Sommerfeld Theory Colloquium (ASC)

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Understanding the LIGO gravitational wave event (GW150914) May 08, 2017

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In February 2016 the LIGO team announced the detection of gravitational waves (GW) created by the merger of two black holes. In addition to confirming a major prediction of general relativity, successful GW detection would provide a powerful new tool for astrophysics. Given their evident importance, the LIGO results and the methods which led to them deserve independent critical analysis. This talk will present the results of one such study in a manner suitable for non-specialists.

The Physics of Active Matter Feb 02, 2017

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Over the past ten years, there has been a growing interest among physicists for ‘active matter’, a codename that encompasses systems in which energy is taken from the environment to generate self-propulsion at the single particle level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic colloids or actin filaments in motility assays, are strongly out-of-equilibrium and exhibit much richer behaviours than their passive counterpart. In this talk I will review recent progresses regarding the physics of active particles. I will show how simple concepts like pressure, the force density exerted by assemblies of particles on their container, play a new role for active systems because of the lack of equation of state. I will also show how new collective phenomena emerge, from the transition to collective motion to the existence of cohesive matter without cohesive forces, that have no counterpart in thermal equilibrium.

Scattering Amplitudes from Geometry Dec 22, 2016

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I will review for a general audience some recent developments in our understanding of the mathematical structure of scattering amplitudes in quantum field theory. Many of these developments involve properties that have been discovered ”experimentally”: not in actual experiments, but by carrying out a tedious calculation and then observing that the result has some remarkable hidden simplicity. I will give examples of this phenomenon, and in particular I will discuss some aspects of the geometry of the ”amplituhedron”, a geometric object that is believed to completely encode certain scattering amplitudes.

The Black Hole Information Paradox Revisited Nov 03, 2016

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I describe the physics of black holes and show how the traditional approach leads to the information paradox. I will then discuss some of the proposed resolutions and the difficulties they need to overcome. I then discuss soft black hole hair and describe how it may help to resolve the information paradox. Finally, I will review the problems that still need to be overcome.

Can a quantum computer solve optimization problems more Efficiently than a classical computer? Oct 27, 2016

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In this talk I will discuss connections between the physics of complex systems such as spin glasses and attempts to solve optimization problems by ”Adiabatic Quantum Computing” (AQC), a version of ”Quantum Annealing” (QA). An optimization problem is one in which one has to minimize (or maximize) an energy function in which there is competition between different terms so no single configuration of the variables minimizes each term in the energy. In statistical physics this competition is called ”frustration”. It leads to a complex energy ”landscape” with many valleys separated by barriers, so simple algorithms easily get trapped in local minima which have a higher energy than the global minimum. Many problems in science, and engineering are formulated as optimization problems. In quantum annealing one tries to avoid being trapped in a local minimum by adding quantum fluctuations so the system can tunnel to regions of lower energy. The strength of the quantum fluctuations is gradually reduced to zero during the annealing schedule. This method applies to problems with binary variables, known as qubits in the quantum case. There is considerable interest in AQC at present, in large part because a company, D-Wave, has produced an actual device, the latest version of which has about one thousand qubits. In addition, there has been considerable theoretical work mainly using computer simulations to see if there is a ”quantum speedup” compared with analogous classical algorithms in which thermal, rather than quantum, fluctuations are used to escape from local minima. In the talk I will discuss difficulties in obtaining a quantum speedup due to (i) (quantum) phase transitions that the system can undergo during the annealing schedule, and (ii) the sensitivity of the state of the system to the precise values of the interactions, i.e. chaos. A related chaotic effect is that the state of the system can change dramatically with small changes in the temperature (temperature-chaos), for thermal annealing, and the strength of the quantum fluctuations, for quantum annealing.

Effects of Dark Matter linear in Interaction Strength Jun 23, 2016

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Low-mass boson dark matter particles produced after the Big Bang form a classical field and/or topological defects. Effects produced by the interaction of ordinary matter with dark matter may be first power in the underlying interaction strength rather than the second power. This may give a big advantage, since the dark matter interaction constant is extremely small. Limits on certain types of dark matter have been improved up to 15 orders of magnitude. New experiments are proposed.

Quantum Critical Points in Metals: Non-Fermi Liquids and their Field Theoretical Description Jun 09, 2016

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Metals are found frequently in nature and their properties are usually very well described within Landaus Fermi liquid theory. Various strongly correlated materials exhibit strange metallic phases which do not fit into the Fermi-liquid framework, however. The theoretical description of such non-Fermi liquids remains one of the main unsolved problems in condensed matter physics. In this talk I will give an introduction to the problem and show how interesting strongly coupled field theories arise in the low energy description of such states, which are still very poorly understood. I will focus on the paradigmatic problem of a metal coupled to fluctuations of a critical Ising order parameter and discuss unexpected scaling properties at finite temperature.

Gone with the wind: The demise of protoplanetary discs and the birth of planets May 12, 2016

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Protoplanetary discs are natural consequence of star formation. These discs hold the left-over material from star formation, which constitutes the reservoir from which new planetary systems may form. The fate of a new planetary is then intimately linked to the evolution and ﬁnal dispersal of the disk from which is born, which determines also the striking diversity observed in extra-solar planetary systems. I will brieﬂy review our understanding of disc dispersal via a photoevaporative wind in the context of planet formation, and show how both processes are ﬁnally dominated by the irradiation from their central star.

From materials science to basic physics May 06, 2016

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Condensed matter provides us deep insights into quantum physics. Giving just two examples, wave-corpuscle duality manifests itself in spectroscopy of strongly correlated systems as coexistence of itinerant and atomic-like features, and graphene and other Dirac materials provide a natural playground to study vacuum reconstruction, Klein tunneling and other fundamental quantum relativistic phenomena. Electron-photon interaction is the key tool to understand this rich and nontrivial physics.

Revisiting the S-matrix bootstrap Apr 28, 2016

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