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Past Colloquium Speakers

Fall 2023

When a Clock (Transition) Is Good for More Than Telling Time

Jonathan R. Friedman, Amherst College
Friday, December 1 at 12:45

The burgeoning field of quantum information requires the precise control of the quantum state of a qubit.  Quantum states are generally described by complex amplitudes in which a well-defined phase is paramount:  When information about the state’s phase is lost to the qubit’s environment – a process known as decoherence – the utility of the state for quantum information applications is also lost.  Thus, protecting a qubit from the effects of its environment is important in nearly all aspects of quantum information science.  One method of achieving decoupling of a system from its environment is through the use of so-called clock transitions, a name arising from their original use in precision atomic clocks. A clock transition occurs at an avoided level crossing where the transition frequency is independent (to first order) of an external perturbation, such as a fluctuating magnetic field. In spin-qubit systems, a bath of environmental spins can lead to decoherence and a suppression of T2, the characteristic phase decoherence time.  In molecular nanomagnets, which are chemically synthesized spin systems, clock transitions can be engineered by exploiting the symmetry of the molecule.  I will discuss how T2 can be markedly enhanced by clock transitions in some molecular nanomagnets, suggesting these systems can be viable spin qubits.  The study of such systems can also elucidate the mechanisms of decoherence since spin-spin interactions are suppressed at a clock transition, allowing us to isolate the decohering effects of such interactions from those due to other mechanisms.

Mapping the Milky Way with Neutrinos

Steve Sclafani, University of Maryland
Friday, November 3 at 12:45

IceCube neutrino observatory is a cubic-kilometer neutrino detector located at the geographic South Pole. It first identified neutrinos from outside our solar system in 2013, and in last decade two active galaxies have been identified as neutrinos sources. However, the majority of the flux remains unexplained. Recently a new sample of neutrinos has been used to make the first observation of the Galactic plane in high-energy neutrinos.  These neutrinos can be produced when cosmic rays interact at their acceleration sites and during propagation through the interstellar medium. This observation was enabled by machine-learning techniques that improved the selection efficiency and angular resolution of cascade-like neutrino events in IceCube.

Detecting Gravitational Waves with Pulsar Timing Arrays

Tristan Smith, Swarthmore College
Friday, September 22 at 12:45

Gravitational wave astronomy became a reality in 2015 with the first detection of gravitational waves from a pair of black holes that had smashed into each other. This milestone (and the hundreds of events detected since then) have all been measured using 4km-long interferometers sitting on the earth’s surface. These instruments are sensitive to gravitational waves with frequencies around 100 cycles per second, which limits the type of black holes they can detect. Pulsar timing arrays constitute a galaxy-sized gravitational wave detector which can potentially probe frequencies as low as one cycle every 10 years, giving us access to black holes with masses as much as one billion times that of the sun. On 29 June 2023, the NANOGrav Collaboration, the European Pulsar Timing Array, the Parkes Pulsar Timing Array, and the Chinese Pulsar Timing Array, all announced the detection of gravitational waves (that earth-based detectors had not sensed). I will discuss how pulsar timing arrays fit into gravitational wave astronomy, point out some curious aspects of these first detections, and describe what we hope to learn about the universe with this new and extraordinary tool.

Spring 2023

The Magnetic Mirror Concept for Fusion Energy

Michael Brown, Swarthmore College
Fri., April 14th at 12:45

There have been several recent developments in the quest for fusion energy: bringing a star to earth.  First, there have been experimental breakthroughs in both of the mainline concepts, laser and magnetic fusion.  Second, enabling technologies have been developed in the past decade that have resurrected previously discarded concepts.  In particular, low-cost high-temperature superconducting tape has enabled the production of 20 Tesla magnets for fusion.  Finally, private investors have started fusion companies with funding far exceeding that by the Federal Government.  In this talk, I'll review the latest developments in fusion energy at NIF and JET, and discuss a new fusion device we are building at the University of Wisconsin.  The device is called WHAM: Wisconsin High-field Axisymmetric Mirror.

The Many Scientific Facets Of Glass: From Diatoms To Nuclear Waste  

Mario Affatigato, Coe College
Fri., March 24 at 12:45

Glass has a long history that mirrors the rise of civilization and technology.  In many cases, it has enabled the development of science, while in others it has become a commodity used by millions.  In this talk we will focus on a variety of applications of glass, emphasizing the scientific reasons why it has reappeared throughout history as a cuttingedge material.  We will discuss its use in biomedical applications like cancer treatments and soft-tissue wounds; the use of glass for nuclear waste encapsulation; the many optical applications of glasses; naturally occurring glasses on Earth and in the Moon; its role in energy storage; and other fascinating roles.  The presentation will leave time at the end for a broad range of questions about this incredible material.  
Time permitting, we will also cover some of the other, ongoing work taking place in the Affatigato research group, especially the manufacturing of glass using aerolevitation.  

Searching for Dark Matter Axions

Samantha Lewis, Fermilab
Wed., February 22 at 4:30
Scheuer Room

Decades of observations and experiments have shown that there is much more matter in our universe than what we can directly see. What this "dark matter" is made of is one of the major outstanding questions of modern physics. In recent years, the axion has become an increasingly popular dark matter candidate. This hypothetical particle is expected to convert to a photon (light) when it enters a magnetic field. Scientists are searching for axions by looking for these converted photons. The mass of the axion is unknown, so scientists must use a variety of techniques to search over the possible range. One type of experiment, the haloscope, aims to detect axions from our galaxy’s dark matter halo using electromagnetic cavities. This talk will provide an introduction to dark matter, axions, haloscopes, and other research efforts dedicated to finding this elusive particle.

Frustrated Magnetism, Spin Liquids, and the Tensor Network Toolbox

Aaron Szasz, Lawrence Berkeley National Laboratory
Thu., January 26 at 4:30

At a macroscopic scale, we identify a liquid by its ability to flow and change shape; at the microscopic level, a liquid is characterized by the randomness or disorder in the locations of its constituent atoms and molecules.  In the same way, the quantum spins of electrons can be disordered even when the electrons themselves are rigidly fixed in place in a solid crystal.  This behavior describes a new and exotic phase of matter, called a “spin liquid.”  Spin liquids can arise due to “geometric frustration,” where due to the specific geometry of a crystal, competing forces on the electrons prevent the otherwise expected formation of magnetic order. 

Many materials that have been identified in experiments as possible spin liquids are approximately described by a particular theoretical model of frustrated electrons, the triangular lattice Hubbard model.  Using state-of-the-art computer simulations based on so-called tensor networks, I show that this model gives rise to a particular type of spin liquid, the chiral spin liquid, alongside metallic and magnetic phases.  I likewise show that small modifications that make the model more realistic lead to additional, distinct types of spin liquids.

Decomposing the Links Between Clouds and the Large-scale Tropical Circulation

Levi Silvers, Stony Brook University
Mon., January 23 at 4:30

Climate models have become a critical tool in our quest to better understand the climate of Earth and how we might expect the climate to change in the future.  Climate models are built by using numerical methods to solve the governing dynamic and thermodynamic equations of the Earth system.  I will give a brief introduction to climate models before describing my research.

The large-scale overturning tropical circulations are intertwined with multiple cloud types including deep convection, cirrus anvil clouds, and low-level stratus cloud decks.  Meridionally oriented overturning circulations are often described in the context of the Hadley circulation and zonally oriented circulations are often described in the context of the Walker circulation.  A simple but common method of defining the Hadley and Walker circulations relies on zonal and meridional spatial averages over the regions of interest.  This method results in substantial overlap between the circulations and complicates the interpretation of each.  

In this work, I partition the horizontal divergent wind field into two independent and orthogonal circulations, an alternate definition of the Hadley and Walker circulations.  Results are then sampled into particular dynamical regimes.  I then combine the Hadley and Walker Circulations with the cloud radiative effect to calculate the cloud characteristics for the Hadley and Walker circulations. With the goal of connecting idealized tropical models such as radiative convective equilibrium (RCE) to more realistic global climate models, we analyze a wide range of model configurations that include simple models of the tropical atmosphere, models of planets with no land, and so-called comprehensive Earth system models.  I will present the range of results across the different types of models.

Journey to the Center of a Neutron Star: Using Astrophysical Observables to Study Ultra-Dense Matter

Carolyn Raithel, Princeton University
Thu., January 19 at 4:30

Neutron stars are extreme objects. Formed in the core-collapse supernovae that mark the deaths of massive stars, they contain the densest matter found anywhere in the Universe, the strongest magnetic fields known to naturally occur, and are the progenitors of the most energetic electromagnetic phenomena observed. As a result, they provide a unique laboratory for studying matter under extreme conditions. In this talk, I will discuss a computational framework for connecting astrophysical observations of neutron stars to the physics governing their interior structure. I will take a multi-messenger perspective, highlighting recent results from X-ray observations, radio pulsar timing, and, especially, gravitational wave (GW) astronomy. I will discuss what we have already learned about neutron star structure from the first few detections of GWs emitted by merging neutron stars, and what we hope to learn from the next generation of proposed experiments. Along the way, I will present recent results from numerical simulations of neutron star mergers to highlight some of the key open questions and challenges that lie ahead.

Fall 2022

X-Raying Stellar Winds to Diagnose Their Mass-Loss Rates

David Cohen, Swarthmore College
Wed., November 30 at 4:30

The massive stars my research group studies have very strong winds – steady and fast outflows of gas from their surfaces – that are driven by the momentum in the light of these stars. The spectrum of light emitted by these winds also carries a huge amount of diagnostic information. In this talk I will tell you about a surprising discovery Victoria Wang ('22) and Gwendolyn Rak ('22) made while analyzing the X-ray emission from the wind of a particular nearby massive star, zeta Puppis, using spectral data from NASA's Chandra X-ray Observatory. Puppis is the Poop Deck – a constellation that used to be considered part of the larger constellation, Argo Navis (think Jason and the Argonauts), but was broken up into Puppis, Vela (the Sail), and Carina (the Keel) in the 18th Century. Astronomy's many connections to culture in addition to a love of colored light are two of the things that started me on my journey from nerdy kid to nerdy scientist and professor.

Using transmission electron microscopy to image atoms and develop new energy materials

Eric Stach, University of Pennsylvania
Fri., November 11th at 12:45

Nanoscience has seen dramatic advances in the past two decades due to significant investments in research and development from governmental and industrial entities. The ability to image and manipulate matter at the atomic scale has been a critical enabler in these advances. In this presentation, I will describe how one tool – the transmission electron microscope – has allowed routine atomic-scale characterization of materials through instrument developments in image, energy, and time resolution. The talk will be general and try to draw parallels to recent advances in imaging the universe via improved telescopes and show how developments in particle physics have played a part in these instrument improvements. Finally, I will talk about some of the work in my research group that uses these instruments to understand better how energy materials such as battery electrodes and catalysts for chemical conversion function.

Pattern Formation Mechanisms on Single Cell Surfaces

Asja Radja, Bryn Mawr College
Fri., October 28 at 12:45 pm

Life is organized at many length scales ranging from the molecular to the macro, and there is an incredible amount of complexity in the morphologies at these various levels of organization. Historically, biologists have looked to the genetic code for instructions to reveal the secrets of self-assembly. However, once DNA is transcribed and translated to proteins and subsequent other macromolecules, there is no known “code” that dictates how these products interact. To discern the principles of this self-organization, we must identify the physical phenomena that govern the soft, disordered, and often far-from-equilibrium living world. Here I discuss the elucidation of a rule of life that dictates how single-cell surfaces are patterned through phase separation that leads to a variety of shapes and adds to our understanding of how life builds itself.

The Lasting Impact of Introductory Physics for Life Sciences (IPLS)

Benjamin Geller, Swarthmore College
Fri., September 23 at 12:45 pm

Two primary goals of Introductory Physics for Life Sciences (IPLS) curricula are (1) to prepare students to effectively use physical models and quantitative reasoning in biological and biomedical contexts, and (2) for students to come to view physics as relevant to the life sciences. To assess whether these goals are being met, we conducted a longitudinal interdisciplinary study of the impact of IPLS on student work in later biology and chemistry courses, and on student attitudes toward physics. In this talk I will report on differences in student reasoning and attitudes that we found to be associated with prior or concurrent enrollment in IPLS. In particular, we found that IPLS students were more likely than non-IPLS students to reason quantitatively and mechanistically about particular biophysical phenomena, even up to two years after leaving the IPLS course, and were significantly more successful at building a physical model that combined ideas in a manner they had not previously seen. We also found that positive changes in IPLS students’ attitudes about the relevance of physics persisted for at least two years after the course ended.

Spring 2022

Galaxies Swept Under the Dust

Jesse Rivera, Visiting Assistant Professor of Astronomy, Swarthmore College
Fri., April 8 at 12:30 pm

Dusty star-forming galaxies (DSFGs) contribute significantly to the cosmic star-formation history of the universe, and are likely progenitors of nearby elliptical galaxies; however, they are challenging to study in detail, in part because their dusty nature makes them difficult to localize at optical wavelengths. Large-area surveys from far-infrared through millimeter wavelengths have proved to be effective at identifying DSFGs that are unusually bright (i.e., gravitationally lensed), and thus easier to study. I will present past and preliminary results on DSFGs selected from a survey of a 470 deg2 equatorial field with the Atacama Cosmology Telescope (ACT), focusing on two systems with spatially resolved mapping of CO(3-2) emission from the Northern Extended Millimeter Array (NOEMA) as well as Hubble Space Telescope (HST) imaging. Lens modelling using a pixel-based algorithm that can be applied to multiple independent velocity channels yields source-plane reconstructions that support the characterization of the morphologies, physical conditions, and kinematics of these extreme star-forming systems. I will also dedicate time to discuss and reflect on my professional trajectory and the many decisions I made throughout my career that led to where I am at presently.

Physics Is More Than Problem-solving: Building Inclusivity and Belonging by Practicing Professionalism

Marty Baylor, Associate Professor and Department of Physics and Astronomy Chair, Carleton College
Thu., March 24, 2022, 4:30 PM

When I’ve asked potential physics majors to reflect on what it takes to be a physicist, their responses essentially write themselves out of the identity that they seem to be striving for. Their narrow definition of what it takes to be a physicist can lead students, particularly students whose identities are under-represented in STEM to feel as if they don’t belong. To help broaden student’s definition of who a physicist is so that they can see themselves in the field now, I have been developing what I am calling the Practicing Professionalism Framework for my courses. This Framework not only helps me help students understand the course content, structures, and skills better, but it also helps motivate in a natural and compelling way that being a physicist is broader than the ability to solve problems and doesn’t require them to win a Nobel Prize. In this talk I will not only present this framework, it’s implementation and its impact on students, I will also describe how I believe it can be expanded beyond physics to other STEM disciplines.

A Random Walk into Optical Signal Processing and Integrated Optofluidic Devices

Marty Baylor, Associate Professor and Department of Physics and Astronomy Chair, Carleton College
Fri., March 25, 2022, 12:45 PM

Light not only has the power to help us learn about the physical world, but it can also help us solve problems that are hard to solve in other ways. I fell into optics through what felt like a winding path to me. As I share about my path from my childhood dreams of being a paleontologist to my current work using photopolymers for integrated optofluidic devices, I will periodically pause to describe the varied optics work I have done along the way at Kenyon College, NASA Goddard Space Flight Center, and CU-Boulder. During this talk, I invite the audience to consider the role of power and privilege in helping me arrive at where I am today.

A Novel Administration of a Sensemaking Instrument in Physics Education

Stephen Hackler, Visiting Assistant Professor, Swarthmore College
Fri., Feb 4, 2022, 12:45 pm

Note: Due to current Covid restrictions, this talk will not be open to visitors. Lunch will be available at 12:20 in the Cunniff Lecture Hall Lobby.

There are numerous survey instruments used to examine student performance over the course of a semester, most of which assess content knowledge. The Math Epistemic Games Survey is a diagnostic instrument for gathering information on student sensemaking in introductory physics course, noted for its length and rigor. Previous research has suggested that student effort (and therefore the fidelity of results) may be improved by shortening the instrument. Through an exploratory factor analysis on MEGS data, we identify 10 key factors and divide each to produce two, half length, mutually exclusive tests. We then administer these half tests to a group of approximately 600 undergraduate introductory physics students at the University of Pennsylvania as pretests and posttests and compare student performance. Our results show that student performance on each test is distributed extremely similarly and that average performance is slightly above that found in previous studies. We therefore conclude that our proposed split administration produces a reliable instrument, which preserves the original validity of the MEGS, and may improve on some of the identified shortcomings of the full MEGS.

Fall 2021

Coordinating Adhesion with Repulsion: How Cells Use Polymer Brushes to Orchestrate Life

Jennifer E. Curtis, Associate Professor, Georgia Tech Physics, College of Sciences
Thu., Nov 18, 2021, 4:30 pm

Multi-cellular organisms rely on reversible adhesions to orchestrate the motion and organization of cells. To date, the physics of tissue has primarily focused on the molecular adhesions between cells and the balance of forces throughout the tissue.  In this talk, I will introduce an important but neglected physical mechanism that cells may use to break or weaken cellular adhesions in a controllable, dynamic fashion. At the heart of this control is cells’ ability to rapidly extrude giant sugar polymers to form a polymer brush-like structure at cell interfaces. I will present experiments confirming that the repulsive forces generated by this compressed brush (glycocalyx) substantially modify the adhesive state of cells. Then I will show how we hijacked the cell’s glycocalyx enzymatic machinery to generate a novel class of ultra-thick polymer brush and in turn, how we have used these tunable brushes as a model system to systematically explore the forces exerted by glycocalyx on adherent cells. Adding to the evidence from our other biophysical assays, these experiments clearly demonstrate that sugar enzymes can push polymer into tight confined spaces and mechanically drive cell deformation. In light of the observed upregulation of glycocalyx synthesis in biological events that require adhesion modulation, ranging from embryogenesis to synaptogenesis, I will conclude with the hypothesis that controlled expression of sweet but repulsive polymers at the cell interface may play a serious role in tissue organization and regulation in multicellular organisms.

Evolutionary dynamics in microbial populations: insights from DNA sequencing and statistical physics

Benjamin H Good
Assistant Professor of Applied Physics, Stanford University
(Swarthmore 2010; Physics and Mathematics major)
Fri., Oct 29, 2021, 12:45 PM, SC 199

Underneath the fascinating biology, evolution is governed by statistical processes that describe how mutations arise and spread within a population. It is now possible to watch this process unfold in real time in rapidly reproducing populations of viruses and bacteria. In this talk, I will show how concepts and approaches from physics (e.g. random walks) can help us make sense of this exciting but counterintuitive new data. I will describe some of our recent efforts to measure the ongoing evolution that takes place within the complex communities of microbes that inhabit the human gut. If time permits, I will show how simple biophysical models can help us understand how these “macroscopic” dynamics emerge from microscopic growth processes along the large intestine.

Searching for Another Earth

Suvrath Mahedevan, Penn State
Fri., Oct 1, 2021, 12:45 PM

Modern astronomical spectrometers are approaching the exquisite sensitivity to detect the signature of an Earth-mass planet around a Sun-like star, and can already do so for such planets around M dwarf stars. In this talk I shall discuss the challenges involved in making these difficult measurements with the Doppler radial velocity technique, and the evolution of the design of these instruments as they seek ever-tighter control of environmental parameters, and increased measurement precision. A suite of new technologies like frequency stabilized laser combs, low drift etalons, and deeper understanding of the detectors is enabling a new level of precision in radial velocity measurements - as well as illustrating new challenges.  I will use two such instruments we have built (HPF and NEID), to illustrate the underlying physics and measurement challenges.  I will then discuss how the stars themselves are the remaining challenge, and how magnetically driven processes create ‘stellar activity’ noise that can masquerade as planets and obfuscate their detection. I shall highlight a few paths the community is exploring to mitigate this – using our star, the Sun, as a guide.

Time permitting, I will also describe how beam-shaping diffusers are now enabling space-quality photometry from the ground to aid in photometric follow-up and confirmation of transiting exoplanets, and how the combination of precision spectroscopy and photometry is necessary to unveil planet properties.

Multi-Wavelength Studies of the Not So Perfect Clocks in the Sky

Natalia Lewandowska, Swarthmore College
Fri., Sept. 17, 2021, 12:45 PM

Pulsars are highly magnetized and fast-rotating neutron stars. They are the stellar remnants of a supernova explosion which takes place when a massive star uses up all ingredients for nuclear fusion in its interior and collapses under the influence of its gravity. A pulsar has a diameter of about 7 miles and about twice the mass of the Sun. It spins several times per second and has a magnetic field that can be a trillion times stronger than the one on Earth. As a pulsar ages it emits electromagnetic radiation ranging from radio waves, visible light possibly up to gamma-rays. Its rotation causes the emission from such a star to be received in the form of pulses on Earth (similar to rays from a lighthouse). Their regularity is the reason why some pulsars are often referred to as clocks in the sky. A small group of pulsars even competes with atomic clocks in terms of regularity.

In the present talk, I will give an overview of multi-wavelength observations of pulsars that also show an irregular form of emission known as radio giant pulses. Currently only observed in a handful of pulsars, giant pulses represent an enigmatic form of radio emission. I will give an overview of how we study them also at other wavelengths like optical, X-rays (with the Neutron Star Interior Composition Explorer, or NICER) and gamma-rays (with Imaging Air Cherenkov Telescopes and satellites like Fermi-LAT) and what such studies can unravel in terms of their potential emission mechanisms.

Spring 2021

Line Intensity Mapping: Modeling & Analysis in the Precision Era

Anthony Pullen, New York University
Fri., Apr. 30, 2021, 1:30 PM

In this talk, I discuss our efforts to increase the precision of modeling and analysis for upcoming line intensity mapping (LIM) science. I will present our work constructing realistic, multi-line LIM mocks and predicting how they can improve LIM galaxy and cosmology science. In addition, I will discuss this work's context for EXCLAIM, an LIM balloon survey that will map CO and [CII] emission.

You are invited to view this pre-recorded research talk in advance of the Zoom Q&A session. 

The talk is available at this video link.

Fall 2020

Note: talks this fall are remote - via Zoom - and are open to anyone in the Tri-Co community.

Superconducting nanowire single-photon detectors: physics and applications

Emma Wollman ('09), NASA Jet Propulsion Laboratory
Fri., Nov. 13, 2020, 1:30 PM - a video of the talk is now available

Superconducting nanowire single-photon detectors, or SNSPDs, are able to detect single photons at wavelengths from the UV to mid-IR. SNSPDs have demonstrated detection efficiencies over 95%, timing resolution below 10 ps, and dark count rates less than 1e-3 counts/s. These characteristics make SNSPDs the detector of choice for applications ranging from fundamental tests of quantum mechanics to deep space laser communication. In this talk, I’ll describe the operational principle of SNSPDs and give some examples of their applications. In particular, I’ll focus on the Jet Propulsion Laboratory’s development of SNSPD arrays for deep space optical communication and for mid-IR astronomy.

Pulsar Timing Arrays: The Next Window to Open on the Gravitational-Wave Universe

Chiara Mingarelli, University of Connecticut
Wed., Oct. 7, 2020, 4:30 PM Register to join via Zoom 

Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz to microhertz regime.  An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent limits which have emerged from international efforts to constrain astrophysical phenomena at the heart of supermassive black hole mergers.

Spring 2020

Testing inflation and constraining cosmology with cosmic microwave background measurements

Kimmy Wu, University of Chicago
Tue., Feb. 11, 2020, 4:30 PM in Science Center 199

Inflation - the leading model for the earliest moments of the time, in which the Universe undergoes a period of rapid, accelerating expansion - generically predicts a background of primordial gravitational waves, which generate a B-mode component in the polarization of the cosmic microwave background (CMB). The measurement of such a B-mode signature would lend significant support to the paradigm of inflation. However, observed B modes also contain a component from the gravitational lensing of primordial E modes, which can obscure the measurement of the primordial B modes. We reduce the uncertainties in the B-mode measurement contributed from this lensing component by a technique called 'delensing.' In this talk, I will give an update on the current delensing effort on the BICEP/Keck data, using data from the South Pole Telescope (SPT) and the Planck satellite. This analysis will tighten the constraint on the amplitude of primordial gravitational waves, parameterized through the tensor-to-scalar ratio r that is related to the energy scale of inflation. For upcoming analyses, efficient delensing relies on high signal-to-noise measurements of the CMB lensing mass map. I will show the current state-of-the-art measurement of CMB lensing using SPTpol data, its inferred cosmological constraints, and its relevance for delensing. I will then discuss on-going efforts and novel methods in making lensing mass maps using new data/simulations from current and next-generation CMB experiments. After discussing the related challenges and opportunities, I will finish with an outlook on constraining r in this decade.

Fall 2019

Observing Planet Formation

Sean Andrews, Smithsonian Astrophysical Observatory and Harvard University
Wed., Nov. 20, 2019, 4:30 PM in Science Center 199

Planetary systems form in the disks of gas and dust that orbit young stars. In the past few years, very high angular resolution observations of disks in nearby star-forming regions have started to uncover some key signatures of the planet formation epoch. This talk will focus on what we are learning about the distribution of disk material on spatial scales of only a few astronomical units, largely based on state-of-the-art measurements with the Atacama Large Millimeter/submillimeter Array (ALMA), and the corresponding implications for the assembly and early evolution of planetary systems.

The Serpent’s Maw: A Physics Perspective on Mouth Function and the Dynamics of Hydra Regeneration

Eva-Maria Collins, Swarthmore College
Fri., Nov. 15, 2019, 12:30 PM in Science Center 199

Named after the monster from the Greek mythology, the freshwater polyp Hydra is famous for its regenerative abilities. While the real Hydra is a few mm small, inconspicuous looking animal, it outperforms the fictional creature by its ability to regenerate from small tissue pieces or from cell aggregates after disintegration into individual cells. This process of self-organization of an initially near-uniform cell ball into an animal with a well-defined head-foot body axis poses fundamental questions regarding the cross-talk of biochemical and physical signaling driving organismal patterning.

Hydra’s self-organizing properties following disintegration into individual cells was first recognized more than 40 years ago. However, what drives cell sorting during regeneration remained debated as existing studies failed to distinguish between different driving mechanisms. Using a combined experimental-theoretical approach, we have recently settled this debate and shown that tissue interfacial tensions drive cell sorting. Furthermore, once sorting is complete or excised tissue pieces have rounded up, the Hydra sphere hollows out and undergoes osmotically driven shape oscillations. The sphere eventually breaks shape symmetry to form an ellipsoid, defining the future head-foot polarity of the adult polyp. It has been proposed that the shape oscillations are necessary for symmetry breaking and successful regeneration. Existing mathematical models assume that a shift in the frequency of shape oscillations of regenerating Hydra spheres coincides with symmetry breaking and axis specification. Recent work in my group breaks this link and suggests that the oscillation pattern is not indicative of symmetry breaking. Instead, the oscillation pattern shift is a direct consequence of mouth function and its use in osmoregulation. As the link between oscillation dynamics and axis specification was a key assumption in current mathematical models of Hydra regeneration, our results require that we reexamine the mechanisms driving pattern formation. 

A Crisis in Cosmology?

Tristan Smith, Swarthmore College
Wed., Oct. 30, 2019, 4:30 PM in Science Center 199

I will summarize the Hubble tension - a statistically significant disagreement between several different ways of inferring the current rate of the expansion of the universe - and my sabbatical work trying to search for a theoretical model to resolve this tension. Planned space and ground observations promise to shed light on this tension and either establish it as a clear disagreement with our standard cosmological model or as an indication that we have significantly underestimated the systematic uncertainties of a range of observations.

The Evolution of Equilibrium: Soft Matter Physics in Biology

Alison Sweeney, Yale University
Wed., Oct. 2, 2019, 4:30 PM in Science Center 199

Condensed matter physics concerns itself with the interplay between phases of matter predicted by physical theory and the realization of those organizations of matter in nature and in experiment. When it comes to soft condensed matter, arguably the best possible laboratory for finding novel phases of soft matter is in the evolution of life on Earth. This talk will focus on the identification of two arguably novel phases of matter predicted by soft matter theory in evolved systems, that of "patchy colloids" in squid lenses, and of "spatially modulated phases" in pollen grains.

Spring 2019

Thermal Effects on the Sedimentation of Macroscopic Grains

Ted Brzinski, Haverford College
Fri., Apr. 12, 2019, 12:30 PM in Science Center 199

In 1851 Stokes solved the motion of a single sphere settling in a viscous liquid at low Reynolds’ number.  It took over a 100 years, until 1961, before Brenner determined the behavior of just 2 spheres! in the following 5 decades, the collective motion of dispersions of settling grains has been the subject of active and sustained study. While we have developed empirical models that describe the dynamics of such systems, our understanding of the underlying mechanics have not much improved since Brenner. A recent metanalysis of sedimentation data revealed that the settling speeds can be collapsed onto a master curve with two distinct branches. This bifurcation suggests an exciting new window into the grain-scale interactions that lead to the bulk settling behavior of these systems - a puzzle over 150 years in the making!  I will explain these results in detail, and describe ongoing experiments intended to explore the implications of this new observation.

The Cassini Mission to Saturn: An Insider's View of an International Journey of Discovery

Richard French, Wellesley College
Thu., Mar. 28, 2019, 4:30 PM in Science Center 181

The Cassini mission to Saturn transformed our understanding of this beautiful ringed planet and its entourage of moons. Share an insider's view of the mission, from the project's inception to the final months of up-close exploration of this giant world, with the the Cassini Mission Radio Science Team Leader.

Electric Power from Earth's Rotation Through Its Own Magnetic Field

Christopher Chyba ('82), Princeton University
Fri., Mar. 8, 2019, 12:30 PM in Science Center 199

If we rotate a permanent magnet about its north-south axis, does its axially-symmetric magnetic field rotate together with the magnet? What about for an electromagnet? If the axially symmetric field does not rotate with a rotating electromagnet, does this mean that the Earth is rotating through the axially symmetric component of its own internally generated magnetic field? Then what happens? Could the resulting Lorentz force be used to drive electrons around a circuit and generate electricity, powered by the Earth's rotational kinetic energy?

Michael Faraday started asking these questions in 1831. I'll review the developments since then, concluding that the Earth does in fact rotate through the axially symmetric component of its own field. I will then present a simple proof that it is impossible to use this effect to generate electric power. Finally, I will demonstrate, and explore, a loophole in that impossibility proof.

Fall 2018

Building an Open Source Python Software Ecosystem for Plasma Physics

Nick Murphy, Harvard-Smithsonian Center for Astrophysics
Fri., Dec. 7, 2018, 12:30 PM in Science Center 199

PlasmaPy is a new community-developed open source Python package for plasma physics.  This project strives to produce the core functionality that is needed to foster the creation of a fully open source Python ecosystem for heliospheric, laboratory, and astrophysical plasma physics.  In this talk, I will tell the story of how PlasmaPy came to be. I will discuss our motivation for the project, the many lessons we have learned along the way, and the importance of community in an open source project.  I will describe the best practices for scientific computing that we are adopting. I will end with a tour of PlasmaPy’s current capabilities and plans for ongoing work.

The Earth's Surface is a Soft-Matter Physics Laboratory

Douglas Jerolmack, University of Pennsylvania
 Fri., Nov. 2, 2018, 12:30 PM in Science Center 199

The Earth's surface is composed of a staggering diversity of particulate-fluid mixtures: dry to wet, dilute to dense, colloidal to granular, attractive to repulsive particles, laminar to turbulent flows, and steady to highly-unsteady forcing. This material variety is matched by the range of relevant stresses and strain rates, from rapid and catastrophic landslides to the slow relaxation of soil over geologic timescales. Geophysical flows sculpt landscapes, but also threaten human lives and infrastructure. From a physics point of view, virtually all Earth and planetary landscapes are composed of ``soft matter''. Geophysical materials, however, often involve compositions and flow geometries that have not yet been examined in physics. I explore how a soft-matter perspective has helped to illuminate, and even predict, the rich dynamics of Earth materials and their associated landscapes. I also highlight some novel phenomena of geophysical flows that challenge, and will hopefully inspire, more fundamental work in physics.

Problems in Physics: What Are They and How Do Physicists and Students Construct Them?

Anna Phillips '09, Tufts University
Thu., Oct., 25, 2018, 4:30 PM in Science Center 181

Physicists treat well-formulated problems as knowledge objects: we discuss them, share them, and include open and solved problems in textbooks. Yet what constitutes a problem, as well as how problems come to be, is an understudied topic in philosophy of physics. In this talk, I discuss examples of historical problems in physics. I also argue that the process of formulating, articulating, and refining problems is central to the inquiry of both physicists and science students at all levels, and present examples of students formulating problems.

Sponsored by the departments of Physics & Astronomy and Educational Studies

The National Science Foundation and Other Federal Science Agencies: Promoting the Progress of Science

Vyacheslav (Slava) Lukin, National Science Foundation
Thu., Oct. 4, 2018, 4:30 PM in Science Center 183 (note time and room)

The National Science Foundation (NSF: is an independent federal agency created by Congress in 1950 "to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense..."  NSF supports fundamental research and education across all fields of science and engineering, with funds reaching all 50 states through grants to nearly 2,000 colleges, universities and other institutions.  Along with the Department of Energy (DOE), National Institutes of Health (NIH), National Aeronautics and Space Administration (NASA), and a number of other US federal agencies supporting scientific research, NSF provides the backbone of support for basic and early applied research across both mission-oriented and academic research and education institutions in the United States.

In this colloquium, I will provide a brief overview of the National Science Foundation, describe some of the frontier research areas NSF is supporting, and I will leave plenty of time for Q&A about the broader US landscape and mechanisms of support for basic scientific research.  My perspective will be based on 20 years of research experience that began in Swarthmore’s Physics & Astronomy Department and has taken me through R1 research universities, DOE & DOD national laboratories, to become a Program Director in the NSF’s Division of Physics.

Computational Methods for Approaching Slow Manifolds, Tracking Extreme Events, and Understanding Multi-Scale Fluid Dynamics

Jeff Oishi, Bates College
Fri., Oct. 5, 2018, 12:30 PM in Science Center 199

Fluid dynamics and plasma physics present a dazzling array of problems with very wide scale separation in both space and time. Such problems are typified by turbulence, which can exhibit factors of 10^12 between the largest and smallest spatial scales and 10^13 between the fastest and slowest dynamical times. These problems, and many others, are modeled using partial differential equations (PDEs). I will present a series of strategies for accurately and expediently solving the PDEs describing the motions of fluids and plasmas in order to extract physical insight into a wide variety of systems. I will focus on the solution to a pair of geophysical and astrophysical problems involving turbulent convection, magnetic fields, and rotation. In particular, I will discuss the application of a new class of models called Direct Statistical Simulation, applicable for a broad class of problems with anisotropic flows, that is those with one direction significantly different than the other two.

In doing so, I will also address the not insignificant but often overlooked challenge of putting new discoveries in applied mathematics into the hands of practitioners in a wide variety of disciplines. I will highlight the use of the Dedalus Project, a flexible toolkit for solving almost arbitrary PDEs that my collaborators and I have developed.

Turbulence and Compression Studies on the SSX Plasma Wind Tunnel

Michael Brown, Swarthmore College
Fri., Sep. 21, 2018, 12:30 PM in Science Center 199

Turbulence in conventional fluids is characterized by stochastic motion, transfer of energy from large scales to small, and ultimately viscous dissipation at molecular scales. Turbulence in highly conductive plasma is complicated by the addition of electrodynamics, motion of charged particles, and the prospect of both viscous and resistive dissipation. In this talk, I will present recent results from the SSX plasma wind tunnel at Swarthmore College. We have measured flow speeds up to 100 km/s, temperatures up to one million K, and magnetic fields up to 0.5 T in SSX. I will discuss projects aimed at understanding the dissipation mechanisms in plasma turbulence, as well as the physics of plasma compression.

Spring 2018

Plasma Science - From Laboratory Fusion to Astrophysical Plasmas

Fatima Ebrahimi, Princeton Plasma Physics Laboratory and Princeton University
Fri., Apr. 20, 2018, 12:30 PM in Science Center 199

Our universe is immersed in magnetized plasma, electrically conducting ionized gas.  Some of the most fundamental and long-standing astrophysical problems, such as the magnetization of the universe, collimation of astrophysical jets, the accretion process and transport in astrophysical disks (surrounding e.g. black holes) and their coronas can only be explored through plasma physics. Our sun as a natural laboratory for plasma physics provides inspiring as well as challenging problems, including its dynamo cycles, heating, and the replication of its core reaction, fusion energy, on earth in a lab. There is an abundance of observational/experimental data emerging from natural phenomena of space and astrophysical plasmas, as well as laboratory plasma experiments, for plasma physicists to explore. I will review some of these topics, in particular magnetic reconnection, the rearrangement of the magnetic field topology of plasmas, which energizes many processes in nature and has been shown to also be critical in the nonlinear dynamics of many processes in toroidal fusion plasmas. Using global simulations, I will demonstrate the instrumental role of magnetic reconnection, which enables an innovative technique for producing current in fusion plasmas.

The Physics of Collisionless Shock Waves

Lynn Wilson, Goddard Spaceflight Center
Fri., Apr. 13, 2018, 4:30 PM in Science Center 101

Collisionless shock waves are an ubiquitous phenomena throughout the universe including solar-generated interplanetary shocks, planetary bow shocks, cometary bow shocks, and astrophysical sources, e.g., supernova.  In traditional thermodynamics of collisional media, a shock wave is the result of a nonlinearly steepening wave balancing steepening with energy dissipation through binary particle collisions. Shock waves generated in an ionized gas – called a plasma – not mediated by collisions posed an interesting challenge when they were first hypothesized.  That is, how does such a phenomena irreversibly transform bulk flow kinetic energy into other forms like random kinetic energy (i.e., heat)?  The problem is further complicated by the fact that the plasma is considered a non-equilibrium kinetic gas, not a fluid, which poses challenges for entropy generation and/or irreversibility.  I will discuss our current understanding of the physics of collisionless shocks from an observational point of view and conclude with some unanswered questions.

Fall 2017

Nonlinear Dynamics in Nature: Mathematics for an Equation-Free W​orld​

Ethan Deyle, University of California San Diego
Fri., Nov. 17, 2017, 12:30 PM in Science Center 199

Equations have a long, proud history in the sciences. However, they are not without limits! In Earth and environmental sciences, the same fundamental laws are at play as are in the physics or chemistry lab, but the behavior we seek to explain is happening at much larger scales. It’s emergent behavior. While a great deal of effort is put into finding equations that describe these phenomena, the various mathematics of non-parametric modeling can be much better suited to the practical reality. In the first half of the talk, I’ll discuss these general ideas and give an introduction to my favored approach, “empirical dynamic modeling.” In the second half, I will describe a specific ongoing study of deoxygenation in Lake Geneva. Despite the physics of the lake being relatively simple, the dynamics of oxygen are also deeply convolved with the lake ecology. Consequently, study of the system with equation-free empirical dynamic modeling finds substantially greater traction than previous equation-based attempts.

Einstein's Last Legacy:  Ripples in Spacetime

Andrea Lommen, Haverford College
Fri., Oct. 27, 2017, 12:30 PM in Science Center 199

100 years ago Einstein made a prediction. He knew a couple things you already know, and I'll show you that you would've made the same prediction. He predicted that Gravitational Waves, ripples in spacetime, exist, but are far too small to ever be detected. He was right about the first part and wrong about the second. Thanks to LIGO we are in the midst of the era of gravitational waves, a rapidly expanding field that will tell us enormous amounts about the universe. Pulsar timing will soon make an analogous detection, but in a complementary part of the gravitational wave spectrum. I'll show you how our experiment is analogous to LIGO's experiment, and convince you that we should actually a expect an overall crinkling of spacetime in the universe from the cumulative effect of tens of thousands of super massive black holes distributed across the universe. Finally, I'll tell you about an x-ray telescope I launched to the International Space Station in June that is going to help us understand the noise in the pulsar timing data.

What Can We Still Learn About Climate from Radiative-Convective Equilibrium?

Tim Cronin, Massachusetts Institute of Technology
Fri., Oct. 6, 2017, 12:30 PM in Science Center 199

Understanding Earth’s climate, and how it may change due to the significant human impact on atmospheric composition, is a key scientific challenge of the 21st century. A large fraction of the uncertainty in predictions of climate change is related to how clouds and their impact on global energy balance may change with warming. Much of this problem in turn owes to our inability to resolve cloud-scale motions (~1km in size) in global climate models, which have grid boxes ~100 km in horizontal size. In this talk, I will focus on the idealized model configuration of radiative-convective equilibrium, and how it is helping us to understand cloud feedbacks on climate change.  Radiative-convective equilibrium (RCE) is the statistical state of the atmosphere and surface that is set by the overall climatic energy balance between absorbed sunlight and emitted infrared radiation, assuming a horizontally uniform surface and horizontally uniform incident sunlight. Although RCE has been used as an idealization of the climate system for over 50 years, increasing computational power in the last two decades has allowed for simulation of RCE that explicitly represents the atmospheric motions that form clouds. One intriguing finding of these simulations is that under some conditions, clouds can undergo a transition from randomly dispersed to highly aggregated. Aggregation of clouds alters the atmospheric energy balance and dries the atmosphere overall, possibly affecting both cloud and water vapor feedbacks on warming. I will talk about simulations and analysis that assess how aggregation of clouds in RCE may affect climate sensitivity, as well as bottom-up theoretical work to understand aggregation of clouds as a linear instability of RCE.  

Magnetothermodynamics: Measuring the Equations of State of a Magnetized Plasma

Manjit Kaur, Swarthmore College
Fri., Sep. 22, 2017, 12:30 PM in Science Center 199

We have explored the thermodynamics of compressed magnetized plasmas in laboratory experiments and we call these studies "magnetothermodynamics." The experiments are carried out in the linear Swarthmore Spheromak eXperiment (SSX) device. In this device, a magnetized plasma source is located at one end of the device and at the other end, a closed conducting can is installed. We generate parcels of magnetized, relaxed plasma and observe their compression against the end wall of the conducting can. The plasma parameters such as plasma density, temperature, and magnetic field are measured during compression, using HeNe laser interferometry, ion Doppler spectroscopy and a linear B-dot probe array, respectively. To identify the instances of ion heating during compression, a PV diagram is constructed using measured density, temperature, and volume of the magnetized plasma. Various equations of state of the magnetized plasma are analyzed to estimate the adiabatic nature of the compressed plasma.

Spring 2017

Searching for the Secrets of the Non-Linear Universe

Tom Giblin, Kenyon College
Fri., Apr. 14, 2017, 12:30 PM in Science Center 199

We have no evidence that general relativity is wrong; every precision test is a resounding confirmation of this elegant and powerful mathematical model.  Trouble is: the greatest cosmological problems of our time (likely require) us to abandon general relativity.  About 95% of the Universe remains a mystery whose solution evades our abilities.  I will talk about how there may still be places in general relativity that have, until now, gone unexplored.  Numerical simulations are a powerful tool that can model the complex non-linear issues of general relativity on cosmological scales. I will present progress that we have made toward modeling the late Universe in its full splendor and outline where there’s hope that we can start to tackle these great questions.

Critical Gravitational Collapse to Rotating Black Holes

Thomas Baumgarte, Bowdoin College
Fri., Mar. 31, 2017, 12:30 PM in Science Center 199

Critical phenomena, i.e. the appearance of universal scaling laws and self-similarity in the vicinity of phase transitions, appear in different fields of physics and beyond.  Critical phenomena in the gravitational collapse to black holes were first observed by Matt Choptuik about 25 years ago - a seminal discovery that launched a whole new field of research.  Until recently, however, much of this research was restricted to spherical symmetry, and therefore could not account for effects that break this symmetry, in particular rotation.  In this talk I will review the appearance of scaling laws and self-similarity close to the onset of black hole formation.  I will then present new numerical relativity simulations of the gravitational collapse of rotating perfect fluids, in the absence of spherical symmetry.  These simulations inform perturbative treatments of the problem, leading to the formulation of generalized scaling laws that take into account the role of angular momentum in the critical collapse to black holes.

The First Observations of Gravitational Waves from Merging Black Holes

Geoffrey Lovelace, California State University, Fullerton
Fri., Mar. 17, 2017, 12:30 PM in Science Center 199

The Laser Interferometer Gravitational-Wave Observatory (LIGO) has made the first direct observations of gravitational waves - ripples of curved spacetime - a century after Einstein predicted their existence. Each gravitational-wave signal originated from a pair of merging black holes over a billion light years away. In this talk, I will discuss LIGO's observations, the methods that made it possible, and the implications for the dawning age of gravitational-wave astronomy. I will also highlight contributions from student and faculty researchers in California State University, Fullerton’s Gravitational-Wave Physics and Astronomy Center, including comparisons of the LIGO observations with numerical calculations of the merging black holes and the gravitational waves they emitted. Near the time of merger, the gravitational waves from merging black holes can only be predicted by numerically solving Einstein's equations of general relativity. I will present new numerical-relativity simulations of merging black holes targeting LIGO's observations, and I will discuss how these and other simulations are helping to maximize our understanding of gravitational waves' astronomical sources.

Dark Matter in the Cosmic Context

Katherine Mack, University of Melbourne
Fri., Feb. 24, 2017, 12:30 PM in Science Center 199

Dark matter forms the foundation for all cosmic structure, and its fundamental nature is one of science's most pressing enigmas. As we search for the most distant galaxies in the universe with radio and infrared observations, we are in a position to explore the particle physics of dark matter — the possibility of annihilation, decay, or other particle interactions — through its effects on early stars and galaxies. I will give an update on the quest to identify dark matter both in the lab and in the sky, major unsolved problems in dark matter theory, and how upcoming observations of the epoch of the first cosmic structures can be used to open a new window on the dark universe.

Fall 2016

Life as a Matter of Chance

Jané Kondev, Brandeis University
Fri., Nov. 11, 2016, 12:30 PM in Science Center 199

The living cell is bustling with nanometer sized protein machines. These machines perform a variety of functions such as the reading of genetic information, the transport of molecules from one side of the cell to the other, the building of the tracks required for this transport, and so forth. Protein machines in cells are very different from man made ones as Brownian motion, the constant agitation of proteins by water and other small molecules, plays a critical role. Brownian motion makes protein machines behave randomly and unpredictably. In this talk I will discuss recent experiments and theory that reveal how this randomness at the molecular scale produces variability in cellular behavior. I will also comment on how these discoveries, which are being made by biologists and physicists working together, has the potential of transforming cell biology and medicine.

The Radiation-Driven Winds and X-ray Emission of Massive Stars

David Cohen, Swarthmore College
Fri., Nov. 4, 2016, 12:30 PM in Science Center 199

The most massive, hot, and luminous stars in the Galaxy drive powerful outflows via the force of their own starlight. These radiation-driven winds return heavy-element-enriched material to the interstellar medium, sculpt beautiful nebulae, and strongly affect the evolutionary paths and end-states of massive stars. In this talk I will describe my research group's work on two categories of massive stars and their winds, focusing on the stellar X-ray emission and what we can learn from it. Both magnetic and non-magnetic massive stars generate copious X-ray emission by converting some of their wind kinetic energy to heat in via dissipation in shocks. The majority of massive stars - the non-magnetic ones - do it via an instability intrinsic to radiation-driving, while the magnetic massive stars channel their winds into a confined magnetosphere where head-on collisions of wind flows from opposite hemispheres cause strong shocks. In both cases, X-ray spectroscopy can be used to diagnose the shock physics and spatial structure of the stellar winds and provide insights about the fundamental physical processes that characterize the most massive stars in the Galaxy. 

Physical Guidance of Cell Migration

Wolfgang Losert, University of Maryland
Fri., Oct. 28, 2016, 12:30 PM in Science Center 199

Cells migrate as individuals or groups, to perform critical functions in life from organ development to wound healing and the immune response.  While directed migration of cells is often mediated by chemical or physical gradients,  our recent work has demonstrated that the physical properties of the microenvironment can also control and guide migration.  I will describe how an underlying wave-like process of the actin scaffolding drives persistent migration, and how such actin waves are nucleated and guided by the texture of the microenvironment.  Based on this observation we design textures capable of guiding cells in a single preferred direction using local asymmetries in nano/microtopography on subcellular scales.  This phenomenon is observed both for the pseudopod-dominated migration of Dictyostelium cells and for the lamellipod-driven migration of human neutrophils.  The conservation of this mechanism across cell types suggests that actin-wave-based guidance is important in biology and physiology.

Sentiment Analysis of Student and Instructor Feedback: Gender Bias and Affective Patterns

Scott Franklin, Rochester Institute of Technology
Fri., Sep. 30, 2016, 12:30 PM in Science Center 199

Sentiment analysis is a computational linguistics tool that characterizes affective meaning, such as positive-negative tone, expressed in language data. In this talk I present two projects that use sentiment analysis to reveal subtle patterns in student and faculty feedback. First, I present a study of more than 5,500 student comments spanning over eight years of biology, chemistry, physics, and math courses and explore differences in sentiment pertaining to instructor competence, organization/presentation, personality/helpfulness, and overall satisfaction. Of particular interest are differences in perception conveyed toward male and female faculty, and between faculty of different disciplines. We also compare automatically extracted sentiment scores with quantitative Likert ratings that students enter alongside their comments, and report on the extent to which the quantitative and qualitative evaluations correlate. The second project analyzes instructor feedback to student Guided Reflection Forms, weekly online reflections about challenges and setbacks students experience. Sentiment analysis supports the development of a stable basis set (rubric) to describe responses that is robust across both introductory and advanced classes. The analysis also reveals the instructor’s subconscious use of the “praise sandwich,” instinctively embedding critiques and suggestions between specific and general encouragements. In both studies, validated, automated, sentiment analysis becomes a useful method by which to analyze large corpuses of written text.

Singing Stars, Eclipsing Stars, and Other Worlds: Imminent Advances from the TESS and Gaia Missions

Keivan Stassun, Vanderbilt University and Fisk University
Wed., Sep. 21, 2016, 4:30 PM in Science Center 199

The upcoming Gaia mission will measure the trigonometric parallaxes to some 1 billion stars with an accuracy of 20 micro-arcseconds. Such precision distance measurements promise to revolutionize our understanding of many areas of stellar astrophysics. However, it is imperative that these distances be benchmarked against independent, accurate distance measurements, which eclipsing binary stars are uniquely poised to provide. The upcoming TESS mission will discover dozens of Earth-link planets around nearby Sun-like stars. However, to determine the physical properties of these "Earth 2.0" with precision requires accurate knowledge of the physical properties of the stars they orbit. The "singing" of stars induced by gas motions at their surfaces provide a powerful way of determining these properties. These exciting, upcoming discoveries represent superb examples of the application of basic physics in astronomical contexts.

Spring 2016

Accelerator Technology for Dark Matter Searches and Quantum Computing

Daniel Bowring, Fermi National Accelerator Laboratory
Mon., Apr. 25, 2016, 12:30 PM in Science Center 183

The accelerator complex at Fermi National Accelerator Laboratory creates, focuses, steers, and collides high-energy/intensity proton beams. The technology developed to accelerate and control energetic beams can sometimes find its way into other areas of research. For example, MRI magnets are now relatively cheap and stable because, in the 1970s, Fermilab had to develop the physics and engineering techniques required to build powerful magnets for steering energetic particle beams. In this talk, we'll discuss the ways in which accelerator technology carries over into other fields. Specifically, we'll discuss how resonant accelerating structures can be used to search for dark matter. This technology can also be applied to quantum computing, in order to improve the coherence of qubits.

Making Model Cells to Study How Proteins Reshape Cell Membranes

Catherine Crouch, Swarthmore College
Fri., Apr. 22, 2016, 12:30 PM

Understanding how cells reshape themselves, and the roles that proteins play in that process, is an active area of current research by scientists across many disciplines. To understand the underlying physics and chemistry of such a complex cellular process requires developing simple model systems. This talk will describe progress toward optimizing a model system in which we can study the role of the M2 protein of influenza A in the reshaping processes required for virus budding. Specifically, the talk will present our optimization of the process of incorporating the protein into synthetic lipid vesicles, including the development of optical techniques for monitoring the incorporation process.

Uncovering the Unbound Stellar Population in and Around the Milky Way

Lauren Campbell, Vanderbilt University
Mon., Mar. 28, 2016, 4:30 PM in Science Center 128

A significant fraction of a galaxy's light may come from a population of unbound stars, and this population can inform us about the dynamical state and stellar content of the galaxy. In galaxy clusters this unbound fraction is known as intracluster light (ICL), identified as a diffuse excess of light in the outskirts of the galaxy. ICL has been observed around distant galaxies and a similar intracluster stellar glow ought to exist around the Milky Way. We developed a technique to identify distant M-giants within the Sloan Digital Sky Survey (SDSS) based solely on color. Using this technique, we identified 700 candidate M-giant stars at intragroup distances, beyond 300 kpc, that we call intragroup stars (IGS). These IGS may constitute rare tracers of an underlying ICL population surrounding the Milky Way. One possible explanation for the origin of these IGS is that they were ejected from the center of the Galaxy through interactions with our supermassive black hole as hypervelocity stars (HVSs). There are only 18 confirmed HVSs and all are young, massive, B-type stars. We identify candidate HVSs from a sample of SDSS G- and K-dwarfs. Using the observed positions and velocities, we calculate the orbits of these candidates in order to determine their place of origin within the Galaxy. We find that nearly half of the candidates exceed their escape velocities with at least 98% probability and no candidate's orbit is consistent with a Galactic Center origin.

Using Stars to Study Galaxy Formation and Evolution

Aaron Grocholski, Louisiana State University
Tue., Mar. 23, 2016, 4:30 PM in Science Center 128

Studies of galaxy formation and evolution generally take one of two main approaches: survey the distant Universe or focus on nearby galaxies. The distant Universe provides myriad galaxies covering a vast range in physical properties, such as mass, luminosity, and structure. The drawback of these studies is that, with current technology, distant galaxies often span only a few tens of pixels on an image, thus leaving the subtle details of these galaxies unresolved. In contrast, the galaxies of the Local Group and other nearby galaxy groups give us a much smaller sample to study. Their proximity, however, allows us to resolve individual stars and thereby study the stellar populations of nearby galaxies at a level of detail that is not possible at large distances. In this talk I will discuss what information stars provide about their host galaxy and present some of the results from my research on galaxies in the nearby Universe.

Identification of Plasma Fluctuations Using Fast Imaging

Adam Light, Earlham College
Thu., Jan. 28, 2016, 4:30 PM in Science Center 128

Fluctuations in magnetized plasmas are ubiquitous and complex. Although they often produce detrimental effects, like increasing heat and particle transport in fusion energy devices, fluctuations also provide a diagnostic opportunity. Identification of a fluctuation with a wave or instability gives detailed information about the properties of the underlying plasma. In addition to introducing plasma as a complex system, I will describe imaging measurements of coherent waves in a cylindrical plasma column. Visible light from Ar II line emission is collected at high frame rates (>50,000 frames/second!) using a fast digital camera. Experimental wave-dispersion-relations are constructed using imaging data alone, and can be compared directly with theoretical models. I will discuss the identification of both electron-drift waves and Kelvin-Helmholtz fluctuations, as well as imaging measurements of nonlinear mode coupling.

Detection and Analysis of Individual Biological Structures at the Nanoscale

James Baker, Cornell University
Tue., Jan. 26, 2016, 4:30 PM in Science Center 128

In his 1959 lecture ‘There’s Plenty of Room at the Bottom,’ Richard Feynman posed an “invitation to enter a new field of physics,” and challenged the scientific community to expand their capabilities to observe, measure, and manipulate materials at the nanoscale. While recognition and manipulation have occurred at the nanoscale in biological systems since the origin of life, our ability to observe and characterize these interactions is still emerging. Today’s physicists continue to play a role in this progress, as the application of fundamental physical principals remains vital in the continued development of new measurement approaches for studying nanoscale systems. In this talk, I will discuss two experimental approaches to the detection and analysis of individual biological structures at the nanoscale: nanophotonic resonators and magnetic tweezers. The applications of such developments range from the study of fundamental biophysical interactions to the development of point-of-care medical diagnostics and beyond.

Fall 2015

Building a Galactic Scale Gravitational Wave Observatory

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.

What Is the Higgs Boson, How Did We Find it, and What’s Next for the LHC?

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.

Nuclear Physics Aspects of Stellar Explosions

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.

Symmetry, Geometry, Cosmology

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.

We Are All Stardust: Understanding Stellar Explosions Using Nuclear Physics

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.

Spring 2015

Was Einstein Right Handed?

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.

See Prof. Alexander's website for more information about his work. Prof. Alexander will be on campus for the Cooper symposium on improvisation: Sound Breaks. You can watch him talk about jazz and physics.

Solar Wind Research at NASA Goddard Space Flight Center

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. 

Fall 2014

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.  

Is There a Universal Character to Plasma Turbulence?

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.

Spring 2014

Positronium: The Simplest Exotic Atom

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.

Manipulating Magnetic States in Nanorings: The Future of Data Storage?

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.

Fall 2013

A Place Like Home?: Revealing the Atmospheres of Super-Earths Orbiting Distant Stars

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.

Water-Based Liquid Crystals: Lessons from the World of LCDs

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.

Why Does Nature Like the Square Root of Negative One?

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.

Order From Disorder: Identifying 'Structure' in Disordered Proteins

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.   

Applications of Diffuse Optics in Non-Invasive Clinical Measurements of Tissue Metabolism

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.  

Spring 2013

Improving Plasma Confinement through Rotational Shear on the Large Plasma Device

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. 

'Smart' Bubbles for Acoustic Contrast

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.

Cracking the Cosmological Code: The Challenge of Dark Energy

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.

Old Quantization from a Modern Perspective

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.

Probing Dark Energy via Electrostatics Analogy

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.

Black Holes and Quantum Gravity

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.

Fall 2012

What is the Higgs Boson?

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.

The Magnetism of Massive Stars

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. 

Why Are We Here? - The Quest of the Long Baseline Neutrino Oscillation Experiments

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.

Magnetic Explosions and Their Role in Space Weather

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.

Spring 2012

Probing Magnetism in Geometrically Frustrated Magnetic Materials Using Neutron Scattering

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."

Dynamical Instabilities in Evolving Star and Planetary Systems

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.

Membrane-Protein Binding: Basic Physics and Two (Fairly) New Experimental Techniques

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.

Fall 2011

When Things Fall Apart: Origins of Mechanical Fragility in Disordered Systems

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.

Looking Inside the Body with Visible Light: Diffusive Optical Tomography of Human Breast Tissue

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.

Have We Broken the Cosmic Speed Limit?

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.

Plasmas: Complexity Spanning 30 Orders of Spatial Scale

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.

Spring 2011

What Can the Smallest Galaxies Tell us About the Universe?

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.

GECCO: Galaxy Evolution in Cluster Cores and Outskirts

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.

The New Age of Exploration: Exoplanets

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.

The Evolution of the Baryon Content of Galaxy Groups

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.

Seeking Signs of Life on Mars

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 available 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)

Fall 2010

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.