Past Colloquium Speakers
Xavier Siemens, University of Wisconsin-Milwaukee
Wed., Dec. 2, 2015, 4:30 PM
For the last decade, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been using the Green Bank and Arecibo radio telescopes to monitor millisecond pulsars. NANOGrav aims to directly detect low-frequency gravitational waves which cause small changes to the times of arrival of radio pulses. In this talk I will discuss the work of the NANOGrav collaboration and our sensitivity to gravitational waves from astrophysical sources. I will show that a detection is possible in the next few years.
See Prof. Siemens's website for more information about his research.
James Saxon ('10), University of Chicago
Fri., Nov. 20, 2015, 12:30 PM
This talk will begin with a brief exposition of the Standard Model, outlining in particular the Higgs Mechanism and the origin of masses of elementary particles. The majority of the talk will focus on the ATLAS experiment and the discovery and early measurements of the Higgs boson. The talk will conclude with an overview of current questions in the field and plans in the second run of the LHC.
Artemis Spyrou, Michigan State University
Wed., Nov. 11, 2015, 12:30 PM
How are the heavy elements synthesized in the cosmos?
Is the merging of two neutron stars a source of heavy element nucleosynthesis?
How do supernovae explode?
Questions like these are driving the field of Nuclear Astrophysics, where astrophysical observations and modeling meet nuclear physics experiments and theoretical calculations. Stellar observations provide new evidence of nucleosynthesis in different astrophysical environments and modeling of these environments requires an accurate description of the nuclear physics involved in these calculations. Nuclear reactions, radioactive decay and the properties of individual nuclei are important components of this complex puzzle and major effort is devoted from experiment and theory to address this need. This talk will focus on the nuclear physics aspects of heavy element nucleosynthesis in different explosive environments. I will present experiments performed at the National Superconducting Cyclotron Laboratory, a rare isotope beam facility, and I will discuss recent results and new initiatives.
See Prof. Spyrou's website for more information about her research.
Marc Kamionkowski, Johns Hopkins University
Fri., Oct. 30, 2015, 12:30 PM
Our existing physical laws are unable to explain several features of the observed Universe. The nature of the dark matter that holds individual galaxies together and of the dark energy that drives different galaxies away from each other both require new physics beyond the Standard Model and general relativity. The preponderance of matter over antimatter likewise requires some new baryon-number violation beyond that in the Standard Model. Explanations for the primordial density inhomogeneities observed in the cosmic microwave background all involve new physics. I will review these questions, discuss some existing avenues to make progress, emphasizing several ways in which considerations of symmetry and geometry may play a role in the quest for new cosmological physics.
See Prof. Kamionkowski's website for more information about his research.
Catherine Deibel, Louisiana State University
Fri., Oct. 2, 2015, 12:30 PM
At the birth of our Universe, the Big Bang produced the initial abundances of hydrogen, helium, and lithium that are seen in our Galaxy today. All other elements, however, were synthesized in stellar environments through nuclear processes. Many of these heavy elements were produced in violent stellar explosions, such as classical novae, X-ray bursts, and supernovae, that are driven by nuclear reactions. This nucleosynthesis, which continues in our Galaxy, can be understood through the combination of stellar observations, computational physics, and experimental nuclear physics. Specifically, the study of these nuclear reactions in the laboratory has undergone signicant advancements with recent developments in radioactive ion beam facilities and detector technology, which have allowed experimental work on isotopes that do not naturally occur on Earth. I will discuss these recent advancements and experimental results using specific examples of key nuclei and nuclear reactions that occur in several types of stellar explosions, including X-ray bursts and classical novae, in the context of the chemical evolution of our Galaxy.
See Prof. Deibel's website for more information about her research.
Stephon Alexander, Dartmouth College
Fri., Apr. 17, 2015, 4:30 PM (in SC 128)
It is interesting that theories beyond the standard model of particle interactions, such as string theory, generically predict modifications to Einstein's theory of general relativity that are left-right asymmetric (parity violating). In this colloquium I provide a pedagogical discussion of the possibility that parity violating primordial gravitational waves, which are produced during the epoch of cosmic inflation, can generate the observed matter anti-matter asymmetry and play a second crucial role in actually ending the epoch of cosmic inflation. I discuss the potential for detecting this form of parity violating gravitational waves in future CMB missions.
Robert Wicks, NASA GSFC
Fri., Feb. 6, 2015, 12:30 PM
The solar wind is the hot, tenuous and turbulent plasma that is emitted by the Sun. I will review how the solar wind is formed and how the solar wind and solar variability cause space weather at the Earth. I will then describe some recent results looking at the small-scale variability and turbulence of the solar wind and what we have learned about plasma physics from launching spacecraft into deep space. We will try to answer questions like: How big is the solar system? What is a solar flare? What is space weather and should we worry about it? Finally, I will discuss upcoming missions and opportunities for students who would like to work at NASA in the future.
See the Goddard Space Flight Center's website for more information about the physics research done there.
Probability Processing, Probabilistic Programming, and Live Chicken Fresh Killed: Stories of Science Entrepreneuring in Graduate School, Industrial Research, and Startups
Ben Vigoda, Gamelan Labs
Fri., Oct. 24, 2014, 12:30 PM
Ben Vigoda (Swarthmore Physics ’96) will talk about his experiences building interesting new things at the MIT Media Lab, industrial research labs, and startups, and will be available for questions and discussion. He and Jake Neely (Swarthmore Physics Class of '13) will also talk excitedly about their newest startup, Gamelan, that combines probabilistic programs, statistical physics, machine learning, and advanced compiler technology to enable “big models”, the next step after big data.
See the Gamelan Labs website for more information.
Michael Brown, Swarthmore College
Fri., Sep. 19, 2014, 12:30 PM
Turbulent fluctuations in conventional fluids like air or water seem to have a universal statistical character. Measurements of statistical turbulence metrics in a wind tunnel or in a tidal basin seem to be the same. It is not known whether plasma turbulence is universal. The Sun launches a turbulent stream of high velocity plasma with imbedded magnetic fields into interplanetary space at about 400 km/s. Properties of this solar wind have been studied for decades and much is known about fluctuations in the velocity, density, and magnetic field. In this talk, I'll discuss measurements in a high velocity plasma wind tunnel at Swarthmore. The SSX MHD wind tunnel features flow speeds up to 100 km/s, magnetic field of 0.5 T, and temperatures of nearly a million Kelvin. Comparisons to measurements from the solar wind will be made.
See Prof. Brown's website for more information about his research.
Gregory Adkins, Franklin and Marshall College
Fri., Mar. 28, 2014, 12:30 PM
Positronium is the exotic atom composed of an electron bound to its own antiparticle, the positron. Positronium is like other atoms, such as hydrogen, in being bound by the Coulombic attraction between particles of unlike charge. It is simpler than hydrogen or other atoms because its constituents are, so far as we know, point-like particles with no internal structure. Positronium is accessible both to high-precision measurements (of energy levels, decay rates, etc.) and to precise calculations based on current theory, so that positronium provides an important test of bound-state methods in Quantum Electrodynamics and a possible window onto new physics.
In this talk I will describe the dominant features of positronium physics in terms of basic quantum mechanics and relativity and will then discuss both refinements in the theory and also the ongoing experimental tests and challenges.
Katherine Aidala, Mt. Holyoke College
Fri., Feb. 7, 2014, 12:30 PM
Magnetic random access memory (MRAM) would combine the benefits of the hard drive (non-volatile, cheap, high density of bits) with the benefits of RAM (fast, mechanically robust). One proposal for MRAM involves the vortex state of nanorings, a state in which the magnetic moments align circumferentially in the clockwise or counterclockwise direction. For a symmetric ring in a uniform field, these states are energetically degenerate and cannot be selected experimentally. A circular field allows us to study the switching behavior between these vortex states. We have developed an experimental technique to apply a local circular field by passing current through the tip of an atomic force microscope. I will discuss how the atomic force microscope works, our experimental results demonstrating switching between the vortex states, and our understanding of the evolution of these states based on our simulations. We predict novel states that arise from both energy minimization and topological constraints.
See Prof. Aidala's website for more information about her research.
Eliza Kempton, Grinnell College
Fri., Dec. 6, 2013, 12:30 PM
Astronomers currently know of close to 1,000 planets orbiting distant stars beyond the confines of our solar system. Of these "extrasolar" planets, most are large gas-rich planets, similar to Jupiter or Saturn. However, more recently, due to improvements in discovery techniques and instrumentation, astronomers have started to discover much smaller extrasolar planets, which are only slightly larger (or more massive) than the Earth. This new class of planets, which have masses of 1-10 times that of the Earth, have come to be known as super-Earths. Super-Earths are particularly interesting because planets in this mass range are not present in our solar system, and they therefore represent a fundamentally new class of planets for astronomers to study. Recently, the first observations of a super-Earth atmosphere were obtained. They reveal a unique planet that does not seem to resemble anything found in our own solar system. In this talk I will begin by presenting an overview of extrasolar planet research, focusing in on what we know about super-Earths and their atmospheres. I will finish by presenting the first observational constraints on super-Earth atmospheric composition and structure, and I will explain some of the challenges to interpreting the available data.
See Prof. Kempton's website for more information about her research.
Peter Collings, Swarthmore College
Fri., Nov. 15, 2013, 12:30 PM
The huge liquid crystal display (LCD) industry relies on oil-based liquid crystals to produce the impressive displays that we use every day. As a result, the properties and behavior of oil-based liquid crystals are well understood. Much less is known about water-based liquid crystals, yet they are being investigated due to novel applications in biology and medicine. One can start to understand the properties and behavior of water-based liquid crystals by exploiting the same conditions used to fabricate liquid crystal displays, but use water-based liquid crystals instead. This has led to new knowledge of how water-based liquid crystals interact with surfaces and how they respond to the addition of twist-inducing agents.
See Prof. Collings' website for more information about his research.
William Wootters, Williams College
Fri., Nov. 8, 2013, 12:30 PM
Quantum mechanics is a probabilistic theory, but the way we compute probabilities in quantum mechanics is quite different from what one would expect from, say, rolling dice or tossing coins. To get a quantum probability, we first compute a complex-valued probability amplitude and then square its magnitude. I begin this talk by looking for a deeper explanation of the appearance of probability amplitudes, or "square roots of probability," in the physical world. It turns out that one can find a potential explanation-it is based on a principle of optimal information transfer-but the argument works only if the square roots are real rather than complex. I then explore a particular theoretical model in which the probability amplitudes are taken to be real and the usual complex phase factor is replaced by a binary quantum variable. One finds that the model leads to a one-parameter generalization of standard quantum theory.
See Prof. Wootters' website for more information about his research.
Elizabeth Rhoades, Yale University
Fri., Oct. 4, 2013, 12:30 PM
In contrast to globular proteins, intrinsically disordered proteins do not form stable, compact structures under physiological conditions. Rather, often their functions are derived from their properties as extended, flexible polymers. It has recently been recognized that intrinsically disordered proteins are involved in a range of functional roles in the cell, as well as being associated with a number of diverse diseases, including cancers, neurodegenerative disorders, and cardiac myopathies. We use single molecule fluorescence approaches to characterize both the 'structures' and dynamics of disordered proteins implicated in the progression of Parkinson's and Alzheimer's diseases. Our goal is to understand how disease-associated modifications to these proteins alter their conformational and dynamic properties and to relate these changes to disease pathology.
See Prof. Rhoades' website for more information about her research.
David Busch, University of Pennsylvania
Fri., Sep. 13, 2013, 12:30 PM
Tissue oxygen delivery is intimately dependent on blood flow, concentration, and oxygen saturation. However, these important physiological parameters are currently difficult or impossible to measure non-invasively in critically ill patients. Diffuse optical techniques utilize near infra-red light to provide a window into tissue hemodynamics. These tools can be integrated into intensive care units and applied to fragile patients. Our current studies include serial measurements of hemodynamics in patients following pediatric stroke and corrective surgery for congenital heart defects. Ultimately, we seek to provide tools to permit physicians measure the effects of their interventions on cerebral oxygen perfusion.
See Dr. Busch's website for more information about his research.
David Schaffner, Swarthmore College
Fri., April 26, 2013, 12:30 PM
The goal of achieving a nuclear fusion reactor has spurred a great deal of investigation into the nature of turbulence and the transport of particles and energy across magnetic field lines in plasmas. Since one of the requirements for having a sustained fusion reaction is the ability to confine plasma particles and heat within a small region, the physics of how particles and heat escape this confinement has been researched extensively. One method for improving confinement in magnetized plasmas is through the application of shear flow which can modify the turbulent fluctuations of a plasma in such a way as to suppress the average transport of particles out of the plasma. The Large Plasma Device (LAPD) at UCLA is a test-bed for conducting basic plasma turbulence experiments and has been utilized in the exploration of improving plasma confinement. The experiment consists of a long, pulsed column of plasma produced by accelerating electrons off of a hot cathode into a 20 m long by 1 m wide chamber filled with helium gas. Recent results have shown that the confinement level of plasma can indeed be improved by induced rotation of the plasma and that increased confinement scales with increased rotational shear.
Wynter Duncanson, Harvard University
Wed., April 17, 2013, 4:30 PM
Ultrasonic imaging is one of the primary means of non-invasively visualizing structures within the human body. Similarly, seismic imaging remains the dominant means of imaging the subsurface geology of oil reservoirs. Both these acoustic imaging techniques can be improved through the addition of a contrast agent. The most suitable contrast agents are gas bubbles; gas bubbles provide the contrast to strongly reflect or scatter sound. However, gas bubbles must be stabilized against dissolution; yet, conventional fabrication techniques do not provide control of their physicochemical characteristics to enable this. I will describe the assembly of stabilized bubbles using droplet microfluidics; this technology provides precise control over the composition of the gas core and a stabilizing shell. I design new types of bubbles by integrating various stimulus-responsive materials into their structure; these 'smart' bubbles adapt to changes in their environment. Moreover, I can easily tune the mechanical properties of the shell to enable bubble deformation and passage through the pore space and I demonstrate methods to accomplish and measure this. In addition, I can create bubble structures that withstand large hydrostatic pressures making it possible to use them as contrast agents in oil reservoir applications. I envisage these new 'smart' bubble contrast agents will lead to improved and more sensitive acoustic imaging, both in the human body and in oil reservoirs.
Further information is available on Dr. Duncanson's website.
Tristan Smith, Lawrence Berkeley National Laboratory and the University of California
Fri., Mar. 8, 2013, 12:30 PM
In the short span of 40 years, cosmology has transformed from a purely theoretical field to one overflowing with increasingly precise data. As a result, our picture of how the universe came into being and how it evolves has come into near-perfect focus: it seems as though, after thousands of years of thought, we may be a few short decades away from understanding the true nature of our ultimate origins. Although correct in certain respects, this sense of understanding may not be as founded as we would hope. Contained within our current picture of the universe are several ideas, which are no more than place-holders waiting for a deeper level of understanding. By far the most puzzling of these place-holders is what we generally refer to as "dark energy": its existence is beyond a doubt and it fundamentally challenges long-held ideas about the fate of the universe and many basic principles upon which all of physics is based. I will introduce the "problem" of dark energy and focus on work I have done in order to explore its fundamental nature using cosmological and astrophysical observations.
Nelia Mann, Reed College
Wed., Mar. 6, 2013, 4:30 PM
The techniques of old quantization were successfully used as an approximation to the real world for many years before the advent of modern quantum mechanics, but are often forgotten or neglected today. I will review the application of this method to simple quantum mechanical systems with known analytic results, and show that old quantization gives useful information. I will then discuss its application to a pair of more sophisticated systems (a logarithmic potential and a Yukawa potential) where analytic solutions are not possible, and compare the results with those obtained using numerical methods. In all cases old quantization provides good predictive power for the main features of the true quantum mechanical problem, and thus can serve as a "back-of-the-envelope" technique for easily providing rough, qualitative information about the system.
Kate Jones-Smith, Reed College
Fri., Mar. 1, 2013, 12:30 PM
According to the current cosmological paradigm, the universe is dominated by a mysterious form of 'dark energy', about which very little is known. This dark energy reveals itself indirectly through astrophysical observations, which are bolstered by a number of theoretical considerations. Many models of dark energy have been proposed, but the astrophysical observations alone are not precise enough to distinguish among them. One class of models that has become popular in recent years is the so-called chameleon scalar field, which can actually be constrained with terrestrial/laboratory experiments. In this talk I will show that the chameleon model of dark energy obeys an electrostatic analogy, and describe how the analogy might be used in experiments to increase their range of sensitivity, thereby opening up the exciting possibility of detecting dark energy in the lab.
Leo Rodriquez, Grinnell College
Wed., Feb. 27, 2013, 4:30 PM
The concept that black holes behave as thermodynamic objects was first realized after the formulation of the laws of black hole mechanics by Bardeen, Carter and Hawking during the 1970's. Since then, black hole temperature and entropy have provided an ample testing bed for most current competing theories of quantum gravity. The widely accepted forms of these thermodynamic quantities are:
T_H = hbar*kappa/2pi (Hawking Temperature)
S_BH = A/(4*hbar*G) (Bekenstein-Hawking Entropy)
where A is the black hole surface area, kappa the black hole surface gravity, hbar is Planck's constant and G is Newton's constant. It is widely believed that any viable ultraviolet completion of general relativity (quantum gravity) should reproduce some variant of the above equations. To date there is a plethora of different approaches for arriving at these formulae, with string theories and loop quantum gravity the predominant competitors. However, no candidate approach has ever yielded a complete formulation of a possible quantum gravity theory and there seems to be no clear consensus which approach to prefer over the other.
In this talk we will outline the problem of quantum gravity, why black holes behave as thermodynamic objects and why they are a useful tool in the study of a yet unsolved problem. In particular we will highlight the AdS/CFT approach pioneered in string theories, for studying two dimensional near black hole horizon quantum conformal field theories relevant to four dimensional black hole thermodynamics.
X-ray Spectral Measurements of the Most Massive Stars: Stellar Wind Mass-Loss Rates and Shock Physics
David Cohen, Swarthmore College
Wed., Feb. 20, 2013, 4:30 PM
The most massive stars in the Galaxy burn brightly, live very short lives, and explode as supernovae. The intensity of the light they emit throughout their lives drives an outflow of material from these massive stars' surfaces. These so-called stellar winds inject momentum, energy, and nuclear-processed matter into the Galactic environment. And they also are strong enough that they can appreciably reduce the mass of a star over the course of its life, affecting supernova statistics and properties. I will discuss how my research group has developed a technique for using high-resolution X-ray spectroscopy to measure the wind mass-loss rates of some of the most massive and luminous stars in the Galaxy. Along the way, we are able to test models of shock-heating and X-ray production in massive star winds and also to constrain the degree of wind clumping and porosity.
Further information is available on Prof. Cohen's website.
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.