Past Colloquium Speakers
Matthew Mewes, Swarthmore College
Fri., Nov. 30, 2012, 12:35 PM
When two teams of physicists announced the probable discovery of the Higgs boson last summer, the particle-physics community buzzed with excitement but struggled to explain why anyone else should care. In many ways, the Higgs, the last of the fundamental particles in the Standard Model of particle physics to be discovered, holds the key to our current understanding of physics at subatomic scales. In this talk, I will discuss what makes the Higgs boson so special.
Véronique Petit, West Chester University
Fri., Nov. 2, 2012, 12:35 PM
The presence of magnetic activity at the surface of our Sun, as well as all similar low-mass stars, is a well established phenomenon. Massive OB stars, around ten times the mass of our Sun, are not thought to be able to sustain dynamo-produced magnetic fields. Nevertheless, the presence of magnetic fields at the surfaces of many massive stars has been suspected for decades, to explain the observed properties and activity of OB stars. However, very few genuine high-mass stars had been identified as magnetic before the advent of a new generation of powerful spectropolarimeters that has resulted in a rapid burst of precise information about the magnetic properties of massive stars. This presentation will review how it is possible to characterize the magnetic fields of stars thousands of light-years away by studying their polarized light, how these magnetic fields differ in properties and origins from those of low mass stars, as well as our current understanding of the impact of magnetism on the lives of the most massive stars.
Ken Heller, University of Minnesota
Fri., Oct. 26, 2012, 12:45 PM
Our Universe started with a Big Bang, creating matter and anti-matter from energy. As the Universe expanded, fundamental particles condensed out. Instead of the matter and anti-matter annihilating, leaving only photons, we find ourselves in a Universe made of matter. This result requires an asymmetry between the properties of matter and anti-matter. Within known physics, the only place where a large enough asymmetry of this type could exist is neutrinos. The signature of this asymmetry, if it exists, can be detected in neutrino oscillations, the propensity for neutrinos to change their identity over time. Such oscillations have been discovered in the past 15 years by a variety of large experiments that detect neutrinos from cosmic rays, the Sun, reactors, and particle accelerators. This talk will review the phenomenon of neutrino oscillations, how oscillations would reveal the matter-antimatter asymmetry that might have created our Universe, the other physics probed by the two large neutrino detectors in the US, MINOS and NOvA, and the construction of the experiments. While MINOS has been taking data for 7 years and has made precise measurement of oscillation parameters, NOvA is the next generation neutrino detector designed with a possibility of measuring the matter-anti-matter asymmetry. The NOvA experiment is currently being installed.
For more information, see Prof. Heller's website.
Mike Shay, University of Delaware
Fri., Sep. 28, 2012, 12:45 PM
What is Space Weather? What is magnetic reconnection? In this talk, I will discuss how activity on the surface of the sun influences the space environment around the earth. Telescopes on satellites monitor solar activity and instruments on satellites directly measure the space environment above the earth's surface. My specific research involves numerical simulations of these processes performed with some of the fastest computers in the world.
For more information, see Prof. Shay's website.
Prof. Michael Brown will give a pre-colloquium on Friday, Sep. 21, at 12:45.
Jeremy Carlo, Department of Physics, Villanova University
Fri., Mar. 23, 2012, 12:30 PM
In materials whose atoms possess nonzero magnetic moments, at sufficiently low temperatures the moments will align into a periodic geometric pattern, giving rise to a static magnetic state. But in recent years a significant amount of interest has centered on so-called geometrically frustrated materials, in which the structural arrangement of moments inhibits the development of magnetic order. This is typically associated with triangular or tetrahedral lattices such as the Kagome, triangular and pyrochlore structures. Although magnetic moments on the very common face-centered cubic (FCC) lattice can also exhibit geometric frustration, relatively few studies have considered such materials. I will discuss neutron scattering studies of one such geometrically frustrated FCC material, Ba2YMoO6. In a neutron scattering experiment, the momentum and energy dependence of a beam of neutrons scattered off of a sample is used to reconstruct both the sample's crystalline and magnetic structure, and the spectrum of energetic excitations from the ground state, yielding a wealth of information. We have confirmed findings of earlier studies which found that to the lowest temperatures studied, Ba2YMoO6 fails to develop static magnetic order, indicating a significant degree of frustration, and have directly observed that Ba2YMoO6 exhibits an intriguing spin-singlet ground state.
For more information, see Prof. Carlo's website.
Prof. Peter Collings will give a pre-colloquium on Friday, March 16, at 12:30, "Determining Structure by Scattering Techniques: A General Scientific Method."
Kaitlin Kratter, Harvard-Smithsonian Center for Astrophysics
Fri., Feb. 10, 2012, 12:30 PM
A large fraction of stars are formed in binary and multiple stellar systems, unlike our own singleton sun. These gravitationally bound pairs and triples are not only common, but contribute immensely to our understanding of topics ranging from nuclear physics to cosmology. In this talk I will describe why these systems are so crucial to our understanding of the universe. Next, I will examine the long term dynamical stability of both stellar and planetary systems. As stars evolve, they lose mass, triggering orbital instabilities. Once the systems become unstable, the stellar and planetary orbits become chaotic. This chaos leads to frequent collisions and ejections. Such events precipitate the formation of exotic astrophysical objects such as the Sirius binary system -- better known as the brightest star in the night sky.
For more information, see Dr. Kratter's website.
Catherine Crouch, Swarthmore College
Fri., Feb. 3, 2012, 12:30 PM
Many critically important biological processes, such as cell growth and division and nerve signaling, involve protein molecules binding to the surface of cell membranes; many diseases, such as Parkinson's disease and ALS, involve changes in these binding processes. The strength and rate of binding is controlled by the basic physics of interaction between the protein and the membrane, such as electrical attractions between the protein and the charged molecules making up the membrane surface, and the energy cost of bending or compressing the membrane in the process. However, making the quantitative measurements needed to develop a detailed physical model of the binding process is challenging, requiring sensitive measurements on individual protein molecules or individual model membranes. This talk will provide an introduction to some of the basic physics affecting the binding of a particular type of protein structure, known as the BAR domain, and describe experimental progress with two techniques for quantitative measurements.
For more information, see Prof. Crouch's website.
Kevin Aptowicz, Department of Physics, West Chester University
Fri., Nov. 18, 2011, 12:45 PM
Within the field of condensed matter physics, our understanding of disordered systems lags far behind our understanding of crystalline ones. In crystalline systems, defects are known to control mechanical fragility of the solid. As an example, if a crystalline solid is stressed, defects will serve as the nucleation site for plastic deformations. However, there are no obvious counterparts to defects in disordered solids. This is the focus of our research. Is there a structural or dynamic property of the material that can be used to identify the nucleation sites of plastic deformation upon being stressed? To put it more concisely, when things fall apart, where does it start?
Our model system to explore this question is a colloidal glass. Colloids have a long tradition as condensed matter analogs of ordinary solids. The length scales and time scales associated with the particles' size and motion are experimentally accessible. Furthermore, colloidal particles undergo thermal motion but are still large enough to allow tracking on the single particle level using video microscopy. Recently, a technique for determining the normal vibrational modes of a disordered colloidal glass has provided new insight into the question at hand. Initial work has shown a correlation between particle participation with low frequency normal modes and those particles that undergo plastic deformation. In this talk, I will describe the experiment design and data analysis that lead to this finding. I will also discuss a newly devised experimental approach (thermal poking) that provides us with a unique experimental pathway to explore the fragility of disordered solids.
Frank Moscatelli, Department of Physics and Astronomy, Swarthmore College
Fri., Nov. 4, 2011, 12:45 PM
Light in the red and near infrared wavelength region can propagate through human tissue for several centimeters with relatively little absorption. Elastic scattering of photons in tissue, however, is extremely high. Indeed, after propagating about 3 millimeters, all information of the photon's initial trajectory is lost. These conditions - low absorption and high scattering - allow for analysis based on a diffusion theory of light transport. Photons diffuse through the medium much as heat diffuses through a solid. The diffusion approximation provides a tractable method for tomographic image reconstruction based on inverse-problem computational algorithms. The absorbing chromophores in human tissue include oxy and de-oxygenated hemoglobin, lipid, and water. Since each species has a distinct absorption spectrum, diffusive optical tomography (DOT), can provide functional maps of tissue oxygen saturation, blood flow, and angiogenesis, in addition to images of cell density anomalies, i.e. tumors. These are important tools in differentiating non-malignant lesions from cancer.
In this talk I will describe our current instrument, which employs six amplitude modulated laser wavelengths, 209 source positions, and a 512 x 512 detector array. This results in about 3 x 108 source-detector pairs, each yielding independent amplitude and phase signals. I will briefly describe the theoretical and mathematical analysis, and then will present some reconstructed images of phantom targets showing perturbing effects due to the chest wall including our approach at remediation.
Matthew Mewes, Department of Physics and Astronomy, Swarthmore College
Wed., Oct. 5, 2011, 4:30 PM
(note the date and time)
A recent measurement by the OPERA experiment suggests that exotic particles called neutrinos may be able to travel faster than the speed of light. As prescribed by Einstein's relativity, the speed of light has long been considered an absolute universal speed limit. Does the OPERA result mean that relativity is wrong? Suggestions have been circulating in the popular press that OPERA implies a breakdown in causality and the potential for time travel. In this talk, I will discuss the experiment and what the result, if confirmed, really says about our current understanding of the universe.
Vyacheslav Lukin, Naval Research Laboratory
Fri., Sep. 30, 2011, 12:45 PM
Plasma, the one in fluorescent lamps rather than in blood vessels, is commonly known as the 4th state of matter and famously comprises 99.9% of the visible Universe. The discipline of plasma physics includes the engineers that design and produce integrated circuits via plasma etching, the astronomers and astrophysicist who observe and interpret the electromagnetic radiation coming from extragalactic jets, and the physicists who try to understand the common underlying properties of plasmas between the former and the latter. As physicists, we believe that in most plasmas we know and understand the elementary interactions between electrically charged particles comprising the plasma, but it is the complexity of the collective mutual interactions of order 10^23 particles that keeps us up at night. In this talk, I will show examples of various natural plasma phenomena with a particular focus on the near-Earth space and solar plasmas, where recently launched satellites are providing us with unprecedented views of the plasma activity above the solar surface. I will also give an overview of the computational methods we use to model these on the modern-day supercomputers, demonstrating our successes and failures, and showing that only with observation, experiment, computation and analytical theory working hand-in-hand do we have any hope of understanding the complexity and finding the fundamental principles behind the observed dynamics of the 99.9% of the visible Universe.
Prof. Michael Brown will give a pre-colloquium talk on Wed., Sep. 28 at 4:30 PM in Cunniff Hall.
Ana Matkovic, Department of Astronomy & Astrophysics, Pennsylvania State University
Tue., Mar. 22, 2011, 4:30 PM
Science Center room 128
Dwarf elliptical galaxies (dEs) are the most numerous galaxies in clusters. However, they are difficult to observe due to their low surface brightness. Although their spherical shapes resemble the bright elliptical galaxies (Es), dEs are significantly smaller, less massive and seem to follow different scaling relations from Es. The shallow potential wells of dwarf elliptical galaxies make them susceptible to a wide range of processes. For example, in dense environments, dEs may experience a larger number of interactions with other galaxies in the cluster. Such effects are reflected in the characteristics of their stellar populations. To study these effects, we compare the underlying stellar populations of dwarf elliptical (dE) galaxies in two regions of different density in the Coma cluster of galaxies. We measure absorption line-strengths for ~70 dE/dS0 galaxies in the core of the Coma cluster and a region just outside the cluster core. Via absorption line-strengths we calculate ages, metallicities and abundances of chemical elements, and investigate scaling relations for these galaxies. We find that, on average, dEs in the core of the cluster are younger, and more metal poor than the ones in the low-density region of the Coma cluster. Furthermore, a surprisingly large number of dEs in the outer region have abundance of elements higher than solar. This implies that dEs have formed their stars in a quick burst of star formation, comparable to more massive ellipticals, while the range in age and metallicity suggests that some of these dE galaxies are unusually young.
Louise Edwards, Infrared Processing and Analysis Center, Caltech
Fri., Mar. 18, 2011, 12:45 PM
Many galaxies often pair together, form low-mass groups or larger, high mass clusters. The cluster mass comes not only from the thousands of member galaxies, but also from extreme amounts of hot X-ray emitting hydrogen gas that permeates throughout the cluster. In this talk I will discuss galaxies inhabiting two particular environments. Looking first in the core of massive clusters, where the density of galaxies and hot gas is highest, I find galaxies are more active under certain conditions. At the other extreme, the edges of clusters, the density of both galaxies and X-ray emitting gas is much lower. By examining the outskirts of one cluster in particular, Abell 1763, our team discovered that it is in fact connected to a second cluster, Abell 1770, by a vast filament of galaxies. Using a wealth of ground and space-based data from optical, infrared, and radio
telescopes I derive physical properties of the individual galaxies. This leads to a comparison of the filament galaxies, to those found in the cluster core, as well as to a novel measurement of the particle density of the surrounding filament gas. Both results are key steps towards a better understanding of the structure of the large-scale
Universe, as well as how this large-scale environment affects galaxy evolution.
More information is available on Dr. Edwards's website.
Joshua Pepper, Department of Physics & Astronomy, Vanderbilt University
Tue., Mar. 15, 2011, 4:30 PM
Science Center room 128
The pace of discovery of exoplanets has been so rapid that our picture of planet formation and evolution has changed radically in the past few years. I will give an overview of the current state of exoplanet discovery, with special attention to extreme planets, as well as the recent results from the Kepler mission. I will also discuss the KELT and MARVELS surveys and how they fit in to the coming flood of new planet discoveries.
More information is available on Dr. Pepper's website.
Eric Wilcots, Department of Astronomy, University of Wisconsin
Tue., Feb. 22, 2011, 4:30 PM
The current state of our understanding of the nature of the baryon content of galaxy groups, derived largely from a large body of X-ray observations, leaves us with two key questions. First, what are the relative fractions of the hot, warm/hot, and neutral gas in galaxy groups, and how is each phase distributed within groups? Second, how has the baryon content of galaxy groups evolved over time and what is its relationship to the dynamical evolution of the group? I will describe a number of recent results from observations probing the baryonic content of galaxy groups using HI, radio galaxies, and diffuse synchrotron emission providing evidence that the intergalactic medium undergoes a tremendous transformation in the group environment.
Note: Prof. Wilcots, along with Franklin Institute planetarium director Derrick Pitts and astronaut Mae Jemison, will be talking about The Future of Space Exploration on Wednesday, February 23, at 4:30 in Science Center 101.
Lisa Pratt, Provost's Professor of Geological Sciences, Indiana University
Sigma Xi & Phi Beta Kappa Talk
Thu., Feb. 10, 2011, 8:00 PM
Cunniff Lecture Hall
Diverse organic molecules containing carbon, hydrogen, oxygen, nitrogen and sulfur are common in meteorites, comets, and interplanetary dust particles. These pre-biotic ingredients rain down on planetary surfaces, providing molecular building blocks for a potential origin of life anywhere where liquid water is present. Ancient Mars appears to be particularly well suited for the transition from pre-biotic to biological activity given increasing evidence of an active water cycle and diverse aquatic environments around 4 billion years ago. The scientific and engineering community has proposed a 2018 rover mission to drill and cache samples as the first step in a campaign to bring Martian rocks and soils back to Earth for study. In preparation for seeking evidence of past or present Martian life, innovative new life-detection instruments are being tested in harsh Arctic and Antarctic environments.
Lisa M. Pratt is Provost's Professor of Geological Sciences at Indiana University. As part of her research into life-sustaining energy for microbes in the deep subsurface of Earth, she has collected samples of water, rock, and natural gas in active gold mines at depths up to 2.5 miles below the surface in South Africa and in the Canadian Arctic. Her research on radiolysis of water as a source of energy for microbial metabolism has been highlighted worldwide. She recently chaired a NASA science advisory group that proposed a 2018 rover (Mars Astrobiology Explorer and Cacher), equipped to drill and encapsulate rock cores as the first step in a sample return campaign.
More information is available on Prof. Pratt's website.
Experimental Dynamos: From Liquid Metal to Plasmas
Cary Forest, Department of Physics, University of Wisconsin, Madison
Fri., Feb. 4, 2011, 12:45 PM
Many astrophysical objects, like the Sun, are composed of highly conducting, turbulent, flowing plasma in which the flow energy is much larger than that of magnetic field energy. Creating such conditions in laboratory plasma experiments is challenging since confinement is required to keep the plasma hot (and conducting), and this requires strong applied magnetic fields. For this reason, laboratory experiments using liquid metals have been addressing fundamental plasma processes in this unique parameter regime. This talk will begin by reviewing self-generation of a magnetic field of energy comparable to the turbulent flow from which it arises--the dynamo process. Then, I'll talk about how experimental studies, using liquid metals, are isolating various processes in the dynamo instability.
This includes liquid metal experiments that (1) demonstrated self-excitation of magnetic fields, (3) intermittent self-excitation and a variety of time dynamics including field reversals, and (3) showed the existence of a turbulent electromotive force (mean-field current generation). Liquid metals are, however, not plasmas: dynamos may differ in plasmas where the relative importance of viscosity and resistivity can be interchanged, and new instability mechanisms, outside the scope of incompressible MHD may be critical in plasmas. This suggests that the next generation of experiments in this important astrophysics regime should be based upon plasmas. The Madison Plasma Dynamo experiment (now under construction) will then be described with an overview of the concept and show how the dynamos might operate in this plasma. Modeling of several experimental scenarios that mimic solar processes will also be described, including experiments on rotating, compressible convection driven by magnetic buoyancy.
More information is availabe on Prof. Forest's website.
Pre-colloquium talk by Prof. Michael Brown on Wed., Feb. 2, at 4:30 in the Physics and Astronomy seminar room (SC 113)
Pencil + Tape = Topological Quantum Computation? - The New Two-Dimensional Universe of Graphene
Paul Cadden-Zimansky, Department of Physics & Astronomy, Columbia University
Fri., Dec. 3, 2010, 12:45 PM
From its isolation in 2004 to this year's Nobel Prize, the impressive material properties of graphene have been widely touted: it's a single atom thick, stronger than steel, a better conductor than copper, and more transparent than glass. But what has intrigued many condensed matter physicists is the unusual charge carriers that can exist in graphene, particularly when it is subjected to high magnetic fields. These "particles" that inhabit graphene's two-dimensional universe can be relativistic, have fractional charge or multiple spins, and may even obey new types of quantum statistics. This talk will present recent experiments demonstrating some of these properties, and explain why the topological nature of these high-field carriers make them a potential building block for quantum computation.
Pre-colloquium talk by Peter Collings in Cunniff
Radiative Properties of Astrophysical Matter: The Quest to Reproduce Astrophysical Matter on Earth
Jim Bailey, Sandia National Lab, Albuquerque, NM
Fri., Oct. 29, 2010, 12:45 PM
Creating stellar interior matter, the extreme radiation near an accretion powered object, and surrogate white dwarf atmospheres are becoming possible with the advent of megaJoule class laboratory facilities. One example is Z, the world's largest pulsed power generator. Z delivers approximately 20 TW electrical power to cm-scale experiments, a power that for a few nanoseconds exceeds the combined capacity of all the world's power plants. The three research topics mentioned above share the feature that ten years ago they were impossible to execute. This talk will describe research at the Z facility demonstrating that all three are now within reach. Significant challenges remain, but as these topics advance toward mature realization, we are motivated to ask: What additional astrophysical research might benefit from newly feasible laboratory measurements?
Pre-colloquium talk by David Cohen on Wednesday, Oct. 27, 2010, 4:30PM in Cunniff
(Nearly) Invisible Galaxies
Beth Willman, Department of Astronomy, Haverford College
Fri., Oct. 22, 2010, 12:45 PM
In the past five years, more than two dozen dwarf galaxies have been discovered around the Milky Way and M31. Many of these discoveries are 100 times less luminous than any galaxy previously known and a million times less luminous than the Milky Way itself. These objects have made astronomers question the very meaning of the word "galaxy". The advent of wide-field, digital surveys (in particular the Sloan Digital Sky Survey) facilitated these discoveries, and hint at a much larger population that will be revealed in imminent and future imaging datasets. These dwarfs have emerged as the most dark matter dominated and most metal-poor galaxies known. As such, they are changing our understanding of galaxy formation at the lowest luminosities and are currently our most direct tracers of the properties of dark matter on small scales. This talk will highlight new and on-going studies of and searches for the very least luminous companions to the Milky Way - those with less than 1000 times the luminosity of the Sun.
More information is available on Prof. Willman's website.
Relaxation, Self-Organization, and Turbulence in Plasmas
Tim Gray, Department of Physics and Astronomy, Swarthmore College
Fri., Sept. 17, 2010, 12:45 PM
Large scale structures can spontaneously emerge from turbulent systems in a process called self-organization. Many systems in nature exhibit this behavior and plasmas are no exception. Recent results in plasma turbulence, self-organization, and relaxation from the Swarthmore Spheromak Experiment will be presented.