Department of Physics and Astronomy Colloquia
Fall 2006 / Spring 2007

(1997/98, 1998/99, 1999/00, 2000/01, 2001/02, 2002/03, 2003/04, 2004/05, 2005/06)
Colloquia are Wednesdays 4:00pm in David Rittenhouse Laboratory (209 South 33rd Street), in room A8, unless otherwise noted. Colloquia are preceded by a department tea at 3:30 outside the lecture hall. All talks are given by eminent scientists, renowned for their speaking ability, at a level that is accessible to all first-year graduate students. This is a department-wide event attended by all students, postdocs, and faculty.
September 13:
Prof. Joel Karp
 University of Pennsylvania, Department of Radiology
Positron Emission Tomography: from the laboratory to the clinic
Positron Emission Tomography (PET) is a powerful imaging tool that is being used to study cancer, using a variety of tracers to measure physiological processes including glucose metabolism, cell proliferation, and hypoxia in tumor cells. As the utilization of PET, in combination with CT, has grown in the last several years, it has become clear that improved lesion detection and quantification are critical goals for cancer studies. While PET is currently used primarily for diagnostic purposes and cancer staging, PET is starting to assume a more prominent role in radiation oncology for therapy assessment and treatment. Advancements have been made recently in development of new scintillation detectors, scanner designs, and image processing algorithms in order to overcome limitations in performance, especially for heavy patients where attenuation and scatter effects are increased. In particular, scanner designs that incorporate scintillators with excellent energy and timing resolution have been shown to improve imaging performance and patient image quality. Improved energy resolution helps to reduce scattered radiation, and improved timing resolution makes it feasible to incorporate the time-of-flight information between the two coincident gamma rays into the image reconstruction algorithm, a technique that improves signal-to-noise. Results of computer simulations and experimental measurements and will be shown to demonstrate these improvements and their impact on clinical patient studies, for both cancer imaging and radiation treatment.
October 11:
Dr. Robert N. Cahn
 Lawrence Berkeley National Laboratory
The 27 Arbitrary Parameters of the Standard Model in your Everyday Life
Contrary to popular conception, the purpose of particle physics is to understand the everyday world. The current theory of fundamental interactions among the quarks and leptons depends on 27 parameters, which are a priori arbitrary. Were these parameters different, our world would be changed dramatically. By exploring the connection between these parameters and everyday phenomena we can better appreciate the challenges confronting contemporary particle physics. Until we can explain the origin of these parameters, we cannot say we truly understand why our everyday world is as it is.
3:00pm Tuesday Oct. 31, DRL-A8
Prof. Licia Verde
 University of Pennsylvania, Department of Physics and Astronomy
Connecting the cosmos with quarks: the promise of precision cosmology
Abstract
November 8:
Dr. Enrico Ramirez-Ruiz
 Institute for Advanced Study
Triggering Gamma-Ray Bursts
Although they were discovered more than 30 years ago, gamma-ray bursts are still a mystery. All that we can be confident about is that they involve compact objects and relativistic plasma. Current ideas and prospects are briefly reviewed. There are, fortunately, several new observations that could help clarify some of the issues.
3:00pm Tuesday Nov. 28, DRL-A8
Prof. Ravi Sheth
 University of Pennsylvania, Department of Physics and Astronomy
The Phenomenology of Structure Formation in the Universe
The next generation of precise cosmological tests will come from large scale galaxy surveys. Interpretation of the measurements in these massive datasets will be based on decades of work spent understanding how gravity has formed and moved objects over the 13 billion years since the Cosmic Microwave Background was formed. I will describe some simple but powerful models of the structure formation process which have played an important role in preparing us for the age of precision astrophysical cosmology. In the models, structure forms hierarchically--small things form first and big things form later by mergers of the small things--there is no fragmentation. I will then use the Sloan Digital Sky Survey to illustrate how such models are now used to interpret the observed abundances and clustering of galaxies and galaxy clusters.
December 6:
Prof. Lance Dixon
 Stanford Linear Accelerator Center
Twistor Spinoffs for Collider Physics
In the coming decade, the search for the Higgs boson, and for physics beyond the Standard Model, will be carried out by colliding protons at the Large Hadron Collider. Because each proton is a complicated bound state of quarks and gluons, proton collisions were described by Feynman as "smashing two Swiss watches together to figure out how they work." In recent decades, we have learned better how the Swiss watches work, using the theory of quark-gluon interactions, quantum chromodynamics. Armed with this knowledge, we can better predict the results of collisions at the LHC, to see whether the Standard Model holds up or fails, or whether new particles are in the data. But a major bottleneck is simply in adding up Feynman diagrams. The rules are well known, yet there can be thousands of extremely complicated diagrams. In fact, the sum of all diagrams is often much simpler than the typical one, suggesting hidden symmetries and better ways to compute. In the past two years, spinoffs from a new theory, "twistor string theory", have led to very efficient alternatives to Feynman diagrams for making such predictions, as I will explain.
3:00pm Tuesday Jan. 9, DRL-A8
Prof. Joshua Klein
 University of Texas at Austin, Department of Physics
Into the Muck: Testing the New Standard Model with Low Energy Solar Neutrinos
The discovery of neutrino oscillations has forced us to revise what used to be the most successful model of the subatomic world ever built. The new model is, however, just that---a model---and we have barely begun to test whether it is a satisfactory description of neutrinos. Neutrino oscillations are themselves a powerful tool for probing the new Standard Model because they provide us with a natural interferometer: anything that distinguishes different neutrino states can alter the observed oscillation pattern, even if its effects are tiny. I will discuss here how low energy solar neutrinos can be used to test some of the most explicit predictions of the neutrino oscillation model, focusing in particular on work being done with data from the Sudbury Neutrino Observatory.
January 10:
Prof. Jay Melosh
 The University of Arizona
Results from NASA's Deep Impact Mission to Comet Tempel 1
In the early hours of July 4, 2005, one of the two of NASA Deep Impact spacecraft splashed down on the surface of Comet Tempel 1 at 10.2 km/sec, creating an impact crater about 100 m in diameter and 30 m deep. The second spacecraft, passing by at the safe distance of 500 km, sent images of the event back to the Earth. The object of this interplanetary kamikaze mission was to dig deep beneath the surface dust deposits and expose the pristine interior 1 to 10 m below the surface. The mission succeeded spectacularly: The impact threw out about 10 million kilograms of fine dust, tarry organic material, frozen water and CO2, after creating an initial jet of melted silicates and vaporized ices. Although results are still evolving from analysis of more than 4,000 images and spectra returned from the two spacecraft, it is clear that old ideas about the structure of comets must be substantially revised.
March 14:
Prof. Charles Marcus
 Harvard University
Quantum Mechanics in the Information Age
We are already experiencing the slowing down of the rate of advance of computer power. Remember when every year, a one-year-old computer was historical (and seemed uselessly slow)? Perhaps we will tell our grandchildren, "for a while, computers got better and better very quickly, then they stopped getting better..." Or, perhaps not. In this talk, I will explore some possibilities for revolutionary (not evolutionary) change in computation and information processing using properties of quantum mechanics that are absent in present-day information processing. These approaches seem to be physically possible but frustratingly difficult to realize. This will be a report on one approach, based on solid state implementation of quantum information, and it will focus on basic physical phenomena that guide our progress.
April 11:
23rd Annual Henry Primakoff Lecture

Prof. David R. Nelson
 Lyman Laboratory of Physics, Harvard University
Spherical Crystallography and Crumpling of Amorphous Shells
Ordered states on spheres require a minimum number of topological defects. The difficulty of constructing ordered states was recognized by J. J. Thomson, who discovered the electron and then attempted regular tilings of the sphere in an ill-fated attempt to explain the periodic table. One set of solutions to this "Thomson problem" requires that regular triangular lattices be interrupted by an array of at least 12 five-fold disclination defects, typically sitting at the vertices of an icosahedron. For R>>a, where R is the sphere radius and a is the particle spacing, the energy associated with these defects is very large. This energy can be lowered, however, either by buckling, as appears to be the case for large viruses, or by introducing unusual finite length grain boundary scars. The latter have been observed recently for colloidal particles adsorbed onto water droplets in oil. New physics arises when amorphous shells subject to thermal fluctuations are deformed by outside mechanical forces.
Rescheduled:
Colloquium - Tuesday April 24th, 4:00pm, DRL-A1
Seminar - Wednesday April 25th, 4:00pm, DRL-A8
Ninth Walter Selove Lecture

Prof. David R. Smith
 Duke University
Invisibility: The Metamaterial Approach
Recently, we presented a theoretical approach to the construction of an "invisibility cloak" [1], a hypothetical structure that could shield an object from detection by causing electromagnetic waves to avoid being scattered by the object. The design of the cloak proceeds by first imagining a coordinate transformation from flat space to a space in which a void exists. The coordinate transformation is then applied to Maxwell's equations, and realized as a complicated set of constitutive parameters. Ideally, this transformation optics approach to electromagnetic design yields a structure that is inherently reflectionless and produces no distortion whatsoever to transmitted waves, although the effect can be achieved only over a narrow bandwidth.
The cloak and other structures designed by transformation optics are electromagnetically complex, requiring anisotropic media with independently varying spatial gradients in nearly all of the permittivity and permeability tensor elements. Such a complicated medium might easily have been dismissed as impractical were it not for the recent developments in the field of metamaterials [2]. Metamaterials are artificially structured materials in which dielectric and metallic inclusions replace the atoms and molecules of conventional materials. By careful design, a much larger range of constitutive parameters can be realized using metamaterials, to the point that even devices that seem to belong more in the realm of science-fiction than science-reality can now be considered. Toward the end of last year, our group reported an experimental confirmation of the invisibility cloak, demonstrating the practicality of the transformation optical approach [3]. In this talk, I will discuss the details of our invisibility cloak in the context of the broader metamaterials effort.
1. J. B. Pendry et al., Science 312, 1780 (2006).
2. J. B. Pendry and D. R. Smith, Scientific American (July, 2006).
3. D. Schurig et al., Science 314, 977 (2006).


Fall 2007
Sep.12 Hitoshi Murayama, Berkeley
Oct.10 Ian Shipsey, Purdue
Nov.14 Ann Finkbeiner, author of "The Jasons"
Dec.5 Noel Clark, Univ CO Boulder

Spring 2008
Jan.23
Feb.20 Robert Ecke, LANL
Mar.19 Walter Kohn, UCSB (Primakoff Lecture)
Apr.16