Title: Decoding the cosmos with gravitational waves

 Title: Where are the supermassive black holes measured by PTAs?

Abstract: Pulsar timing arrays (PTAs) consist of a set of regularly monitored millisecond pulsars with extremely stable rotational periods. The arrival time of pulses can be altered by the passage of gravitational waves (GWs) between them and the Earth, thus serving as a galaxy-wide GW detector. Evidence for the first detection of low-frequency (~nHz) gravitational waves has recently been reported across multiple PTA collaborations, opening a new observational window into the Universe. Although the origin of the GW signal is yet to be determined, the dominant sources are expected to be inspiralling supermassive black holes. I will discuss what we are learning from mapping the nano-hertz GW sky, focusing on a recent work in which we compare the GW detections by PTAs with the expected signal implied by existing electromagnetic observations in a simple but robust manner. We highlight that there is a simple upper limit to the GWB amplitude and that the currently measured GW amplitude is somewhat larger than expected. I will then show that additional information regarding the typical number of sources contributing to the background can already be inferred from current PTA data. 

Title: What’s wrong with QED?

Abstract:  Quantum electrodynamics (QED) is the extraordinary relativistic quantum theory of electromagnetic interactions of particles.  It is extraordinary because it makes incredibly accurate predictions that all appear to be consistent with experiment.  For example, QED gives the ratio of the magnetic moment of the electron to the Bohr magneton with an uncertainty of less than 2 parts in 1013 or with about 14 significant figures.  It would thus appear that nothing is wrong with the theory and we can skip this talk.  However, despite its overwhelming success, QED is deeply flawed, at least in principle.  The problem is that to get such extraordinarily accurate results, it is necessary to do calculations with expressions that are actually infinite and to subtract another slightly different expression that is also infinite.  If things are done according to a well-defined set of rules, the answer comes out to be finite and, perhaps surprisingly, also correct, meaning in agreement with experiment.  One could take the position that this is just an inconvenience because there are practitioners who know how to sweep the infinities under the rug, and many share this point of view.  However, being one of those practitioners, I, along with many other physicists find this to be unsatisfactory.  In this talk, you will see videos of Paul Dirac and Richard Feynman, two of the most important contributors to the development of QED, expressing the same concern.  Also in the talk, I will point out general features of QED that may be hiding the problem.  Even though this sounds like a very technical topic, I will attempt to make it clear to people who may have no familiarity with QED.

Title: Chemistry without Chemical: An Introduction to Computational Quantum

Chemistry through Applications to Hydrogen Bonding & Concerted Proton Transfer

Abstract:  The subjects of solvation, molecular recognition and supramolecular self-assembly provide some of the motivation and impetus for the work that is the focus of the talk. Convergent approaches to quantum mechanical (QM) ab initio electronic structure calculations have provided tremendous insight into the structures, energetics and spectroscopic signatures of molecular clusters held together by relatively weak, non-covalent interactions (London dispersion forces, hydrogen bonding, halogen bonding, π-stacking, etc.). Unfortunately, the computational demands associated with the most accurate and reliable QM methods often prohibit their application to large molecular systems. Part of this talk will focus on strategies that systematically converge toward exact numerical solutions of the electronic Schrödinger equation via methodical application of correlated wave function methods and Gaussian atomic orbital basis sets. That will set the stage for an overview of computational techniques for non-covalent clusters that take advantage of the many-body expansion (MBE) of the total energy. A layered, ONIOM-like approach to the MBE is one such technique that we have developed to extend demanding QM electronic structure computations, such as the CCSD(T) method, to larger systems. If time permits, some additional applications will be discussed that examine concerted proton transfer processes in cyclic hydrogen-bonded clusters composed of H2O,