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Physics Colloquium


Spring 2015 Colloquium Schedule

Date Speaker Title Host
January 12 Ken Nollett, University of South Carolina Speeding things up and slowing them down in the big bang Geoff Greene
January 26 Jim Hack, Oak Ridge National Laboratory Climate Change Research: Computation as an Enabling Technology Geoff Greene
February 2 No Colloquium NA  
February 9 Larry Nagahara, NIH CANCELLED Ali Passian
February 16 Lu Deng, NIST Light Wave Mixing and Scattering in Quantum Gases (CANCELLED due to inclement weather) Ray Garrett
February 23 Art Ramirez, UC Santa Cruz The Frustration Paradigm for Materials Haidong Zhou
March 2 Ron Gilman, Rutgers The Proton Radius Puzzle (CANCELLED) Geoff Greene
March 9 Seamus Riordan, Stony Brook Studying the Dark Side of the Nucleus: From Neutron Skins to Neutron Stars Nadia Fomin
March 23 Richard Milner, MIT Precision Study of the Standard Model at Low Energies Nadia Fomin
March 30 Karl van Bibber, Berkeley The Curse of the Bambino Geoff Greene
April 6 John Marko, Northwestern Micromechanical studies of DNA-protein interactions and chromosome organization Jaan Mannik
April 13 William Noel, U. Penn. EUREKA! The Archimedes Palimpsest Nadia Fomin
April 20 Honors Day Celebration Talk by Dr. Wanda Ferrell, 2015 Distinguished Alumni Honoree  


January 12, 2015

Speeding things up and slowing them down in the big bang

Kenneth M. Nollett, University of South Carolina and San Diego State University

General relativity provides us with a connection between the expansion rate of the universe and the energy density that it contains. This connection has been applied successfully to times extending back 13.8 billion years, to within a second of the (apparent) initial singularity. Weak-interaction physics and nuclear reactions between about 1 second and 30 minutes after the singularity established a nuclear composition (by mass) of 25 percent helium-4 and 75 percent hydrogen, along with some trace species. Quantitative details of this "big bang nucleosynthesis" are determined by competition between microphysical timescales and the expansion timescale, so the chemical composition of very primitive matter probes these timescales. I will describe advances and challenges in determining the microphysical inputs to big bang nucleosynthesis, as well as the limits that composition measurements place on any MeV-scale particle species that have eluded detection in the laboratory. These limits become especially powerful when combined with data from the cosmic microwave background, which have recently come to probe the expansion timescale 400,000 years after nucleosynthesis.

January 26, 2015

Climate Change Research: Computation as an Enabling Technology

James J. Hack, Director, National Center for Computational Sciences at Oak Ridge National Laboratory

The challenge of quantifying the consequences of climate change over the next few decades is motivated by an increasingly urgent need to adapt to near term trends in climate features and to the potential for changes in the frequency and intensity of extreme events. There are significant uncertainties in how climate properties will evolve at regional and local scales. The large background signal of natural variability makes sorting out and minimizing these uncertainties particularly difficult. Consequently, the climate change research community continues to face major challenges and opportunities in its efforts to rapidly advance the basic science and its application to policy formation. Meeting the challenges in climate change science will require qualitatively different levels of scientific understanding, numerical modeling capabilities, and computational infrastructure than have been historically available to the community. This talk will touch on how the challenge of simulating the climate system given the complexities present in the system and the need to better understand all the processes that regulate the observed climate system. There is an unprecedented urgency to improve predictive capabilities on the evolution of the climate system with a fidelity required by policy makers and resource managers.

February 16, 2015

Light Wave Mixing and Scattering in Quantum Gases

Lu Deng, NIST

Nonlinear and quantum optics with high-density, ultra-cold quantum gases present exciting new directions for research. Recent studies have shown that highly efficient narrow-bandwidth ultra-violet radiation can be generated in quantum gases in directions that are impossible to achieve in normal gases. In this talk recent progress in coherent light-matter wave mixing and introduces some new concepts will be presented. These new findings will certainly reframe future research directions. Indeed, most well-known nonlinear optical-wave effects should be carefully reexamined in the context of these exotic states of matter where the fundamental excitations in condensed matter physics begin to contribute and manifest themselves in light-matter interaction processes.

February 23, 2015

The Frustration Paradigm for Materials

Art Ramirez, Physics Department, University of California Santa Cruz

Frustration is loosely defined as the incompatibility of interacting degrees of freedom and the lattices they occupy. This incompatibility suppresses conventional long range order, allowing other states to emerge. I will discuss realizations of frustration in magnetic systems resulting in spin ice and spin liquid ground states. While readily studied in magnets, the concept ties together seemingly disparate systems such as water ice, negative thermal expansion dielectrics, and candidate materials for quantum computation. I will illustrate some basic concepts with examples of known compounds and present ideas for future directions.

March 2, 2015

The Proton Radius Puzzle

Ron Gilman, Rutgers

In 2010, measurements of muonic hydrogen determined the proton radius about an order of magnitude more precisely than previous measurements with electrons - but the result was about 5 sigma smaller. This inconsistency has become known as the proton radius puzzle. More recent measurements have confirmed the puzzle and increases the discrepancy to over 7 sigma. Various explanation have been proposed, including novel beyond-standard-model physics, novel conventional physics, and issues in determining the radius from experiments. No explanation has majority support in the community. I will describe the puzzle and the efforts underway to resolve it.

March 9, 2015

Studying the Dark Side of the Nucleus: From Neutron Skins to Neutron Stars

Seamus Riordan, Stony Brook University

The neutron densities in atomic nuclei are notoriously difficult to measure to high precision: the standard tool of electromagnetic interactions which has been used to map out the nuclear charge distributions simply doesn't see them. In fact, it has only recently been experimentally confirmed that the neutron-rich lead nucleus even has a neutron skin, which is a fraction of a neutron radius thick. Encoded in these distributions is a wealth of important information about how the strong nuclear force builds up systems where the number of protons and neutrons are unequal. This information has bearing not only for our understanding of asymmetric nuclei, but also in the construction of the fascinating and complicated systems that are neutron stars. Fortunately, nature gives us a novel way to image this side of the nucleus: through fundamental weak force interactions, which couple primarily to neutrons rather than protons. In this talk I will discuss why these neutron distributions play an important part in our understanding of nuclear physics and astrophysics, how one images such tiny systems with electron beams, and the recent and upcoming experimental efforts for such measurements.

March 23, 2015

Precision Study of the Standard Model at Low Energies

Richard Milner, MIT Department of Physics

Elastic electron-proton scattering at GeV energies is a fundamental process in the Standard Model. Precision experiments at modern accelerators have unearthed some surprises. The number of photons exchanged in the elastic electron proton scattering is a subject of interest. The completed OLYMPUS experiment, a measurement of the ratio of positron-proton to electron-proton scattering to determine the contributions beyond single photon exchange, will be discussed. A future experiment, called DarkLight, to search for a massive photon in the10-100 MeV mass range, will be motivated and discussed.

April 6, 2015

Micromechanical studies of DNA-protein interactions and chromosome organization

John F. Marko, Northwestern University Department of Physics and Astronomy and Department of Molecular Biosciences

The centimeter-long DNAs in our cells are folded up into micron-scale chromosomes through an array of protein-DNA interactions. Our group uses single-DNA micromanipulation – stretching and twisting of the double helix – as a tool to analyze a variety of protein enzymes that act on DNA. I will describe a few different kinds of “magnetic tweezers” experiments our group is involved focused on enzymes that package DNA and which change its topology, based on piconewton-scale force measurements. I will also discuss statistical-mechanical models relevant to the experiments. I will then describe analogous but larger scale (nanonewton force scale) micromanipulation approaches to study the large-scale structure of chromosomes. I will discuss experiments that show us how chromosomes behave as gels of underlying protein-DNA "chromatin" fibers. Results of recent experiments studying "condensin" complexes - thought to be major "crosslinkers" of chromosomes - will be presented. The role of condensins in the control of DNA entanglements will also be discussed.

March 30, 2015

The Curse of the Bambino: History and Future of the Microwave Cavity Search for Dark Matter Axions

Karl van Bibber, University of California, Berkeley

After nearly four decades, the axion, a hypothetical elementary particle, still represents the best solution to the Strong-CP problem, i.e. why the neutron has a vanishingly small electric dipole moment. Should the axion exist, it is expected to be extremely light, and possess extraordinarily feeble couplings to matter and radiation, far beyond the reach of conventional particle physics experiments. Such very light axions would also have been produced abundantly during the Big Bang, and thus the axion represents a well-motivated dark matter candidate. This talk will describe the development of the world’s most sensitive spectral radio receiver to detect the axion, and related searches for axions in the laboratory and from the Sun’s burning core.

April 13, 2015

EUREKA! The Archimedes Palimpsest

William Noel, Director of the Kislak Center for Special Collections, Rare Books and Manuscripts, and Director of the Schoenberg Institute for Manuscript Studies at The University of Pennsylvania

The Archimedes Palimpsest, a 10th century manuscript, is the unique source for two of Archimedes treatises, The Method and Stomachion, and it is the unique source for the Greek text of On Floating Bodies. All these texts were erased in the thirteenth century, and written over. In private hands throughout much of the 20th century, the manuscript was sold at auction to a private collector in 1998, and subsequently deposited at The Walters Art Museum in Baltimore, Maryland, by the owner a few months later. Since that date the manuscript has been the subject of conservation, imaging, and scholarship. Entirely new texts from the ancient world have been discovered, and transcribed, using many different imaging techniques, including X ray flourescence imaging at SLAC. This lecture will describe the history of the Palimpsest, its imaging, and the recent discoveries.

Previous Physics Department Colloquia


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