Physics Colloquia 

Colloquia are held on Mondays at 4:00pm (Refreshments at 3:30pm) Barus & Holley 168

---

PAST COLLOQUIA

---

SPRING 2013 

April 22, 2013 - A. O. Williams Public Lecture (MacMillan 117)

Lord Martin Rees (University of Cambridge)

“From Mars to the Multiverse”

Astronomers have made astonishing progress in probing our cosmic environment. We can trace cosmic history from some mysterious 'beginning' nearly 14 billion years ago, and understand in outline the emergence of atoms, galaxies, stars and planets.

Unmanned spacecraft have visited the other planets of our Solar System (and some of their moons), beaming back pictures of varied and distinctive worlds. An exciting development in the last decade has been the realization that many other stars are orbited by retinues of planets -- some resembling our Earth.

Looking further afield, we are understanding galaxies and their nuclei in fuller detail, and can check models of their evolution by detecting objects all the way back to an epoch only a billion years after the 'big bang'.  Indeed we can trace pre-galactic history with some confidence back to a nanosecond after the 'big bang'.

But the key parameters of our expanding universe -- the expansion rate, the geometry and the content -- were established far earlier still, when the physics is still conjectural but can be pinned down by future observations. These advances pose new questions: What does the long-range future hold? Should we be surprised that the physical laws permitted the emergence of complexity? and Is physical reality even more extensive than the domain that our telescopes can probe? This illustrated lecture will attempt to address such issues.

April 8, 2013 - Gunther Roland (MIT)

"Trillion Degree Matter"

In this talk I will discuss a very simple question: What are the properties of matter at extremely high temperature, in excess of several trillion Kelvin? Experiments at large particle colliders like RHIC at Brookhaven Lab and LHC at CERN have shown that such temperatures can be achieved in collisions of heavy nuclei, creating a plasma of quarks and gluons resembling the universe shortly after the Big Bang. We have found that the produced state of matter exhibits fascinating and somewhat surprising properties: Although its density exceeds that of water by 16 orders of magnitude, the quark-gluon plasma behaves like a near-perfect liquid.  I will review the most striking observations made in recent LHC data and discuss the unexpected connections to strongly coupled systems in other areas of physics, ranging from string theory to ultra-cold atoms.

April 1, 2013 - Lia Krusin-Elbaum (City College of New York)

“Searching for topological superconductivity of disordered Dirac fermions”

 In the last few years there has been an explosive development in materials’ science – the discovery of a new class of insulators in three dimensions (3D) having a fully insulating electronic gap in the bulk but with metallic Dirac states on their surfaces which can carry spin/charge currents that are topologically protected against (time-reversal-invariant) disorder and perturbations. This discovery launched the search for a topological superconducting analogue characterized by a fully gapped odd parity (p-wave) pairing state that could support Majorana bound states. Such a superconducting phase has important implications for a fault-tolerant quantum computing owing to the non-Abelian exchange statistics obeyed by Majorana fermions. Recently, superconductivity below ~4 K was reported in two 2^nd generation 3D topological insulators (TIs): in Bi₂Se₃ doped with copper and in Bi₂Te₃ under ~6 GPa pressure. In both cases, however, the topological nature of the superconducting phase has not yet been proved. In my talk I will discuss our most recent results on another 2^nd generation TI, Sb₂Te₃, where we have discovered superconductivity with T_c = 8.3 K, the highest transition temperature among the TIs reported thus far.  This state is obtained when the material is synthesized under modest pressure (in the MPa range) in the narrow dome-like pressure range, reminiscent of quantum criticality in other exotic superconductors. The diamagnetic state of this new superconductor is very unusual, since even in the normal state the system supports large orbital currents, while robust van Vleck-like spin response of the disordered Dirac fermions still persists in the superconducting state. We will discuss our observations in the context of recent ideas of how arbitrarily weak disorder can assist fluctuation superconductivity and turn Dirac semimetal into a superconductor.

March 18, 2013 - Harry Themann (Brookhaven National Lab)

"Observation of Electron-antineutrino Disappearance at Daya Bay Nuclear Reactor Neutrino Experiment"

At the Daya Bay Reactor Neutrino Experiment we have measured the last unknown neutrino mixing angle, θ13, to be non-zero at the 6.2σ level, this is the most precise measurement to date. This was done using a careful optimization of the detector systems, the measurement technique and choice of experimental site.  

I will give a short introduction to neutrinos, what we know about them and how we know these things. I will then talk of the mathematical formalism of neutrino mixing and how we might measure the parameters of mixing.

March 11, 2013 - Mehran Kardar (MIT)

"Levitation by Casimir Forces In and Out of Equilibrium"

A generalization of Earnshaw's theorem constrains the possibility of levitation by Casimir forces in equilibrium. The scattering formalism, which forms the basis of this proof, can be used to study fluctuation-induced forces for different materials, diverse geometries, both in and out of equilibrium. In the off-equilibrium context, I shall discuss non-classical heat transfer, and some manifestations of the dynamical Casimir effect.

March 4, 2013 - Rajan Gupta (Los Alamos National Laboratory)

"India's Energy [In]Security, China's Growing Consumption, Spain's Options"

Energy security, economic development and climate security are three urgent, pressing and interconnected goals.  This talk will examine how different regions of the world aim to meet their energy needs, and sustain development. This presentation will then focus on three major regions, China, India and Europe (using Spain as an example) to illustrate challenges and opportunities. Finally, it will end with some philosophical thoughts on climate change and sustainable development.

February 25, 2013 - Scott Ransom (NRAO)

"Detecting Gravitational Waves (and doing other cool physics) with Millisecond Pulsars"

The first millisecond pulsar was discovered in 1982. Since that time their use as highly-accurate celestial clocks has improved continually, so that they are now regularly used to measure a variety of general relativistic effects and probe a variety of topics in basic physics, such as the equation of state of matter at supra-nuclear densities. One of their most exciting uses though, is the current North American (NANOGrav) and international (the International Pulsar Timing Array) efforts to directly detect nanohertz frequency gravitational waves, most likely originating from the ensemble of supermassive black hole binaries scattered throughout the universe. In this talk I’ll describe how we are using an ensemble of pulsars to try to make such a measurement, how we could make a detection within the next 5-10 years, and how we get a wide variety of very interesting secondary science from the pulsars in the meantime.

February 11, 2013 - John D. Monnier (University of Michigan)

"Imaging the Surfaces of Stars"

Under even the best atmospheric conditions, telescope diffraction fundamentally limits the angular resolution for astronomical imaging. Using interferometry, we can coherently combine light from widely-separated telescopes to overcome the single-telescope diffraction limit to boost our imaging resolution by orders of magnitude. I will review recent technical and scientific breakthroughs made possible by the Michigan Infrared Combiner of the CHARA Array on Mt. Wilson, CA, with baselines of 330 meters allowing near-infrared imaging with sub-milli-arcsecond resolution. I will present the first resolved images of main sequence stars besides the Sun, focusing on the oblate and gravity-darkened photospheres of rapidly rotating stars.  We can now also resolve the interacting components of close binary stars for the first time and I will highlight the recent remarkable eclipse of epsilon Aurigae as viewed by CHARA.    

---

FALL 2012 

December 3, 2012 - Eric Siggia (Rockefeller University)

"Geometry and Genetics"

Developmental genetics has been very successful in attaching a function to thousands of genes whose mutation can lead to developmental defects. This makes the task of quantitative modeling effectively impossible. We have instead used geometric methods from dynamical systems theory plus genetics to posit simple models of phenotype that collapse the activity of many genes onto a few parameters. 

November 26, 2012 - 

 Joshua Frieman (University of Chicago/Fermilab)

 November 19, 2012 - Stuart Raby (Ohio State University)

"The Puzzle of Charge and Mass"     

Beginning with the seminal work of Rutherford, Geiger and Marsden in 1911, physicists have investigated the atom using particle beams  (alpha particles, and protons) as probes. They developed new detection methods; the geiger counter, scintillators, cloud and then bubble chambers.  This new paradigm for probing matter and new detectors led to many discoveries.

To make a long story short, by 1974 the chaos of discovery lead to the Standard Model describing all observed particle phenomena in terms of three fundamental forces (4 including gravity) and the fundamental building blocks of matter, quarks and leptons.

Only now, after the dust of this chaotic discovery settles, are we able with hindsight to recognize the underlying principles which define the theory we call the Standard Model.  It is these principles and their logical extension which I will attempt to describe in this talk. 

November 12, 2012 - David Hammer (Tufts University)

"What do the students need?"

 The instruction we offer students, at any level, presumably reflects what we believe will help them understand physics.  But we don’t often subject our beliefs to scrutiny.  Most physics instructors work from common sense assumptions about what students need—clear explanations, demonstrations, motivation and practice.  As in physics, however, common sense ideas (e.g. 'objects move because they are pushed') aren't always correct.  I will present evidence that these usual assumptions are insufficient and offer an expanded set of possibilities, focusing in particular on how students understand knowledge and learning.  In many cases what students most need is help taking a different approach to learning, to start thinking of science as a “refinement of everyday thinking” (Einstein, 1936) rather than as a body of new information to memorize.

November 5, 2012 - Geralyn Zeller (Fermilab)

"Neutrino Physics: This Decade and Beyond"

Neutrinos are one of nature's strangest and most elusive particles. For more than 50 years, they have surprised us: not only by their mere presence, but also by the revelation that these ghostlike particles can oscillate from one type to another. This stunning discovery has opened up a host of new questions about neutrinos and their properties; questions which we will be trying to answer in the next decade and beyond. In this talk, a survey of recent experimental results will be presented, along with projections for what the future holds. If history is any indication, we are in store for an exciting ride.

October 29, 2012 - Mehran Kardar (MIT) 

CANCELLED DUE TO STORM             

October 22, 2012 - David Griffiths (Reed College)  

"Hidden Momentum"

 Electromagnetic fields carry energy, momentum, and even angular momentum. The momentum density is ε_0 (E×B), and it accounts (among other things) for the pressure of light. But even static fields can harbor momentum, and this would appear to contradict a general theorem: if the center of energy of a closed system is at rest, then its total momentum must be zero.  Evidently in such cases there lurks some other momentum, not electromagnetic in nature, which cancels the field momentum. But finding this “hidden momentum” can be surprisingly subtle. I’ll discuss a particularly cute example.

October 15, 2012 - Douglas Finkbeiner (Harvard University)

"The Galactic Center 130 GeV Line:  WIMP or Artifact?"

 The recent claims of a gamma-ray line in the Galactic center at 130 GeV
has generated excitement, not least because it could be a signal of dark
matter annihilation.  I will summarize the current state of the
observations of the Galactic center, clusters, and unassociated halo
objects, and speculate about models of particle dark matter that could
explain the data.

Special Colloquium

October 10, 2012  - Greg Landsberg (Brown University)

"Discovery of the Higgs Boson(?) at the Large Hadron Collider"

 On July 4th, 2012 the ATLAS and CMS Collaborations at the Large Hadron Collider announced a discovery of a new boson with the mass of 125 GeV, a likely candidate for a long-sought Higgs boson. In this special colloquium I'll give an inside track of the events that lead to this fundamental discovery, explain its importance, and discuss the next steps in uncovering the true nature of the new particle.

October 1, 2012 - Brendan Casey (Fermilab)

"The Next Muon g-2 Experiment"

 There is a discrepancy between the measured and expected values of an intrinsic property of the muon, its anomalous magnetic dipole moment, aka g-2.  The discrepancy could be an indication of new physics beyond the current standard model of particle physics and leads to predictions that new particles will be found at the LHC.  As an example, within weeks of the discovery of the Higgs at the LHC, papers were submitted correlating the Higgs to gamma gamma branching fraction with the g-2 anomaly.   An extraordinary opportunity has arisen with the successful completion of the Tevatron program at Fermilab.  The infrastructure used to create antiprotons can readily be repurposed to create muons.  The Fermilab accelerator complex will be able to produce close to twenty times more muons than recorded by the last experiment at Brookhaven in less than two years allowing us to measure g-2 to a precision of 140 parts per billion. I will discuss the motivation for the new experiment, the technical difficulties involved in the prediction and measurement, and the progress towards mounting the new experiment.

September 24, 2012 - 

 Robert Hallock (University of Massachusetts, Amherst)

"Is Solid Helium-4 an Example of a 'Supersolid'?  Come, Listen, and See What You Think"

 About 40 years ago it was predicted that it might be possible for solid helium to display some of the properties of a superfluid, thus there might be a 'supersolid'.  Experiments in the mid to late 1970’s were negative and the prediction languished.  Then in 2004 an experiment was done that was interpreted as positive evidence for the presence of a 'supersolid' in solid helium.  But, the interpretation has been very controversial and other evidence has complicated the picture.  After a review of some background and a few key experiments, experiments will be described that are conceptually different from all the others. These provide direct evidence to that it is possible to pass helium through a sample cell filled with solid helium .  Some of the recent predictions for 'supersolid' behavior due to theorists differ substantially from the original theory of 40 years ago and these are rather consistent with our observations.  This will be an experimentally oriented talk, designed to be accessible to all.

September 17, 2012 - Rouven Essig (SUNY - Stony Brook)

"The Hunt for Dark Matter"

 Dark matter makes up 80% of the matter in our Universe, but we have yet to learn its identity. Astrophysical probes are becoming powerful enough to inform us on some of its fundamental properties and a wide array of experiments are probing its non-gravitational interactions with ordinary matter.  In this talk, I will review the astrophysical evidence for dark matter and give a broad overview of current and future direct and indirect detection experiments, as well as collider probes.  I will also discuss some of the hints for dark matter at currently running experiments.