Sommerfeld Theory Colloquium (ASC)

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, The observed deviations from the laws of gravity of Newton and Einstein

in galaxies and clusters can logically speaking be either due to

the presence of unseen dark matter particles or due to a change in

the way gravity works in these situations. Until recently there was

little reason to doubt that general relativity correctly describes gravity

in all circumstances. In the past few year insights from black hole

physics and string theory have lead to a new theoretical framework in

which the gravitational laws are derived from the quantum entanglement

of the microscopic information that is underlying space-time.

An essential ingredient in the derivation is of the Einstein equations

is that the vacuum entanglement obeys an area law, a condition that

is known to hold in Anti-de Sitter space due to the work of Ryu

and Takayanagi. We will argue that in de Sitter space due to the

positive dark energy, that the microscopic entanglement entropy also

contains also a volume law contribution in addition to the area law.

This volume law contribution is related to the thermal properties of

de Sitter space and leads to a total entropy that precisely matches the

Bekenstein-Hawking formula for the cosmological horizon. We study

the effect of this extra contribution on the emergent laws of gravity,

and argue that it leads to a modification compared to Einstein gravity.

We provide evidence for the fact this modification explains the

observed phenomena in galaxies and clusters currently attributed to

dark matter.

Sommerfeld Theory Colloquium, 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.

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, 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, 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.