The David and Edith Harris Physics Colloquium Archives

Fall 2009 Schedule

September 10, 2009
Stanford University

"Topological Insulators and Topological Superconductors"

Recently, a new class of topological states has been proposed and experimentally realized. These topological insulators have an insulating gap in the bulk, but have topologically protected edge or surface states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. I shall review the theoretical prediction of the QSH state in HgTe/CdTe semiconductor quantum wells, and its recent experimental observation. The edge states of the QSH state supports fractionally charged excitations. The QSH effect can be generalized to three dimensions as the topological magneto-electric effect (TME) of the topological insulators. Bi2Te3, Bi2Se3 and Sb2Te3 are theoretically predicted to be topological insulators with a single Dirac cone on the surface. I shall present a realistic experimental proposals to observe the magnetic monopoles on the surface of topological insulators. Finally, I shall discuss topological superconductors and superfluids in two and three dimensions, and discuss various experimental proposals.

September 17, 2009

"Dark Matter is from Cygnus: in search of a wind of Dark Matter in the Milky Way"

Astronomical and cosmological observations indicate that Dark Matter is responsible for 23% of the energy budget of the Universe and 83% of its mass. Yet, little is known about the identity of Dark Matter and its interactions, because Dark Matter particles have never been directly observed in the laboratory.

In this talk I will discuss the challenges and rewards of direct detection of Dark Matter. In particular, I will discuss how a direction-sensitive detector can lead to the unambiguous observation of Dark Matter particles and provide unique information about the distribution of Dark Matter in the Milky Way.

September 24, 2009
Harvard University

"Four Famous Myths of the Copernican Revolution"

2009 is the 400th anniversary of both the first astronomical use of the telescope and of Kepler's great Astronomia Nova, truly the "new astronomy."  These events and their predecessors are now full of mythology, perhaps how things should have happened but not how they actually occurred.  For example, many times this year we have heard that with his telescope Galileo PROVED the Copernican heliocentric cosmology. A widely reported myth!  Or we are told that Kepler found the elliptical orbit of Mars by fitting a curve through triangulated points.  Wrong again!  Not even Copernicus and Tycho escape the mythmaking!

Professor Owen Gingerich, an astrophysicist turned historian, will try to straighten us all out!

October 1, 2009

"The General Relativistic Two-body Problem"

A simple problem in Newtonian gravity, the motion of two bodies about one another is far more challenging in general relativity.  Motivated largely by the importance of compact binaries as gravitational-wave sources, many years of effort have produced a suite of tools for modeling binaries in general relativity.  In this talk, I will present an overview of what we have learned from GR's two-body problem, focusing on recent advances in analytic and semi-analytic methods.  I will also describe how we can exploit the general relativistic two-body problem as a tool for studying compact bodies in our universe, especially black holes, in exquisite detail.

October 8, 2009

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

"Squeezing Quantum Noise to Improve Precision Measurements"

Atomic clocks, the most sensitive instruments ever made by mankind, are now approaching a mindboggling absolute stability of 10-17. The performance of the best instruments is limited by the quantum noise in the final readout measurement of the clock, a situation referred to as the standard quantum limit. This limit arises from the projection postulate when applied to an ensemble of independent particles, i.e. it arises from single-particle quantum mechanics. I will discuss how quantum mechanically correlated (entangled) states of the many-body system can be used to overcome the standard quantum limit, and how we generate such states in an ensemble of laser-cooled atoms using light. These experiments also demonstrate explicitly how the projection postulate emerges as an initially weak measurement is made stronger and stronger.

October 15, 2009
Senior Executive Producer, PBS-NOVA, and Director of Science Unit, WGBH (Boston)

"The Art of Science Television"

Using clips from the internationally acclaimed and award winning NOVA science series, Paula Apsell will discuss the art of science television.  How are topics chosen? How does one make difficult material not only accessible but enthralling?   How does one educate, entertain, and stimulate creative and critical thinking, all at the same time?   And what particular challenges face science journalists and educators today, when scientific and cultural literacy are more at risk than ever in a computer-game-oriented society with literally hundreds of choices  at their remote control? 

October 22, 2009
Theory Division, Los Alamos

"Quantum Theory of the Classical"

I discuss three insights into the transition from quantum to classical. I will start with (i) a minimalist (decoherence-free) derivation of preferred states. Such pointer states define events (e.g., measurement outcomes) without appealing to Born's rule (Ρκ=|Ψκ|2) . Probabilities and (ii) Born’s rule can be derived from the symmetries of entangled quantum states. With probabilities at hand, one can analyze information flows from the system to the environment in course of decoherence. They explain how (iii) robust “classical reality” arises from the quantum substrate by accounting for objective existence of pointer states of quantum systems through redundancy of their records in the environment. Taken together, and in the right order, these three advances elucidate quantum origins of the classical.

October 29, 2009

"From Black Holes to Strange Metals:  Many-body Physics through a Gravitational Lens"

In the last ten years string theory has revealed a surprising and deep connection between gravity and  many-body physics.  Puzzling issues in quantum gravity can now be formulated as questions in a many-body system without gravity, where conventional quantum mechanics applies. Alternatively,  difficult questions in some strongly coupled many-body systems can be answered by simple calculations in classical gravity.

The physical intuition behind this connection will be briefly described, as well as new  insights into the quantum nature of black holes that have been obtained. I will then focus on  some recent work where the connection has been used to find  universality classes of non-Fermi liquids, with a possible application to the strange metal phase
of the cuprate high temperature superconductors.

November 5, 2009
California Institute of Technology

"Neutrino Oscillations: Recent Triumphs and Future Challenges"

Recent studies of neutrino oscillations have established the existence of finite neutrino masses and mixing between generations of neutrinos. The combined results from studies of atmospheric neutrinos, solar neutrinos, reactor antineutrinos and neutrinos produced at accelerators paint an intriguing picture that clearly requires modification of the standard model of particle physics. These results also provide clear motivation for future neutrino oscillation experiments as well as searches for direct neutrino mass and nuclear double-beta decay. I will summarize the status of experimental and theoretical work in this field and discuss the future opportunities that have emerged in light of recent discoveries.

November 12, 2009
California Institute of Technology

"The Origin of the Universe and the Arrow of Time"

Over a century ago, Boltzmann and others provided a microscopic understanding for the tendency of entropy to increase.  But this understanding relies ultimately on an empirical fact about cosmology: the early universe had a very low entropy.  Why was it like that? Cosmologists aspire to provide a dynamical explanation for the observed state of the universe, but have had very little to say about the dramatic asymmetry between early times and late times.  I will argue that the observed breakdown of time-reversal symmetry in statistical mechanics provides good evidence that we live in a multiverse.

November 19, 2009

"Stability in a Turbulent (Fermi) Sea:  The Ever More Remarkable High Temperature Superconductors"

For over two decades high temperature superconductivity has captured the attention of scientists the world round.  However, rather than finding a simple explanation for the properties of these materials, as was done for their low temperature cousins half a century ago, intensive research has instead led to an increasingly complex picture of materials characterized by an intricate phase diagram, full of competing or coexisting states, yet still dominated by a superconducting state which persists, at least in some materials, almost half way to room temperature.

In this talk I will describe nanoscale investigations of the electronic structure of high temperature superconductors using scanning tunneling microscopy (STM). We have recently found that a still not understood high temperature phase in these materials, the pseudogap, is characterized by strong charge inhomogeneity. Surprisingly, although this disorder persists into the superconducting state, it does not seem to perturb coexisting homogeneous superconductivity. The resolution of this apparent contradiction gives new insight into the onset of superconductivity and its relationship with the pseudogap phase.

December 3, 2009
Columbia University

"Ricci Flow and the Topology of 3-manifolds"

Ricci flow is a natural evolution equation for Riemannian metrics on a manifold, which is a non-linear heat-type equation for symmetric 2-tensors. Intuitively, one expects this equation to equally distribute curvature of the metric around the manifold and hence one expects the flow of metrics to converge to homogeneous metric. At least for 3-dimensional manifolds this happens in some special cases. 

But the non-linear aspect of the equation can prevent this from always happening. In regions of high curvature, the non-linear terms can dominate and drive the evolving metric to a singularity in finite-time.

Because of the revolutionary work of Perelman, building on earlier work of Hamilton, how this occurs for 3-manifolds is now completely understood. Out of this understanding, one sees how to extend the 3-dimensional Ricci flow through the singularities by doing surgery, cutting out the regions where singularities develop. Amazingly, this process is tailor made to study the topology of 3-manifolds, and it leads to a complete classification, or listing, of compact 3-manifolds in terms of those with a homogeneous geometry.

December 10, 2009
University of California at Santa Cruz

"A Sharper Image: Nearby Active Galactic Nuclei seen with Adaptive Optics"

Mergers between galaxies are thought to play an important role in galaxy evolution, and may be key to understanding the observed correlation between the mass of central supermassive black holes and the properties of their host galaxies. We are using the new technology of adaptive optics to observe nearby galaxies in the messy process of colliding with each other, and to search more distant galaxies in order to find signs of dual active black holes. I will discuss recent results on NGC 6240, an ongoing merger between two disk galaxies. Our high-resolution near-infrared images, combined with radio and x-ray positions, reveal the location and environment of two central supermassive black holes. Spatially resolved spectroscopy is allowing us to directly measure the black hole masses.