Sommerfeld Theory Colloquium (ASC)

In this lecture I propose to discuss some issues concerning the founda- tions of quantum mechanics and its interplay with space-time physics. I start by clarifying the distinction between ’realistic theories’ and ’probabilistic theories’ of Nature and sketch how the latter can often be viewed as ’deformations’ of the former. I then briefly recall some of the intriguing features of atomistic Quantum Mechanics, which belongs to the second class of theories. I attempt to describe, in con- ceptual terms, what it is that Quantum Mechanics predicts about Nature when appropriate experiments are done. I try to sketch some implications of this discussion for our views of space and time. I will conclude by sketching some recent results on the ’effective quantum dynamics’ of Open Systems, in particular on ’Quantum Brownian Motion’.

Are there plausible extensions of the Standard Model that could lead to early discoveries at the LHC? To address this general question on a concrete example, I will consider a class of minimal models with an extra massive neutral gauge boson Z'. I will first review different theoretical motivations for extending the SM gauge group with an extra U(1) factor, possibly broken near the TeV scale. I will then discuss the interplay between the bounds from electroweak precision tests and direct searches at the Tevatron, to identify the early LHC discovery potential. I will finally comment on the peculiar features of models where the Z' couples non-universally to lepton flavors and of string models with intersecting or magnetized branes.

One of the simplest ways to make gauge fields massive is to add them a mass "by hand". Intuitively, one could expect that the corresponding massless theory would then be easy to recover. Yet, conventional methods indicate that such a limit is singular. In this talk, we will explore the massless limits of several massive gauge theories. We will identify the source of the apparent discontinuities and show that they are, in fact, simply an artifact of the perturbative approach. Then, we will discuss the consequences of this study on the relations between different gauge fields. Finally, we will conclude with a comment on the latest insights about these theories and their prospects.

Our search for a quantum theory of gravity is aided by a unique and perplexing feature of the classical theory: General Relativity already knows" about its own quantum states (the entropy of a black hole), and about those of all matter (via the covariant entropy bound). The results we are able to extract from classical gravity are inherently nonperturbative and increasingly sophisticated. Recent breakthroughs include a derivation of the entropy of Hawking radiation, a computation of the exact integer number of states of some black holes, and the construction of gravitational holograms in our universe using techniques from single-shot quantum communication protocols.

String theory is around 50 years old and for much of that time it has been proclaimed as a quantum theory of gravity unified with all forces and matter. However, we still don’t know its fundamental formulation, although we do now know it is not just a theory of strings. Nonetheless, it has led to many new and surprising insights, with concepts that were once seen as absolute now seen as dependent on the “duality frame". In this talk I survey some of these insights and discuss their implications for physics and the fundamental formulation of string theory.

The question, whether a local, realistic theory can be a valid description of nature led to Bell's formulation of a clear cut experimental test. In spite of the many measurements performed and the numerous violation of Bell's inequality, all these tests relied on assumptions opening loopholes for local realistic theories. We present experiments which attempted to close as many as possible loopholes during the recent years, and what still might be left to do. In the experiment, as Bell's inequality limits preshared knowledge about possible measurement results, it can be used on the one hand to now confirm random numbers deduced from measurement results or the security of the devices used for quantum key distribution. On the other hand we can use the techniques developed for this experiment as the basic link for future quantum networks distributing entanglement efficiently over larger distances.

The apparent crisis of black holes inconsistency with foundational physical principles provides a sharp focus for the conflict between quantum mechanics and classical spacetime. Various resolutions have

been proposed; a very plausible one is that small interactions can transfer sufficient information between the black hole and outgoing radiation, with a quantum enhancement from the enormous number of black hole states. Such interactions must however violate conventional notions of locality, perhaps as a symptom of the more basic subtlety of information localization in quantum gravity, and hinting at aspects of the fundamental structure of quantum gravity. An intriguing question is whether further clues can be found from new observational windows on black holes.

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.

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.

In driven open quantum matter, coherent many-body quantum dynamics, drive, and dissipation play equally significant roles. These systems span a wide range of examples, including cold atomic gases, exciton-polaritons in solid state, and quantum devices designed for quantum information applications. These setups break the conditions of thermodynamic equilibrium on the microscopic scale, prompting questions about how this impacts macroscopic behavior, such as phases and phase transitions. We examine two key points: First, we showcase that a minor out-of-equilibrium perturbation on the microscopic level can lead to substantial macroscopic effects, including the emergence of novel non-equilibrium universality classes. This paves the way to active quantum matter scenarios in solid state physics. Second, we argue that drive and dissipation can be used constructively to maintain or even create fragile quantum mechanical correlations such as phase coherence, entanglement or topological order by carefully engineering the system. A topological quantum phase transition far from equilibrium can be induced in this way, exhibiting intriguing analogies to the problem of directed percolation.