Sommerfeld Theory Colloquium (ASC)

Sommerfeld Theory Colloquium, The upshot of extensive studies of �uctuations in condensed matter systems is that

their qualitative importance is typically con#ned to isolated critical points of

continuous transitions between phases of matter. This conventional wisdom also

predicts the number of low energy Goldstone modes based on the so-called “G/H”

pattern of symmetry breaking. I will discuss a class of systems, some quite wellknown,

that violate this standard paradigm. Namely, they exhibit a fewer than “G/H”

number of low-energy modes due to an emergent Higgs mechanism. Even more

spectacularly, such systems exhibit “critical” ordered phases, with universal power-law

properties reminiscent of a critical point, but requiring no #ne-tuning and extending

throughout the ordered phase. One exciting recently discovered state is the heliconical

nematic that in addition to above phenomena also exhibits spontaneous chiral

symmetry breaking.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, The conjectured relation between higher spin theories on anti de-Sitter (AdS) spaces and weakly coupled conformal �field theories is reviewed. I shall then outline the evidence in favour of a concrete duality of this kind, relating a speci�c higher spin theory on AdS3

to a family of 2d minimal model CFTs. Finally, I shall explain how

this relation �ts into the framework of the familiar stringy AdS/CFT correspondence.

Sommerfeld Theory Colloquium, 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.

Sommerfeld Theory Colloquium, 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.