Unless otherwise noted, all physics seminars and colloquia are held on Tuesdays from 4:45 to 6:00, in lecture room 3 of Merrill Science Center. Tea and snacks will be served before seminars at 4:15 in 204 Merrill. If you would like to be mailed seminar announcements, please send an email to Ellen Feld.
Contact colloquium organizer Jonathan Friedman with any questions about colloquia.
Tue, Sep 15, 2015
Tue, Sep 22, 2015
"Waves, Currents, Beaches, and All That: Fluid Dynamics of the Coastal Ocean"
How do ocean waves, currents, and coastlines evolve and interact? The answer lies in the fascinating dynamics of the coastal ocean: wind-generated ocean waves refract, steepen, and break as they enter shallow water, drive strong currents, transport sand, and change the shape of the beach. Improving our understanding of these processes is essential to making better predictions of flooding and erosion during storms like Hurricane Sandy. I’ll describe a set of experiments in which we “perturbed” the seafloor by dredging big holes and channels underwater at the beach, then studied the response to learn about the feedbacks between waves, currents, and seafloor change. The field observations, combined with theory and numerical simulations, reveal how gravity smooths out sandcastles and sandbars, and why rip currents turn on and off.
Tue, Sep 29, 2015
"Experimental Studies in 1D Quantum Magnetism: Old Physics, New Materials, New Physics"
Quantum physics is the study of the behavior of nature at very small length scales, atomic size and smaller. The explosive development of nanoscience over the past thirty years enables people to synthesize, manipulate, image, and control very small materials.
This presentation will offer a different perspective. Quantum behavior can also be studied and understood using appropriately-designed macroscopic materials. This premise will be illustrated by recent developments in one-dimensional magnetism in which magnetic behavior is dominated by purely quantum effects.
Tue, Oct 6, 2015
"Does the Big Bang Prevent Us from Being in Two Places at Once?"
Quantum mechanics tells us that atoms and even molecules can display both particle-like and wave-like properties. By wave-like properties, we mean for example that an atom can be in two different places at once. Only when we probe the atom does its position "collapse" into having one value or the other, and we have no way of knowing beforehand which value it will be. However, there is nothing in the theory of quantum mechanics that rules out larger objects, such as a ball or even you, being in two places simultaneously. So why don't we observe everyday objects in such bizarre quantum states? We shall argue that the answer may ultimately lie in the presence today of a largely invisible, leftover remnant from the Big Bang that created our Universe.
Tue, Oct 13, 2015
"Atomic Sensors for Precision Measurements"
Modern atomic physics techniques use lasers to interact with and manipulate atoms in a gas. Quantum mechanics predicts atomic structure to high precision and demonstrates that atoms of the same species are truly identical. These features provide an attractive platform for the use of atoms as sensors for a variety of fundamental and applied precision measurements. I will describe a fundamental symmetry test using warm atomic vapors of interacting K and 3He that set new limits on a Lorentz-violating background field interacting with the neutron spin. I will also describe instrument development of a novel atom interferometer in an optical cavity using Cs atoms in a cold atomic gas. Such an apparatus could be used for proposed fundamental gravity tests for use in inertial navigation without the aid of GPS.
Tue, Oct 20, 2015
"Breaking the Myth of the 'Non-Traditional' Physicist: Preparing Physics Graduates for the Real World of Employment"
Do you want to get a job after you graduate? Perhaps an internship? I will relay a recent talk by Crystal Bailey from the American Physical Society that detailed the statistics of employment for physics majors and how to land that perfect job (title, above, and abstract, below, are from Bailey's recent talk at an AAPT meeting). If you are a major in physics, biophysics, or a student in the Dartmouth 3-2 Engineering program, you won't want to miss this talk.
"Physics degree holders are among the most employable in the world, often doing everything from managing a research lab at a multi-million dollar corporation, to developing solutions to global problems in their own small startups. Employers know that with physics training, a potential hire has acquired a broad problem-solving skill set that translates to almost any environment, as well as an ability to be self-guided and -motivated so that they can learn whatever skills are needed to successfully achieve their goals. Therefore it's no surprise that the majority of physics graduates find employment in private sector, industrial settings. Yet at the same time, only about 25% of graduating PhDs will take a permanent faculty position--yet academic careers are usually the only track to which students are exposed while earning their degrees. In this talk, I will explore less-familiar (but more common!) career paths for physics graduates, and provide resources to help faculty mentors give their students better information and training for a broader scope of career possibilities."
Tue, Oct 27, 2015
"Physics Education Research on Laboratory Skills and Careers in Physics"
Physics education research (PER) offers a set of methods and ideas to better understand and improve students' learning of physics and has established itself as a vibrant and growing sub-field within the physics community. This talk will apply PER to study the professional practices of physicists in academic research and industrial physics careers and describe findings that can be applied to the undergraduate curriculum. One specific application will focus on the complexity of students' model-based reasoning in an upper-division physics laboratory, where students encounter both sophisticated apparatus and theoretical ideas . A second application will discuss on-going research on workplace usage of physics, math, and communication within academic and industry jobs in physics-related fields, such as photonics. The talk will provide a welcoming introduction to physics education research and approach PER in a way that draws many parallels to conducting research in the natural sciences.
Tue, Nov 3, 2015
"Interfacing Photons and Atoms with (really, really thin) Optical Fibers"
A major effort in quantum physics is the engineering of strong interactions between photons and atoms. The atoms need not be "real" and the photons need not be visible, but the underlying goal is the same. These strong interactions have applications in metrology, spectroscopy, quantum control, and quantum computation, to name a few. In this talk, I will summarize several implementations before focusing on one particular system: cold atoms interacting with photons via an optical nanofiber. An optical nanofiber has a diameter thinner than the wavelength of the light propagating through it, resulting in an evanescent field with relatively large amplitude that is the source of the atom-light interaction. I will outline how we make nanofibers with ultrahigh transmission, and how we use them for atomic-cloud thermometry, to measure atomic level decay, and to confine atoms for long times.
Tue, Nov 10, 2015
"Tabletop-Scale Probes for TeV Physics"
The Large Hadron Collider (LHC) is seeking to observe as-yet undetected, very massive particles. Their strategy involves colliding protons at unprecedentedly high energies, in order to directly create new particles according to E = mc^2. I will describe experiments using an alternate but powerful strategy to detect new particles. In quantum field theories, all ordinary particles are continuously emitting, and then quickly reabsorbing, every type of other particle that exists in nature; this “cloud” of “virtual particles” can modify the apparent properties of ordinary matter. We are searching for an asymmetric charge distribution of this cloud—an electric dipole moment—along the spin axis of an electron or a proton, which can be induced by the existence of certain types of new particles. Although our experiments can fit in a single room, they are sensitive to masses of 3 TeV or even higher, exceeding the direct reach of the LHC. This talk will explain the concepts and techniques of our experiments, including a new effort inspired by Prof. Hunter’s recent work at Amherst.
Tue, Nov 17, 2015
"Astrophysical Searches of Dark Matter Decay and Annihilation"
The abundance of ubiquitous dark matter is now well quantified by observations, yet its nature remains unknown. Dark matter is believed to be composed primarily of an elementary particle. Searches for the dark matter particle are one of the major efforts in particle physics and astrophysics today. X-ray observations of dark matter dominated objects have the potential to reveal a signal from decaying or self interacting dark matter. A well motivated dark matter candidate sterile neutrino oscillates into an active neutrino by emitting an X-ray photon. Another viable candidate is a self interacting dark matter, which could also produce an X-ray signal. I will summarize constraints on decaying and self interacting dark matter models with a focus on the detection of an unidentified emission line 3.55 keV in observations of clusters of galaxies.
Tue, Dec 1, 2015
"Investigating the Collective Motility of Dynein Motor Ensembles Using Structural DNA Nanotechnology"
Kinesin and cytoplasmic dynein are microtubule-based motors that drive intracellular cargo transport in eukaryotic cells. Many intracellular cargos are propelled by small groups of these motors working in ensembles. Yet many of the biophysical mechanisms that govern ensemble motility remain unknown. Building upon our previous work using the techniques of DNA origami (Derr & Goodman et al Science 2012), we have designed a programmable synthetic cargo “chassis” that allows us to control both the number of motors in the ensemble and the rigidity of the cargo chassis that links them. On this chassis, motors within an ensemble are conjugated together through variable length cargo “linker” regions comprised of parallel instances of either single- or double-stranded DNA. The configuration of these regions control the number of independent steps each motor can take before exerting forces on the other motors within the ensemble. This design enables us to investigate how the “communication" of motor stepping through the cargo structure affects interactions within the ensemble and how multiple linked, but independently acting, motors can coordinate to haul cargo. Using TIRF microscopy, we have observed dynein ensembles in vitro transporting these cargo chassis along microtubules. In contrast to our previous results using a rigid chassis (Derr & Goodman et al Science 2012) we find that ensembles of dynein on non-rigid cargos move faster as more motors are added. Our results suggest that cargo rigidity, and the ability for motors to communicate their steps to one another, play important roles in determining the collective motility of multiple molecular motors.
Tue, Dec 8, 2015
- Edward Kleiner - "Building Toward Quantum Control of Be+ Ions"
- Ji Hoon Lee - "A Geometric View of Dynamical Symmetries in the Kepler Problem"
-Wonjae Lee - "A Crazy Synthetic Magnetic Field in a Spinor Bose-Einstein Condensates"
Tue, Jan 19, 2016
"The Formation of Stars: Revealing the Physical Processes of Stellar Assembly"
Stars are the fundamental building blocks of galaxies, and the primary night sky objects visible to the naked eye. As a result the study of stars is one of the oldest fields of astronomy. While many aspects of stellar astrophysics are now well understood, the formation of stars remains an area of active investigation. In particular, the astronomy community still does not fully understand the physical processes governing how stars gain their mass. I will give a basic overview of how stars form and discuss my recent work studying the very youngest stars still in the process of forming, focusing on a resulting paradigm shift in our understanding of the stellar mass assembly process. I will end with an overview of future directions needed to further advance our understanding of stellar formation.
Fri, Jan 22, 2016
"Laser-Induced Breakdown Spectroscopy on Mars: The Long Road toward Calibrating ChemCam to Lase the Red Planet"
The Curiosity rover has been touring the surface of Mars in Gale Crater since 2012, and has amassed nearly 300,000 laser shots of Martian rock and soil targets. From those LIBS spectra, chemical compositions with high spatial resolution are obtained almost daily from targets at distances up to 7 m from the rover, providing a wealth of insights into martian geology and geochemistry. This talk follows the development of the LIBS and ChemCam from the behind-the-scenes perspective of a science team member. Over the past decade, LIBS has gone from an essentially-untested analytical method to a well-studied (if often intractable) new technique that might soon vie with the tricorder used in Star Trek! We will follow the path of LIBS technique development as it intertwines with daily life on the Curiosity science team, through pre-flight testing, launch, landing, and daily science operations.
Tue, Jan 26, 2016
"The Milky Way Laboratory"
Myriad physical processes shape galaxies throughout the distant cosmos, far outside the reach of detailed study with our telescopes. Luckily for us, we can study these processes up close and personal in the laboratory that is our own Milky Way Galaxy. On a tour through our Milky Way Laboratory, I will discuss 1) how we can use long, skinny molecular clouds, potential "Bones of the Milky Way," to trace our Galaxy's spiral structure, 2) how large surveys of our Galaxy have revealed that star clusters continue to grow even as they are forming, and 3) how observing the Central Molecular Zone (the inner few hundred parsecs of our Galaxy) can help us learn more about gas is converted into stars during the peak of cosmic star formation (z~2).
Fri, Jan 29, 2016
"Tracing the Cosmic Shutdown of Star Formation in Massive Galaxies"
Over the last few decades, astronomers have progressed from archeological studies of nearby galaxies to direct observations of the early universe. We have uncovered billions of years of cosmic growth that present new challenges to galaxy formation theories. In this talk, I will review the recent innovative techniques developed to probe the distant universe, and the key observations constraining the formation histories of galaxies over the past 11 billion years. We have discovered a population of surprisingly compact and massive “red and dead” (quiescent) galaxies that are no longer actively forming stars. The physical mechanisms responsible for shutting down star formation and the subsequent buildup of this quiescent population at such early times is one of the most outstanding questions in astrophysics today. We don’t yet understand why these enigmatic galaxies are so compact, with sizes a factor of 5 smaller than nearby galaxies of similar mass. I will present promising paths forward towards solving this puzzle that leverage the capabilities of the Hubble Space Telescope, as well as a look toward the future with exciting upcoming public facilities.
Tue, Feb 2, 2016
"The Extravagant Universe: Big Stars, Big Explosions, and Big Black Holes"
Our Universe has a curious proclivity for producing big, beautiful, monstrous
things. Here I will describe our feeble efforts to characterize what She has
created, and our even feebler attempts to understand why. Along the way, I
will highlight my group's entrance into the study of supermassive black holes,
which appear to lurk at the centers of most (all?) galaxies, as well as our
ongoing -- and recently revealing -- investigation of the explosion geometry of
Nature's most massive stars. The value of constantly interacting with
inquisitive and tenacious students will also be emphasized.
Tue, Feb 9, 2016
"The Birds and the Bees - Exoplanet Edition: How Planets are Made"
How do we find planets around other stars? What do these discoveries tell us about the physics of planet formation? In this talk, I will discuss the fast-growing field of exoplanet astrophysics. I will begin with a brief introduction to the field in general, including the revolutionary techniques that in recent years have enabled discoveries of planetary systems very unlike our own. I will then discuss how my subfield of "exoplanet direct imaging" fits into the larger picture and how my work, including the discovery of the first actively-forming planet, helps to address several important open questions. I will conclude by discussing the current direction of the field, including specific research opportunities for Amherst undergraduates.
Fri, Feb 12, 2016
"The Surprisingly Complex Lives of Massive Galaxies: Cannibalism, Growth, and Evolution through Cosmic Time"
Massive galaxies reside in the densest and most evolved regions of the Universe, yet we are only beginning to understand their formation history. Once thought to be relics of a much earlier epoch, the most massive local galaxies are red and dead ellipticals, with little ongoing star formation or organized rotation. In the last decade, observations of their assumed progenitors have demonstrated that the evolutionary histories of massive galaxies have been far from static. Instead, billions of years ago, massive galaxies were morphologically different: compact, possibly with more disk-like structures, and on-going star formation. The details of this observed evolution can place constraints on the physical processes that have driven massive galaxy evolution through cosmic time. I will discuss on-going observational studies of the structure, dynamics, and compositions of massive high-redshift galaxies. Finally, I will outline prospects for further understanding of the history of these intriguing objects with next generation observatories and instruments.
Tue, Feb 16, 2016
"Particle Physics: From Grains To Cells"
Is toothpaste a liquid or solid? Why is it so difficult to run on sand? What makes squishy stuff, well, squishy? The answers to these questions are all related to physics at the natural “mesoscale” of these materials: the constituent particles, cells, droplets, or polymers. In this talk, we will explore the behavior of several of these systems (hard grains, soft colloids, and living cells) in flowing or nonequilibrium states. We will introduce novel experimental techniques and metrics for characterizing motion. The systems show certain universalities in their behavior; the systems' discreteness and density beget the emergence of similar dynamics, even though the particles and interactions in each system are different. (We will also likely mention robots.)
Fri, Feb 19, 2016
"Spectroscopy of Ultracold 85Rb2"
Spectroscopy has helped us understand the nature of matter for over two hundred years. Using modern physics techniques, extremely precise measurements can be made of molecular structure, and we can even manipulate the state of these molecules. I will give an overview of ultracold physics techniques, and describe experiments to form interesting states of rubidium diatomic molecules primarily using photoassociation. I will show experimental results on short-range photoassociation and forming pure long-range Rydberg molecules.
Tue, Feb 23, 2016
Tue, Mar 1, 2016
Talk has been cancelled.
Tue, Mar 8, 2016
"Nature’s Own Trapped Atom: Coherent Control of Atom-Like Defects in Diamond"
The nitrogen-vacancy (NV) center, an atom-like defect in diamond, has recently emerged as a promising platform for applications in quantum information science and quantum metrology. NV centers can be manipulated much like single trapped atoms. They can be isolated and addressed individually using optical microscopy; microwave radiation can be used to control their electrons’ spins and tunable lasers can be used, at cryogenic temperatures, to drive coherent transitions between electronic orbital states. However, the fact that an NV center is surrounded not by vacuum but by a solid diamond lattice presents both experimental challenges and new opportunities.
We use the NV center’s atom-like properties to probe its interactions with the diamond lattice. I will describe how phonons, quantized vibrations of the diamond lattice, drive the intersystem crossing process, a decay mechanism that plays a crucial role in the NV center’s optical dynamics. I will also discuss our recent efforts to use coherent population trapping to probe and manipulate nuclear spins embedded in the diamond lattice.
Tue, Mar 22, 2016
"Molecular Physics and Chemistry at Micro-Kelvin Temperature"
I will give an overview of how we prepare molecules at ultracold temperatures
for exciting applications such as ultracold reactions, quantum simulations, and
quantum information processing. I will also introduce a new on-going effort to
make molecules in an arrary of optical tweezers.
Tue, Mar 29, 2016
"Why Does Nature Like the Square Root of Negative One?"
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 discuss a few of the ideas people have put forward to try to understand why nature favors complex amplitudes. At present no such idea has gained wide acceptance—indeed, it is conceivable that the question has no answer—but the effort to find an answer has nevertheless produced insights into the structure of quantum theory.
Tue, Apr 5, 2016
"Quantum Simulation with Trapped Calcium Ions"
Improvements in coherent control over the quantum states of physical systems has given
rise to a wealth of applications ranging from metrology to quantum information process-
ing. And yet, our ability to simulate quantum behaviors remains in its infancy; the very
features which make encoding information in quantum systems powerful (superposition
and entanglement) make these systems fantastically difficult to simulate on an ordinary
computer. One solution lies in the development of a practical quantum computer, but for
now this remains only a distant vision. Fortunately, analog quantum simulation - wherein
one quantum system is used to emulate the behaviors of a second with similar interactions
- offers a path forward. After a broad overview, I will discuss ongoing efforts at Williams
to build a trapped-ion quantum simulator and describe application of such a simulator to
the study of thermal conductivity in nano-scale systems.
Tue, Apr 12, 2016
"What is the MUSCEL Behind Low Surface Brightness Galaxies?"
Low surface brightness galaxies are disk galaxies that are characterized by their gas richness, low gas surface densities, low past and current star formation rates, blue colors, low metal abundances, and kinematics that are dominated by dark matter. While they span the same range of masses as their high surface brightness "normal" counterparts, they have clearly taken a different evolutionary path. Why is this? Why do they have so few stars? Why do their dark matter halos look the way they do? I will present spectroscopic and photometric observations that explore the properties of these galaxies. I will also discuss our efforts to answer these questions as part of the MUSCEL, MUltiwavelength observations of the Structure, Chemistry, and Evolution of LSB galaxies, Program.
Tue, Apr 19, 2016
"Dots for Dummies"
Quantum dots involve a confluence of three problems: finite size, strong interactions and randomness. Yet they are amenable to exact solution in certain cases thanks to ideas from Random Matrix Theory and the Renormalization Group, both of which will be introduced from scratch for the audience in this blackboard talk.