The David and Edith Harris Physics Colloquium Archives

Fall 2010 Schedule

September 9, 2010
Microsoft Research New England
Hosted by Mehran Kardar

"Interdisciplinarity in the Age of Networks"

Everywhere we turn these days, we find that dynamical random networks have become increasingly appropriate descriptions of relevant interactions. In the high tech world, we see mobile networks, the Internet, the World Wide Web, and a variety of online social networks. In economics, we are increasingly experiencing both the positive and negative effects of a global networked economy. In epidemiology, we find disease spreading over our ever growing social networks, complicated by mutation of the disease agents. In problems of world health, distribution of limited resources, such as water, quickly becomes a problem of finding the optimal network for resource allocation. In biomedical research, we are beginning to understand the structure of gene regulatory networks, with the prospect of using this understanding to manage the many diseases caused by gene mis-regulation. In this talk, I look quite generally at some of the models we are using to describe these networks, and at some of the methods we are developing to indirectly infer network structure from measured data. In particular, I will discuss models and techniques which cut across many disciplinary boundaries.

September 16, 2010
NASA Ames Research Center
Hosted by Josh Winn

"Planet Formation and Exoplanets in the Kepler Era"

Modern theories of star and planet formation are based upon observations of planets and smaller bodies within our own Solar System, extrasolar planets (a.k.a. exoplanets) around normal stars and of young stars and their environments. Planets accumulate from disks of gas and dust surrounding young stars.   Formation models predict that rocky (terrestrial) planets should form in orbit about most single stars.  Observations provide the best constraints on the abundance of giant planets, because most protoplanetary disks may dissipate before solid planetary cores can grow large enough to gravitationally trap substantial quantities of gas.

Two decades ago, no planets orbiting stars other than the Sun were known. The inventory of known exoplanets is now approaching 500, and consists primarily of massive planets within a few AU of their stars.

NASA's Kepler Mission, launched in March 2009, is searching for a broad range of planets around more than 100,000 stars using the transit method. Identification of Earth analogs in the Kepler data will take years, but large, closer-in planets have already been found.

September 23, 2010
Hosted by Peter Fisher

"Fundamental measurements of the proton’s sub-structure using high-energy polarized proton-proton collisions"

Understanding the structure of matter in terms of its underlying constituents has a long tradition in science. A key question is how we can understand the properties of the proton, such as its mass, charge, and spin (intrinsic angular momentum) in terms of its underlying constituents: nearly massless quarks (building blocks) and massless gluons (force carriers). The strong force that confines quarks inside the proton leads to the creation of abundant gluons and quark-antiquark pairs (QCD sea). These ‘silent partners’ make the dominant contribution to the mass of the proton. Various polarized deep-inelastic scattering measurements have shown that the spins of all quarks and antiquarks combined account for only 25% of the proton spin.

New experimental techniques are required to deepen our understanding on the role of gluons and the QCD sea to the proton spin. High energy polarized proton-proton (p + p) collisions at RHIC at Brookhaven National Laboratory provide a new and unique way to probe the proton spin structure using very well established processes in high-energy physics, both experimentally and theoretically.

A major new tool has been established for the first time using parity-violating W boson production in polarized p + p collisions at √s = 500 GeV demonstrating directly the different polarization patterns of different quark flavors, paving the path to study the polarization of the QCD sea. Various results in polarized p + p collisions at√s = 200 GeV constrain the degree to which gluons are polarized suggesting that the contribution of the gluons to the spin of the proton is rather small, in striking contrast to their role in making up the mass of the proton.

September 30, 2010
Hosted by Patrick Lee

"Exploring nanophotonics to tailor the laws of physics"

By nano-structuring materials at length scales smaller than the wavelength of light, one can create effective materials, exhibiting optical properties unparalleled in any naturally occurring materials. The power of this approach is illustrated with two particularly important examples.

Firstly, it is shown that the control over the density of photonic states via such effective materials provides a control over black body emission, which can now be tailored almost at-will. It is also shown that such materials offer unprecedented opportunities for tailoring the near-field.

In the photonic near-field, thermal transfer can be orders of magnitude stronger at a given temperature than the black-body thermal transfer. And since over 90% of all primary energy sources are converted into electrical and mechanical energy via thermal processes, exciting energy-related applications could be enabled. Secondly, by exploring time-reversal symmetry breaking in non-trivial topological states, it is shown that one can create a very unusual optical phenomenon: photonic edge states that propagate in only one direction. This phenomenon is closely related to quantum-Hall edge states, and is in marked contrast to the conventional behavior of light, whose propagation is always bi-directional. This effect is directly implemented to create one-way waveguides, in which backscattering (and hence Anderson localization) cannot exist. In such peculiar waveguides, light is immune to disorder and can even travel right around large obstacles without any loss in energy.

October 7, 2010
Hosted by Mehran Kardar

"How Do Cells Pack Their DNA?"

We study how a DNA molecule of about 2 meters long is folded inside a cell nucleus of about 5 microns in diameter. Packing and processing of DNA by the cells pose several intriguing problems for polymer physics.

Recently developed experimental methods of chromosome conformational capture provide a comprehensive view at DNA organization inside the cell. Analysis of these data demonstrated that statistical characteristics of this organization are not consistent with well- known polymer states such a random coil or an equilibrium globule, but are consistent with the “fractal globule” state first proposed by Grosberg et al in 1988. The fractal globule is a dense, self-similar, and unknotted conformation of a polymer. Using theory and simulations, we study physical properties of the fractal globule, demonstrating that it constitutes a non-equilibrium state, which can serve as an attractive model for DNA architecture inside a cell. Comparison of available data on DNA organization in human, yeast and bacterial cells suggests that the same physical principles lead to formation of different DNA architectures in these organisms.

October 14, 2010
Stanford University
Hosted by Dan Kleppner

"Physicist W. W. Hansen at MIT and Stanford: Microwave Radar, NMR and Accelerators"

Once only a physics curiosity, microwave technology is ubiquitous today. This talk recounts the life and contributions of Stanford’s W. W. Hansen (1909-1949), an outstanding physicist and the founder of microwave electronics. He conceived the microwave cavity resonator and the klystron, and his discoveries in nuclear magnetic resonance and linear accelerators brought Nobel Prizes to his colleagues Felix Bloch and Robert Hofstadter. Before his untimely death at the age of 39, Hansen also played historic roles in the MIT development of WWII microwave radar, the rise of Stanford from a regional college to its present stature, and the emergence of Silicon Valley. The MIT Radiation Laboratory was staffed with cyclotron physicists, and Hansen introduced them to microwaves.  His lectures are preserved in a previously unpublished 1200-page manuscript, Hansen’s “Notes on Microwaves,” the first book on microwaves and the “bible” of the Rad Lab. After WWII Hansen and his protégés returned west to found Varian Associates and Stanford’s Microwave Laboratory, spawning NMR, megawatt klystrons, the Stanford Linear Accelerator, Stanford’s emergence as a major research university, and Silicon Valley.

October 21, 2010
University of Michigan
Hosted by The Society of Physics Students

"The Large Hadron Collider: Early Research Results, Research Plans and Priorities, and New Challenges and Opportunities for U.S.  Science Education"

Over the past year the high energy physics community has witnessed the commissioning and startup of the Large Hadron Collider, one of the most ambitious research projects ever undertaken.  Though the energy and luminosity of the collider are not yet at design levels, several early physics measurements portend a very interesting period of observation and discovery at the Terascale in the near future. The speaker will report on the status of the research done to date, discuss plans for the continued ramp-up of the machine toward its design configuration, and review the high priority physics studies planned. He will also discuss the special challenges and opportunities for U.S. science education when international frontier research facilities are located at distant sites.

October 28, 2010
Naval Research Laboratory
Hosted by Deepto Chakrabarty

"The Fermi Gamma-Ray Space Telescope, Relativistic Jet Sources, and the Origin of the Ultra-high Energy Cosmic Rays"

With the launch of the Fermi Gamma ray Space Telescope in 2008, gamma-ray astronomy has undergone a period of extraordinary advances. After reviewing the instrument, its capabilities, and some major discoveries, I focus on black-hole sources of GeV and TeV radiation. Blazar active galactic nuclei (supermassive black holes with jets) and gamma-ray bursts (transient emissions from newly born black holes) are argued to be the most probable sites for the acceleration of the ultra-high energy cosmic rays (UHECRs), which are subatomic particles that individually carry Joules of energy. According to the scenario proposed here, UHECRs are energized through shock acceleration processes in the jets of rotating black holes.

November 4, 2010
Hosted by Ed Bertschinger

"Isaac Newton and the Counterfeiter: Greed, Crime and the Birth of Modern Idea of Money"

In 1696 Isaac Newton left Cambridge – his home and workplace for thirty five years – to move to a new job in London as Warden of the Royal Mint.  It was supposed to be a sinecure, a no-show job.  It wasn’t.  Instead, Newton found himself in charge of remaking England’s entire supply of money, of hard currency -- and required to serve as the nation’s top currency cop.

In performing both duties, Newton confronted, and then helped to transform the notion of what money actually is.  That transformation, the heart of what is often called the financial revolution, drew directly on the themes Newton and others developed in the contemporaneous scientific revolution.  This talk will use the story of Newton’s most celebrated criminal case to trace this process, the way in which new approaches to thinking about nature so rapidly exerted their influence on matters seemingly far removed from the mathematical principles of natural philosophy.

November 18, 2010
Weizmann Institute of Science
Hosted by The Physics Graduate Student Council

"Patterning an Embryo: What Makes It So Robust?”

During embryonic development, uniform fields of cells differentiate into a reproducible pattern of organs and tissues.  To do so, cells need to receive information about their position within the field, and this information is often conveyed by spatial gradients of signaling molecules, termed morphogens. We are interested in the mechanisms which make these morphogen gradients robust to genetic or environmental fluctuations.

The formation of a morphogen gradient is a dynamic process, influences by the kinetics of morphogen production, diffusion and degradation. These processes are tightly controlled through intricate networks of positive and negative feedback loops. In the past three decades, many of the components comprising the morphogen signaling cascades have been identified and sorted into pathways, enabling rigorous analysis of underlying mechanisms.

I will discuss insights we gained by quantitatively analyzing the process of morphogen gradient formation, focusing in particular on mechanisms that buffer morphogen profiles against fluctuations in gene dosage. I will discuss general, theoretical ideas, and will describe their implementation specific biological systems, supported by experimental validations. Of particular emphasis will be a diffusion-based "ligand-shuttling" mechanism that we identified previously. I will present new results describing a self-organized implementation of this mechanism, and discuss experimental evidence supporting its function during the establishment of dorso-ventral polarity of the fruit-fly Drosophia embryo.

December 2, 2010
Columbia University
Hosted by Pablo Jarillo-Herrero

"Pseudo Spins in Graphitic Carbon Nanostructures: From Analogy of Relativistic Quantum Mechanics to Carbon Based Electronics"

Carbon based graphitic nanomaterials have been provided us opportunities to explore fundamental transport phenomena in low-energy condensed matter systems to reveal interesting analogy to the relativistic quantum mechanics. The unique electronic band structure of graphene lattice yields a linear energy dispersion relation where the Fermi velocity replaces the role of the speed of light and pseudo spin degree of freedom for the orbital wavefunction replaces the role of real spin in usual Dirac Fermion spectrum. In this presentation we will discuss experimental consequence of charged Dirac Fermion spectrum with pseudo spin structure in two representative low dimensional graphitic carbon systems: 1-dimensional carbon nanotubes and 2-dimensional graphene. Combined with semiconductor device fabrication techniques and the development of new methods of nanoscaled material synthesis/manipulation enables us to investigate mesoscopic transport phenomena in these materials. The exotic quantum transport behavior discovered in these materials, such as ballistic charge transport and unusual half-integer quantum Hall effect both of which appear even at room temperature.

December 9, 2010
Lawrence Livermore National Laboratory
Hosted by Janet Conrad

"Cooperative Monitoring of Reactors with Antineutrino Detectors"

Reactors are a copious and well understood source of antineutrinos. The Nobel prize winning experiment of Cowan and Reines, in which neutrinos were first discovered, used a reactor source. In the ensuing decades, reactors have enabled a rich set of experimental studies of neutrino properties, including  oscillations and searches for coherent scattering from nuclei. In a useful inversion, antineutrinos can be used as a kind of wireless window into the operations of nuclear reactors, with potential implications for the global nonproliferation regime. The current state-of-the-art in antineutrino detection is such that it is now possible to  monitor the operational status, power levels and fissile content of nuclear reactors in real-time at standoff distances of a few tens of meters, well outside of the reactor containment. This has been demonstrated at civilian power reactors in both Russia and the United States, with several more experiments now being deployed worldwide. In the last few years, the International Atomic Energy Agency has begun to consider the potential of this technology for its reactor monitoring regime. In this talk, I describe the state of the art for this application, and emphasize the natural overlap with ongoing fundamental physics experiments using reactor sources.