Sommerfeld Theory Colloquium (ASC)

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, 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.

Sommerfeld Theory Colloquium

Sommerfeld Theory Colloquium, Do quantum many-body systems necessarily come to thermal equilibrium

after a long enough time evolution? The conventional wisdom

has long been that they do and that, in the process, any quantum

information encoded in the initial state is lost irretrievably. Thus the

dynamics of many-interacting particles becomes effectively classical.

But these ingrained notions of thermalization and ergodicity have

recently been called into question. In this talk I will discuss how

ergodicity can break down in disordered quantum systems through

the phenomenon of many-body localization. In contrast to thermalizing

fluids, quantum correlations can persist through time evolution

of the localized state even at high energy densities. Thus, investigating

the many-body localization transition offers a concrete route

to address fundamental unsolved questions concerning the boundary

between classical and quantum physics in the macroscopic world. I

will emphasize the important role that quantum entanglement plays

in current attempts to understand this fascinating dynamical phase

transition. Finally I will present recent progress in confronting the

emerging theoretical understanding of many-body localization with

experimental tests using systems of ultra-cold atoms.

Sommerfeld Theory Colloquium, Highest resolution Event Horizon Telescope (EHT) observations will

probably soon tell us more about the supermassive black hole at

the Galactic Centre (Sgr A*) and the cores of active galactic nuclei

(AGN). It might also help to clarify the long-standing question

whether the central massive objects in AGN are instead close pairs

of black holes. Mergers of supermassive black hole pairs would provide

the strongest gravitational wave signals. I will present examples

of how we identify potential close binary black hole candidates

based on the combined analysis of high resolution radio interferometric

(VLBI) observations and multi- wavelength data. I will also

provide an outlook on the scientific prospects with regard to future

EHT-observations.

Sommerfeld Theory Colloquium, 2015 is the International Year of Light, and of its purposes is “to raise

awareness of optical technologies”. One such technology, high-power

lasers of the petawatt class and beyond, provides the most intense light sources created by humankind so far. The intensities and field

strengths in question are in excess of 1022 W/cm2 and 1014 V/m, respectively

magnitudes that correspond to concentrating the total solar

radiation on a pinhead. This talk will present an overview of the uses

and consequences of such extreme environments within the realm of

particle physics. The relevant theory is strong-field QED, with the

laser beams providing a rather peculiar electromagnetic background

field. The magnitudes above are such that a nonperturbative treatment

of the background becomes a necessity. Using appropriate theoretical

tools, a number of phenomena will be addressed, including

radiation reaction, nonlinear Thomson/Compton scattering, laserstimulated

pair production and photon-photon scattering. A particular

incarnation of the latter, polarisation flip forward scattering,

or vacuum birefringence, will be discussed in some detail, with an

outlook on a planned experiment at the European XFEL at DESY.

Sommerfeld Theory Colloquium, All flows show a transition from a laminar phase to a turbulent one for sufficiently high flow speeds. In many cases turbulence develops in a succession of instabilities that create flows of increasing temporal and spatial complexity (Lord Rayleigh, Sommerfeld, Heisenberg, Taylor, Landau etc), For the classroom example of pipe flow and

several other flows, the linear stability analysis of the laminar profile does not reveal any instability, so that the very first point in that cascade of instabilities is absent. Over the last decade much of the mystery of the transition in pipe flow has been resolved, primarily

thanks due to suitable adaptations and extensions of ideas from nonlinear dynamics. Numerical and experimental data corroborate a scenario where the appearance of new classes of fully 3d solutions and their increasing entanglement provides the key ingredients for the transition. Further studies on the spatio-temporal dynamics in the transition region, where turbulence is not space-filling, reveal

intriguing similarities to the directed percolation transition in statistical

mechanics.

Sommerfeld Theory Colloquium, The hard-disk model has exerted outstanding influence on computational

physics and statistical mechanics. Decades ago, hard disks

were the first system to be studied by Markov-chain Monte Carlo

methods and by molecular dynamics. It was in hard disks, through

numerical simulations, that a two-dimensional melting transition was

first seen to occur even though such systems cannot develop long-range

crystalline order. Analysis of the system was made difficult

by the absence of powerful simulation methods. In recent years, we

have developed powerful Monte Carlo algorithms for hard disks and

related systems. I will in particular show how the event-chain Monte

Carlo algorithm has allowed us to prove that hard disks melt with a

first-order transition from the liquid to the hexatic and a continuous

transition from the hexatic to the solid. I will finally describe how a

new factorized Metropolis filter transforms the event-chain algorithm

into a paradigm for general Monte Carlo calculations. First results

with the generalized algorithm have allowed us to establish the phase

diagram for two-dimensional soft disks and Yukawa particles.