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The David and Edith Harris Physics Colloquium Series
SPRING 2016 Schedule
The Spring 2016 David and Edith Harris Physics Colloquium Series is now concluded.
Please visit this summer for the upcoming Fall 2016 schedule.
PAST SPRING 2016 COLLOQUIA
"Bulk Locality from Quantum Error Correction in AdS/CFT"
The Anti-de Sitter/Conformal Field Theory correspondence has given us a non-perturbative description of quantum gravity in asymptotically Anti-de Sitter space, as a quantum field theory (without gravity) living in one lower dimension. This proposal has passed many tests, but the emergence of the extra dimension has remained somewhat mysterious. In this talk I will discuss recent work relating this emergence to the theory of quantum error correcting codes, originally introduced to solve the seemingly unrelated problem of protecting a quantum computer from decoherence. I'll introduce some puzzles in AdS/CFT that are naturally resolved in this language. We will also see that it makes precise some recent speculations relating boundary entanglement to the emergence of bulk geometry. I will illustrate these ideas using an explicitly soluble model of AdS/CFT, constructed using tensor networks. The talk should be quite accessible to beginning graduate students!
"Atom Trap, Krypton-81, and Global Groundwater"
The long-lived noble-gas isotope 81Kr is the ideal tracer for water and ice with ages of 105 - 106 years, a range beyond the reach of 14C. 81Kr-dating, a concept pursued over the past five decades, is finally available to the earth science community at large. This is made possible by the development of the Atom Trap Trace Analysis (ATTA) method, in which individual atoms of the desired isotope are captured and detected. ATTA possesses superior selectivity, and is thus far used to analyze the environmental radioactive isotopes 81Kr, 85Kr, and 39Ar. These three isotopes have extremely low isotopic abundances in the range of 10-16 to 10-11, and cover a wide range of ages and applications. In collaboration with earth scientists, we are dating groundwater and mapping its flow in major aquifers around the world. We have also demonstrated for the first time 81Kr-dating of old ice.
“Second-Order Phase Transitions and Conformal Field Theories”
We review some recent progress on the long-standing problem of characterizing second-order phase transitions. The symmetries of second-order transitions are now better understood. In addition, there are new general results about critical exponents, a better control of what happens when relevant operators are turned on, and some constraints on higher correlation functions. These new ideas can be tested in systems ranging from boiling water to quantum gravity in Anti-de Sitter space.
"100 Years of Gravitational Waves: The Observation of a Binary Black Hole Collision"
The complex history of gravitational waves from Einstein's initial hypothesis to this month's direct measurement will be briefly presented. The instrument that made the detection and the measured waveforms will be described followed by a vision for the future of the field of gravitational wave astronomy.
"The Event Horizon Telescope: Imaging and Time-Resolving a Black Hole"
A convergence of high bandwidth radio instrumentation and Global mm and submm wavelength facilities are enabling assembly of the Event Horizon Telescope (EHT): a short-wavelength Very Long Baseline Interferometry (VLBI) array, which can observe the nearest supermassive black holes with Schwarzschild Radius resolution. Initial observations with the EHT have revealed event horizon scale structure in SgrA*, the 4 million solar mass black hole at the Galactic Center, and in the much more luminous and massive black hole at the center of the giant elliptical galaxy M87. Over the next 2 years, this international project will add new sites and increase observing bandwidth to focus on astrophysics at the black hole boundary. The EHT will have an unprecedented combination of sensitivity and resolution with excellent prospects for imaging strong GR signatures near the horizon, detecting magnetic field structures through full polarization observations, time-resolving black hole orbits, testing GR, and modeling black hole accretion, outflow and jet production. This talk will describe the project and the latest EHT observations.
"New Particles and Emergent Phases in Quantum Materials Assembled Atom-by-Atom"
The observation of massless Dirac fermions in monolayer graphene and topological insulators has propelled a new area of science and technology seeking to harness relativistic charge carriers within solid-state materials. Using low-temperature scanning tunneling microscopy and spectroscopy, we show the emergence of Dirac fermions in fully tunable condensed-matter systems—molecular graphene and related structures—assembled via atomic manipulation of a two-dimensional electron gas. We embed, image, and tune the symmetries underlying the two-dimensional Dirac equation into these electrons by sculpting the surface potential with manipulated molecules. By distorting the electron hopping parameters into a dimerized Kekulé pattern, we find that these natively massless Dirac particles can be endowed with a tunable mass engendered by the associated scalar gauge field, in analogy to the Higgs field. With altered symmetry and texturing, the Dirac fermions can be dressed with gauge electric or magnetic fields such that the carriers believe they are in real fields and condense into the corresponding ground state, as confirmed by tunneling spectroscopy. Using these techniques we have realized a quantum Hall state that preserves time-reversal symmetry, in which electrons quantize into Landau levels in a gauge magnetic field ramped up to 300 Tesla with zero applied laboratory field. We also observe spontaneous broken-symmetry nematic states emerging at very high band filling factors and within flatbands where kinetic energy is effectively quenched and Coulomb interactions dominate. These and other chiral phases can be used to guide or confine charge in nontrivial ways and to synthesize new particles.
"Evolutionary Dynamics in Microbes"
The basic rules of evolution are well known: mutations generate variation, while genetic drift, recombination, and natural selection change the frequencies of the variants. Yet even in very simple and well-defined circumstances, it is often surprisingly difficult to predict what is possible in evolution, over what timescales and in which conditions. This is particularly true in microbial populations, where natural selection faces a key problem: there is too much going on at once. Many mutations are often present simultaneously, and selection cannot act on each individually. Rather, mutations are constantly occurring in a variety of combinations linked together on physical chromosomes, and selection can only act on these combinations as a whole. This dramatically changes how evolution can act. I will describe both theoretical and experimental worked aimed at understanding evolution in these populations.
"Quantitative Biology: A Fusion Between Physics and Biology"
Advances in biology have presented a multitude of opportunities for physicists and for physics. I will illustrate the different types of opportunities using examples encountered during my personal journey as a theoretical physicist. At the molecular scale, a maximum entropy principle turns variations in the sequence composition of related proteins into a procedure to inform the prediction of protein structures and protein-protein interactions. At the cellular level, discovery and application of phenomenological "growth laws" lead to quantitatively accurate predictions on bacterial response to genetic and environmental perturbations. At the population level, physical expansion of population and tissue open up simple dynamic mechanisms to generate spatiotemporal patterns.
"Inside the Iran Deal: The Technical Evolution of a Historic Agreement"
This talk will describe the evolution of the technical negotiating strategy behind the Iran nuclear deal, the trade-offs that were made during the negotiations, and calculations on the ultimate ability of Iran to make nuclear weapons given the constraints of the deal. It concludes that while the deal has several loopholes that might allow Iran to get closer to a weapon than negotiators had hoped, the deal nonetheless creates a stable technical barrier against proliferation for the next eleven years; after which point the politics of the region must become the sole source of nonproliferation stability.
"Dirac Semimetals, Weyl States, and the Chiral Anomaly*"
A host of interesting electronic phenomena associated with the 2D Dirac states have been observed in graphene. The Dirac nodes in graphene are rigorously protected by time reversal symmetry (TRS) and inversion symmetry (IS). Starting in ~2012, interest turned to 3D Dirac materials. The combination of TRS, IS and point group symmetry leads to protected Dirac nodes, provided they lie on a high-symmetry axis. Predictions that the two semimetals Na3Bi and Cd3As2 are topological Dirac semimetals were quickly confirmed. I will describe the successful growth and refinement of these materials, focusing on Na3Bi. The existence of Dirac states in 3+1 dimensions provides an exciting platform for searching for the chiral anomaly in a crystal.
I will briefly explain the chiral anomaly and its mysterious niche in quantum field theory, and then describe its recent detection in Na3Bi. Spectral flow between Weyl states of opposite chiralities engenders an enhanced current plume that is locked to the direction of the applied magnetic field. I will outline a new approach that may turn up the chiral anomaly in strong spin-orbit-interaction, zero-gap semimetals which do not a priori have Dirac states in zero magnetic field (this is a large class). A strong magnetic field creates Weyl nodes. Again, the spectral flow in an electric field leads to the chiral anomaly. The thermoelectric properties of the chiral anomaly current are described.
*Supported by ARO, ARO-MURI, Moore Foundation and NSF
Physics in the Interest of Society Colloquium
"Sustainable Electricity: A Generational Change in the Making"
The electric grid has been called a system that works in practice, not in theory. While large power networks based on 19th century technology have served us remarkably well, new challenges await: a transition to carbon-neutral energy sources is now imperative. This introduces the problem of coordinating heterogeneous, temporally intermittent and spatially distributed resources at an unprecedented scale, and with greater precision than ever imagined by early grid architects. Solutions must reconcile the constraints of legacy infrastructure with a spectrum of new opportunities created by advanced sensors, controls and information technology.
This talk will outline the integration challenges from a physical perspective, starting with an elementary characterization of a.c. electric power systems and what is (surprisingly) not obvious about them. It will also introduce research on new tools such as micro-synchrophasors that support a qualitative leap in how we may observe, understand and manage the grid, as a critical infrastructure and enabler of timely change across the energy sector.
"Rebels of the Standard Model: Majorana Neutrinos and the Search for Neutrinoless Double-Beta Decay"
The neutrino is unique among the Standard Model particles. It is the only particle that could be its own antiparticle, a Majorana particle. A Majorana neutrino would acquire mass in a fundamentally different way than the other particles and this would have profound consequences to particle physics and cosmology. The only feasible experiments to determine the Majorana nature of the neutrino are searches for the rare nuclear process neutrinoless double-beta decay. These are very difficult experiments and it is still not clear which techniques are the best to pursue. In this talk, I will review the physics of the neutrino and the experimental challenges involved in searching for neutrinoless double-beta decay. I will highlight the achievements of the R&D program here at MIT.
"Ultracold Dipolar Gases: From Chromium to Lanthanides”
Dipolar interactions in gases are fundamentally different from the usual van der Waals forces. Besides the anisotropy, the dipolar interaction is nonlocal and as such allows for self organized structure formation. Ten years ago, the first dipolar effects in a quantum gas were observed in an ultracold Chromium gas. By using a Feshbach resonance, a purely dipolar quantum gas was observed three years later. Currently, dipolar interaction effects have been observed in lattices and also polar molecules. Recently, it became possible to study degenerate gases of lanthanide atoms among which one finds the most magnetic atoms. The recent observation of their collisional properties include the emergence of quantum chaos and very broad resonances. Similar to the Rosensweig instability in classical magnetic ferrofluids, a self organized structure formation was expected. In our experiments with quantum gases of Dysprosium atoms we have recently observe the formation of a droplet crystal. In contrast to theoretical mean field based predictions the superfluid droplets did not collapse. We find that this unexpected stability is due to beyond mean field quantum corrections of the Lee-Huang-Yang type.
Last updated on May 6, 2016 1:07 PM