Department of Physics and Astronomy Colloquia
Fall 2008 / Spring 2009

(1997/98, 1998/99, 1999/00, 2000/01, 2001/02, 2002/03, 2003/04, 2004/05, 2005/06, 2006/07, 2007/08)
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. 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 10:
Cristiano Galbiati
 Princeton University
Results from the Borexino Solar Neutrino Experiment
The Borexino Solar Neutrino Detector at Gran Sasso Labs (Italy) is a low energy solar neutrino detector operating since 2007 at Gran Sasso with 300 tons of liquid scintillator serving as an active target. The extremely low background achieved in the target has allowed the first real-time observation of low energy solar neutrinos, below the natural radioactivity barrier. I will report on recent results form Borexino and their implications for neutrino physics and neutrino oscillations.
October 8:
David DeMille
 Yale University
Tabletop probes for TeV physics: searching for the electric dipole moment of the electron
Time-reversal (T) symmetry is observed to be broken in K- and B-mesons, in a manner consistent with the Standard Model (SM) of electroweak interactions. Violation of T-invariance makes it possible for elementary particles such as the electron to have an electric dipole moment (EDM) along their spin. Although the SM prediction for the electron EDM is unobservably small, nearly every viable extension to the SM gives EDMs within a few orders of magnitude of the current limits. A new class of tabletop-scale experiments is now poised to test the predictions of these theories. At the projected level of sensitivity, these experiments will effectively probe CP-violating phenomena at the 100-1000 TeV scale, and could provide the key to understanding how matter came to dominate over antimatter in the universe. This talk will describe the basic concepts of the field, the experiment giving the current best limit on the electron EDM, and the new generation of experiments now underway.
November 12:
Prof. Marija Drndic
 University of Pennsylvania
Nanosculpting and Nanoelectronics
Manipulation of matter on the scale of atoms and molecules is an essential part of realizing the potential that nanotechnology has to offer. In this talk I will describe a method to nanosculpt matter by controllably exposing it to an intense and highly focused beam of electrons. Electron irradiation can be used to controllably displace or ablate regions of the material, such as thin metal films and graphene sheets, with nearly atomic resolution. I will discuss the impact of this work in nanoelectronics and nanofluidics, including molecular translocation studies through nanopore-based transistors.
December 3:
Mehran Kardar
 MIT/KITP UCSB
The elusiveness of knots in polymers and proteins
The abundance of knots in human art and culture indicates a long history of fascination and interest. The scientific classification of knots, however, begins with the work of three Scottish physicists, Thompson, Maxwell, and Tait. This talk will examine knots in polymers, synthetic or biological. Somewhat contrary to intuition, physical considerations (energy or entropy) are found to favor tight knots, with the entanglements localized on relatively small segments. Knots are also statistically rare in swollen (coil) polymers, but abundant in compact (globular) configurations. Yet, investigation of the Protein Data Bank reveals very few knotted structures in globular proteins. I shall discuss some intriguing examples of this set, including the most complicated protein knot (circa 2007) appearing in Human ubiquitin hydrolase, and new knots from the 2008 additions.
January 14:
Steve Ritz
 NASA Goddard
Fermi (nee GLAST) at Six Months
The Fermi Gamma-ray Space Telescope, formerly called GLAST, is a mission to measure the cosmic gamma-ray flux in the energy range 20 MeV to >300 GeV, with supporting measurements for gamma-ray bursts from 8 keV to 30 MeV. In addition to breakthrough capabilities in energy coverage and localization, the very large field of view enables observations of 20% of the sky at any instant, and the entire sky on a timescale of a few hours. With its recent launch on 11 June 2008, Fermi now opens a new and important window on a wide variety of phenomena, including pulsars, black holes and active galactic nuclei, gamma-ray bursts, the origin of cosmic rays and supernova remnants, and searches for hypothetical new phenomena such as supersymmetric dark matter annihilations. In addition to early results and the science opportunities, this talk includes a description of the instruments and the mission status and plans.
February 11:
Paul McEuen
 Cornell University
Nano Carbon: From String Theory to Atomic Drums
Carbon takes many forms, from precious diamond to lowly graphite. Surprisingly, it is the latter form that is the most prized by nano physicists. Graphene, a single layer of graphite, is incredibly strong, robust, and has the electronic structure of massless 2D Dirac fermions. Rolled up into a nanometer-diameter cylinder--a carbon nanotube--the fermions can acquire a mass and make great 1D transistors, quantum dots, and nano-solar cells. In this talk, I will present some of our recent results on these remarkable materials, including ultrafast measurements of ballistic transport, studies of topological spin-orbit coupling, and measurements of very efficient electron-hole creation by impact ionization. We will also discuss how to play a graphene drum and inflate a graphene balloon that is one atom thick.
25th Annual Primakoff Lecture
March 4:
Jim Peebles
 Princeton
Finding the Big Bang
I will review how people hit on the idea that the universe may be expanding, how it was discovered that there is a fossil remnant --- thermal radiation --- from a denser hotter state of the universe, some of the other steps toward the tight web of evidence that now convincingly shows the relativistic expanding cosmology is a good approximation to what actually happened, and a few of the issues now under discussion that might lead us to a still better approximation to the large-scale nature of the universe.
April 8:
Arthur Wolfe
 CASS/UCSD
Finding the Gas and Magnetic Fields that make Galaxiesk
Damped Lyman-alpha absorption systems (DLAs) are a population of gas layers that dominate the neutral-gas content of the Universe in the redshift interval z=[0,5] and serve as neutral-gas reservoirs for star formation in early galaxies. I discuss evidence for the evolution of neutral-gas content with time and its implications for the cycling of gas through stars at high redshifts. I then focus on our recent measurement of a magnetic field in a DLA with redshift z=0.692 using Zeeman splitting of the redshifted 21 cm absorption line. The absorbing gas exhibits a strong magnetic field, low metallicity, low electron fraction, and quiescent kinematics. These conditions pose a challenge for understanding the origin of the magnetic field.
SPECIAL COLLOQUIUM:
Date: April 22
Time: 3:30pm

Sandra Faber
 UC Santa Cruz/Lick
Galaxy Evolution Over the Last Two-Thirds of Cosmic Time

Major surveys have now thoroughly sampled the galaxy population at redshifts below z ~ 1.4. This talk will summarize broadly what has been learned about galaxy evolution over these epochs, focussing on results from DEEP2 and Sloan. The main conclusion is that galaxy evolution is highly correlated with stellar mass, which in turn is plausibly correlated with dark halo mass. A useful working model is that galaxy evolution is driven principally by halo mass and that the star formation rate is given uniquely by halo mass at each redshift over these epochs.