Colloquium

In 1983, Nielsen and Ninomiya predicted that the Adler-Bell-Jakiw (or chiral) anomaly should be observable in a crystal that has protected Dirac states in the bulk (3+1 D). Following recent progress in the field of Topological Quantum Matter, the anomaly has now been observed, most clearly in the two semimetals Na3Bi and GdPtBi. I will discuss the general problem of realizing Weyl Fermions in semimetals, and explain what the chiral anomaly is in condensed matter. I will remark on its historical context, starting with pion decay. Several tests that support our conclusion will be described.

*Supported by the Moore Foundation, ARO and NSF.

Population expansions are ubiquitous in nature. They control the speed of many important dynamical processes, including multicellular development, biological evolution and epidemic outbreaks. Yet, the theoretical description of spreading behaviors has been limited largely to mean-field models that ignore the randomness inherent to living systems. In this talk, I present theoretical arguments and experimental results that elucidate how noise influences spreading processes on many scales, ranging from cellular scales, where jamming cells impede their own expansion, to global scales, where epidemic spread relies on rare long-range jumps.  Our results underscore that carrying excellent genes does not guarantee success in evolution - the pure luck of being in the right place at the right time can be equally, or more, important.

It has long been believed that Stradivari and his contemporaries in 18th Century Italy built violins with playing qualities unmatched by later makers. However, a team of researchers led by Claudia Fritz and Joseph Curtin have shown that under double-blind conditions neither professional violinists nor experienced listeners can tell Old Italian violins from new ones at better than chance levels. Moreover, players and listeners tend to prefer the new. Violin-maker, researcher, and MacArthur Fellow Joseph Curtin will discuss recent developments in violin science, and his own interest in measuring violin sound in order to establish objective parameters for violin quality.

With two confident binary black hole mergers already detected in their first observing run, the advanced LIGO detectors are expected to detect hundreds more in coming years.  We are poised to learn more about compact binary (e.g., BNS, NSBH, BBH) formation than ever before, but doing so will demand both detailed characterization of individual events, and inference of the properties of the underlying population as a whole.  I will present the methods behind the detection and characterization of LIGO's binary black hole mergers, what they have taught us about compact binaries thus far, and what they will potentially teach us about compact binary formation in the future.

In 2015 the LIGO detectors observed gravitational waves from two distinct stellar-mass binary black hole mergers. This long awaited feat now opens avenues to explore astrophysical questions which cannot, or are difficult to, be answered purely by electromagnetic means. Massive stars which end their lives in a pair-instability supernova are not thought to leave a remnant behind, meaning there should exist a gap in the black hole mass spectrum. In this talk I will discuss whether LIGO observations can tell us something about this apparent mass gap. 

LIGO is currently operating in its second science run, and like its first, alerts are sent to electromagnetic partners whenever a candidate event is identified. I will also discuss the efforts being made to capture the signature of an electromagnetic counterpart, some of the facilities involved and the hurdles which need to be overcome to make a confident association between a gravitational-wave signal and an electromagnetic transient.

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 both continuous gravitational waves and 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.

The spectral action functional of noncommutative geometry provides a model of Euclidean (modified) gravity, possibly coupled to matter. The terms in the large energy asymptotic expansion of the spectral action can be computed via pseudodifferential calculus. In the case of highly symmetric spacetimes, like Robertson-Walker metrics and Bianchi IX gravitational instantons, there is a richer arithmetic structure in the spectral action, and the terms in the asymptotic expansion are expressiblein terms of periods of motives and of modular forms. This reveals a new occurrence of interesting periods and motives in high-energy physics.

By a celebrated theorem of Jacob Lurie, an extended TQFT is entirely determined by what it assigns to a point. It is natural to ask whether this theorem applies to TQFTs of physical interest. And, if yes, what do these theories assign to a point? In this talk, I will propose an answer for the case of 3-dimensional Chern-Simons theory. I will then
explore the relation to line defects in chiral WZW models, and boundary conditions for full WZW models. At last, I will present a conjectural classification of the above mentioned line defects and boundary conditions.

Laser-cooled trapped ions are among the most versatile experimental platforms for exploring quantum information. In this talk, I will give a brief overview of this system and its capabilities to simulate non-trivial interacting quantum models. Internal states of these ions, such as hyperfine states, constitute well isolated qubit (or spin-1/2) states, with quantum coherence demonstrated up to fifteen minutes. Individual qubits states can be detected by laser beams with near perfection. Quantum logic gates and interacting spin Hamiltonians are engineered by coupling internal states of multiple ions to their collective vibrational modes using optical forces. By suitably tailoring these spin-phonon couplings, interactions between ion-spins can be tuned in magnitude, range, and sign. The laboratory for Quantum Information with Trapped Ions at IQC aims to build a highly flexible quantum simulator with dozens of spins, and explore problems in quantum information and many-body physics in a regime that is intractable with classical computers.

The discovery of the Higgs boson at the LHC in 2012 was a watershed in particle physics. Its existence focuses attention on the outstanding questions about physics beyond the Standard Model: is `empty' space unstable? what is the dark matter? what is the origin of matter? what is the explanation for the small masses of the neutrinos? how is the hierarchy of mass scales in physics established and stabilized? what drove inflation? how to quantize gravity Many of these issues will be addressed by future runs of the LHC, e.g., by studies of the Higgs boson, and also motivate possible future colliders.

Black holes of 1 million to 20 billion solar masses have been found at the centers of galaxies.

New observational data and improved orbit models in the past several years have substantially expanded the sample of black holes with dynamically measured masses.  I will describe recent progress in discovering a new population of ultra-massive black holes and its impact on our understanding of the symbiotic relationships between black holes and galaxies.  I will discuss the implications for the ongoing pulsar timing array experiments searching for nano-Hertz gravitational waves from merging supermassive black hole binaries.