Sommerfeld Theory Colloquium (ASC)

The Massive Black Hole and Nuclear Star Cluster of the Milky Way, Evidence has been accumulating for several decades that many galaxies harbor central mass concentrations that may be in the form of black holes with masses between a few million to a few billion time the mass of the Sun. I will discuss measurements over the last two decades, employing adaptive optics imaging and spectroscopy on large ground-based telescopes that prove the existence of such a massive black hole in the Center of our Milky Way, beyond any reasonable doubt. These data also provide key insights into its properties and environment. Future interferometric studies of the Galactic Center black hole promise to be able to test gravity in its strong field limit. I will also briefly discuss the cosmological evolution of massive black holes.

Maximal Supersymmetry, Light-Front Dynamics and Exceptional Symmetries, In this talk I will describe the light-front formulation of maxi- mally supersymmetric theories. This is a formalism where only the physical degrees of freedom is used. This means that the SuperPoincaré invariance is non-linearly realized, a fact we use to build the models. In this formalism the N=4 Yang-Mills and the N=8 Supergravity are treated in a very similar fashion and the close relationship between them is obvious. I will also show that the exceptional symmetry E_7(7) is very naturally connec- ted to the N=8 SuperPoincaré algebra. This formalism is an al- ternative to the ordinary covariant formalisms and is very use- ful to investigate the UV properties of these models.

Imaging Astrophysical Turbulent Convection and Dynamo Action in the Sun, The Sun is a most remarkable object: it is filled with vibrantly evolv- ing magnetic fields, well-mixed hierarchically-arranged turbulent con- vective cells, and also poorly-mixed sunspots that persist with im- punity. Solar variability has direct consequences for the earth and space weather, an important reason to develop an appreciation for the physics of the solar cycle (dynamo).

Direct observation of the solar subsurface is impossible due to the high degree of optical scattering by the partially ionized plasma that inhabits the near-surface layers of the Sun. The deepest part of the Sun visible to us, known as the photosphere (also the solar surface), appears as a roiling, turbulent, radiative, magnetized, convecting plasma. At first glance, it would seem therefore that subtle ques- tions relating to the subsurface constitution of the Sun seem com- pletely unanswerable and the interior properties unknowable. How- ever, analogous to geoseismology, a great deal can be gleaned about the internal structure and dynamics of the Sun by carefully observ- ing and analyzing the surface wavefield. This has been made possible over the last few decades through the development and application of techniques of helioseismology.

In this talk I will outline some of the major results in this area over the last two decades and discuss some recent developments pertaining to the properties of turbulence in the deep-convection zone of the Sun.

Testing Strings at the LHC?, A theory with such a mathematical beauty cannot be wrong: this was one of the main arguments in favor of string theory, which unifies all known physical theories of fundamental interactions in a single coherent description of the universe. But no one has ever observed strings, not even indirectly, neither the space of extra dimensions where they live. However, there are good reasons to believe that the ``hidden" dimensions of string theory may be much larger than what we thought in the past and they become within experimental reach in the near future, together with the strings themselves. In my talk, I will give an elementary introduction of this framework and describe the main experimental predictions.

Anderson Localization: Looking Forward, Localization of the eigenfunctions of quantum particles in a random potential was discovered by P.W. Anderson more than 50 years ago. In spite of its respectable maturity and rather intensive theoretical and experimental studies this field is by far not exhausted. Anderson localization was originally discovered in connection with spin relaxation and charge transport in disordered conductors. Later this phenomenon was observed for light, microwaves, sound, and more recently for cold atoms. Moreover, it became clear that the domain of applicability of the concept of localization is much broader. For example, it provides an adequate framework for discussing the transition between integrable and chaotic behavior in quantum systems. We will discuss current understanding of the Anderson localization and its manifestation in different physical situations.

Gravitational Waves from Coalescing Binary Black Holes: Theoretical and Experimental Challenges, A network of ground-based interferometric gravitational wave detectors (LIGO/VIRGO/GEO/...) is currently taking data near its planned sensitivity. Coalescing black hole binaries are among the most promising, and most exciting, gravitational wave sources for these detectors. The talk will review the theoretical and experimental challenges that must be met in order to successfully detect gravitational waves from coalescing black hole binaries, and to be able to reliably measure the physical parameters of the source (masses, spins, ...).

Testing the Dark Energy Paradigm and Beyond, The next generation of surveys, e.g. the Dark Energy Survey, PanSTARRS, LSST, Euclid and others, aim to study the nature of Dark Energy and alternatives.

The talk will discuss how the Dark Energy paradigm evolved over the past 20 years, and the cosmic probes which will help us to test it.

In particular the surveys rely on accurate of photometric redshifts for the determination of cosmological quantities such as Dark Energy parameters and neutrino masses.

The talk will describe photometric redshift methods and their impact on analysing galaxy clustering and weak lensing data and on the derived cosmological parameters.

The Inverse Ising Problem, The "Inverse Ising Problem" refers to finding the parameters (the Jij's and the hi's) in an Ising model given the first and second moments (the magnetizations mi and the correlation functions cij).

This is of considerable interest in machine learning and data analysis whenever the data set and the number of variables is large, but the values taken by the variables can be taken to be "high" and "low". The maximum entropy distributions with given first and second moments then has the Ising form where the hi's and Jij's are Lagrange parameters.

Several perturbative methods to solve the inverse Ising problem approximately have been proposed in the last few years. I will give a survey of the situation, with a focus on what we have know about the applicability of these methods to data such as gene expression and recordings from many neurons, where an underlying exact description is surely not of the Ising form.

This is work with John Hertz and Yasser Roudi (Frontiers Comp Neuroscience, 2009) and work in progress.

The search for a perfect fluid: is string-theory relevant for ultracold atoms?, Based on a calculation of the shear viscosity η and the entropy density s of certain strongly coupled field theories via the AdS/CFT-correspondence, Kovtun, Son and Starinets conjectured that their ratio is bounded below by the universal number ħ/4πkB, a bound that appears to hold for all existing fluids. The substances that come closest to being perfect in the sense of a minimum value of η/s are the quark gluon plasma and ultracold atoms at infinite scattering length, the so called unitary Fermi gas. The talk provides an introduction to the origin and interpretation of this bound from a simple physics perspective. Moreover, quantitative results for η/s are presented in the normal phase of the unitary Fermi gas. They are consistent with the Kovtun, Son and Starinets bound and are also in good agreement with experiments.

A simple proof of the Kochen-Specker theorem on the problem of hidden variables, In this talk I present a simple derivation of an old result of Kochen and Specker, which is apparently unrelated to the famous work of Bell on hidden variables, but is presumably equally important. Kochen and Specker showed in 1967 that quantum mechanics cannot be embedded into a classical stochastic theory, provided the quantum theoretical probability distributions are repro- duced and one additional highly desirable property is satisfied. This showed in a striking manner what were the difficulties in implementing the Einstein programme of a `complete' version of quantum mechanics.