter range-precision. By determining the exact shape of the lunar orbit, it will be possible to test the equivalence principle, the time-rate-of-change of the gravitational constant, gravitomagnetism, and geodetic precession to at least ten times better precision than currently tested. In addition, APOLLO will be sensitive to departures from the inverse-square law of gravity and can potentially probe the effects of extra dimensions to which only gravity has access. APOLLO's recent record-breaking successes will be reported.

We have recently analyzed the most complete record of marine animal fossils ever compiled, the "Compendium" of Jack Sepkoski, which lists all known fossil marine animal genera back 542 million years. When the fossil diversity (number of distinct genera) is plotted, it shows a very strong 62 Myr cycle. The cycle is particularly evident in the species that endured for relatively short times. We examined many possible causes for this cycle, including movement of the Earth through the Galaxy, volcanic eruptions from the core of the Earth, variations in the energy output of the Sun, comet bombardments from solar system perturbations, an intrinsic biological cycle, and more. No known mechanism accounts for the cycles. There is one other strong cycle in the data, with period 140 Myr. This matches a previously known climate cycle, and may be due to the passage of the Earth through the arms of our galaxy. Our initial results were published in Nature, vol 434, 208-210, 10 March 2005.

A. DOUGLAS STONE, Carl A. Morse Professor of Applied Physics; Director, Division of Physical Sciences
Yale University

In 1917, Einstein authored a little-known paper on the problem of generalizing the old quantum theory to problems with several degrees of freedom that are not separable. This paper was his only published work on the correct quantization rule for matter, which was, of course, not known at that time. His work laid the foundation for a method which is completely correct (within its sphere of applicability), now known as Einstein-Brillouin-Keller quantization, a multi-dimensional generalization of the WKB approximation. However, he pointed out that the method fails if there do not exist a number of integrals of motion equal to the number of degrees of freedom, i.e., unless the system is integrable. Einstein suggested that non-integrable classical dynamics is typical and presents an open problem for quantum theory. This brilliant insight was ignored until the late sixties, when it became well-known to physicists that partially chaotic motion is indeed generic in classical mechanical systems. The problem noted by Einstein is fundamental and has never been fully overcome; but alternative semiclassical approaches to the quantum mechanics of classically chaotic systems have been developed and applied to interesting problems in atomic, condensed matter and optical physics. I will review Einstein's arguments and place them in a modern context, then I will describe one application of semiclassical methods to a chaotic system:s dielectric microcavity lasers.

"From Network Biology to Human Dynamics"

The emergence of many networks of scientific of technological interest are driven by self-organizing processes that are governed by simple but generic laws. Here I will show how the organizing principles of biological networks, best described using the tools of statistical physics, impact our ability to understand cellular functions and human diseases. Finally, I will use human dynamics to illustrate the next frontier in network research: capturing the detailed time resolved behavior of each node in a system. In this context I will demonstrate that the timing of events in human activity driven systems are described by non-Poisson processes, well captured by simple queuing models.

We review the experimental evidence for Einstein's general relativity. Tests of the Einstein Equivalence Principle support the postulates of curved spacetime, while solar-system experiments strongly confirm weak-field general relativity. We describe the status of the recently concluded Gravity Probe B experiment. Binary pulsars provide tests of gravitational-wave damping and of strong-field general relativity. Recently operational laser interferometric gravitational-wave observatories, and a future space interferometer, may provide new tests via the properties of gravitational waves.

Please visit this page in late August for a listing of physics colloquia speakers for the 2006-07 academic year.