Department of Physics and Astronomy Colloquia
Fall 2005 / Spring 2006

(1997/98, 1998/99, 1999/00, 2000/01, 2001/02, 2002/03, 2003/04, 2004/5)
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 should be accessible to all first-year graduate students. This is a department-wide event attended by all students, postdocs, and faculty.
September 14:
Prof. Mark Goulian
 University of Pennsylvania
Perturbing, Modeling, Imaging, and Evolving Cell Signaling Circuits in Bacteria
All cells sense and respond to physical and chemical cues in their environment. They do this through elaborate networks of interacting molecules that detect, interpret, and carry out responses to specific input signals. Although many of these biochemical circuits have analogs in electronics and other control systems familiar from physics and engineering, there is, as yet, no "circuit science" for cell signaling. My lab has been studying a particularly simple and well-characterized class of circuits in bacteria in order to understand the principles underlying their design. I will describe our results for several of these systems, with an emphasis on those aspects that are likely to be broadly applicable to biochemical circuits in general. I will also describe some of our more recent work in which we have re-engineered circuits to change their input signals and internal topology.
Host:
October 12:
Eighth Walter Selove Lecture

Dr. Jay C. Davis
 National Security Fellow
Lawrence Livermore National Laboratory
The Role of Science and Technology in National Security

The end of the Cold War and the arrival of large scale terrorism as a major concern for the domestic security of the United States and our Friends have caused a re-examination of the role of science and technology in contributing to security. The spectrum of threats is broader, the boundaries between civil and military responsibilities are less clear, and the impacts on unclassified research and education are of concern. Jay Davis has played a role in the technical and operational response to these threats since the middle 1980s, and for three years built and led a Department of Defense combat support agency whose responsibilities included the control of and response to the use of weapons of mass destruction. In that latter role, he created the national program for attribution of unclaimed nuclear detonations. He will discuss policy matters associated with these new challenges and give some personal examples of technical capabilities, all based in unclassified research programs that are contributing to deterrence through forensic and attribution technologies.
Additional lecture Thursday October 13th: Acceleration Mass Spectrometry and Physics in the Service of Other Disciplines
The discovery in the late 1970s that tandem electrostatic accelerators could be configured for accurate single atom detection of rare isotopes produced a revolution in many scientific disciplines. The technique depends on the efficient production of negative ions in sputtering sources, a technology to which Roy Middleton of Pennsylvania made so many fundamental contributions. This accelerator method produces a gain in sensitivity of up to six orders of magnitude relative to scintillation counting for either naturally occurring or man-made isotopes such as 14C, 10Be, 26Al, 36Cl and 41Ca, producing great advances in archaeology, the study of climate records and processes, and the reconstruction of dosimetry from events such as Hiroshima, Nagasaki and Chernobyl. The gain in sensitivity is such that any previous experiment using an isotopic tracer or marker must be reconsidered to see if it can be improved by using this method. Beginning in 1987 at Livermore, the group at the Center for Accelerator Mass Spectrometry (CAMS) pioneered the use of labeled organic compounds coupled with AMS, allowing for novel experiments in the health and nutritional sciences. In particular, the high sensitivity and low risk for 3H-, 14C-, or 41Ca-labeled compounds make possible research with human subjects to understand fundamental processes of metabolism and disease not accessible before now. The work has now expanded to the point that the LLNL spectrometers can measure isotopes from tritium to the actinides, opening new possibilities for forensic research in support of national security. Jay Davis was the founding director of CAMS and will provide an introduction to the technique and to the research programs currently carried out at Livermore.
Host:
November 16:
Dr. Rashid Sunyaev
 Max-Planck Institute for Astrophysics, Garching
The Hard X-Ray Sky
A selection of recent results from INTEGRAL, the INTErnational Gamma-Ray Astrophysics Laboratory, will be discussed. These include i) The supermassive black hole at the center of our Galaxy was a million times brighter 400 years ago. A molecular cloud near it can be used as a natural mirror. ii) Extremely bright and narrow electron-positron annihilation line from the central parts of our Galaxy give new information about the properties of interstellar matter. iii) A neutron star in our Galaxy was more powerful than a quasar for 0.7 seconds. INTEGRAL is an European Space Agency mission in cooperation with Russia and the United States. It is the most sensitive gamma-ray observatory ever launched, and is detecting some of the most energetic radiation that reaches us from space.
Host:
December 7:
Prof. Jay Kikkawa
 University of Pennsylvania
Magnetic Surprise in Carbon Nanotubes
Carbon nanotubes are being heavily studied for applications ranging from molecular electronics to high performance composites. Their high aspect ratio leads to anisotropies which are of fundamental and practical interest, but can be difficult to characterize. In this talk I will describe how we have been able to measure the optical and magnetic anisotropies of single-walled carbon nanotubes. While the former confirms theoretical predictions, the latter continues to yield unexpected results.
Host:
January 11:
Prof. Young-Kee Kim
 The University of Chicago
E=mc2 Opening Windows on the World
The profound discovery of Einstein a century ago, that particles can both be made from energy and disappear back into energy, inspires the experiments that provide our knowledge of the smallest building blocks of matter. The experiments, done at enormous accelerators, have led to a consistent theory of the origins of our world up to a certain point. However, at an energy scale not far above what we can attain at existing accelerators, this picture is predicted to break down. Moreover, the theory of the very small is intimately connected to cosmology -- the ultimate cause and structure of our universe. Cosmological observations again point to the need for a new theory in this energy range. In this colloquium, I will trace out the path from where we are at the Tevatron and what we need to do to take the next step towards understanding the nature of space and time. The discovery of new particles will open up windows on this world.
Host:
February 8:
Ralf Bender
 University of Munich
The Triple Nucleus and Supermassive Black Hole of M31
I report Hubble Space Telescope observations of the nucleus of M31, also known as the Andromeda galaxy. Andromeda is the nearest large galaxy to us. It is shown that the red double nucleus P1+P2 is well explained by Tremaine's eccentric disk model. Inside of P2 resides a third, very blue nucleus, P3, which contains the supermassive black hole. The blue light of P3 is due to A-type stars, presumably formed in starburst 200 million years ago. Photometric and dynamical modeling reveals that P3 has the structure of a thin disk in Keplerian rotation. The disk has an exponential profile and a scale length of about 1 lightyear. It rotates with 1000 km/s at this radius, resulting in a black hole mass of about 1.4*10**8 Msun, considerably higher than previous measurements indicated. I discuss black hole alternatives and show that clusters of dark objects are excluded.
Host:
March 1:
The 22nd Annual Henry Primakoff Lecture

Prof. Paul G. Langacker
 University of Pennsylvania
The Standard Model and Strings - Can They be Connected?
In the last 30 years, there has been a tremendous advance in our understanding of the elementary particles and their interactions. We now have a mathematically consistent theory of the strong, electromagnetic, and weak interactions - the standard model - which is almost certainly the correct description of Nature down to a distance scale 1/1000th the size of the atomic nucleus. However, nobody believes that the standard model is the ultimate theory - it is too complicated and arbitrary. There are many ideas for improvements, the most promising being superstrings. The string energy scale is close to the Planck scale, some sixteen orders of magnitude higher than the reach of foreseeable experiments. Nevertheless, careful examination of a variety of string models hints at observable signatures, such as new particles and interactions, and possibly supersymmetry. Implications for accelerator physics, dark matter, the origin of the baryon asymmetry in the universe, and the masses of neutrinos will be discussed.
Host:
April 5:
Prof. Jayanth R. Banavar
 Pennsylvania State University
Geometry and Physics of Proteins
Physics has provided an understanding of the essential features underlying the phases of inanimate matter in terms of the principles of geometry and symmetry. Living matter is also governed by physical law. The hallmarks of life are information, replication, natural selection, and functionality. The DNA molecule is the repository of information. Base-pairing and the structure of the DNA molecule provides a clear logical mechanism for replication. A similar simple understanding of protein molecules is lacking. We are developing approaches for the representation and analysis of protein folding patterns that can distill the essential features common to all globular proteins. We aim to elucidate the interactions that dictate why a polypeptide sequence folds into a particular topology, and to construct a framework for a quantitative description of protein folding, amyloid formation, protein interactions and natural selection.
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