Virtual Seminars

I will sketch the toolbox and achievements of modern lattice quantum gravity, aka solving the full 4D Lorentzian gravitational path integral, using causal dynamical triangulations (CDT). It provides us with a long-sought window on physics in a near-Planckian regime, where geometry is highly nonclassical and not describable in terms of standard tensor calculus. With concrete answers to longstanding issues of how to properly implement diffeomorphism invariance, causal structure and Wick rotation, we are now focusing on interesting, approach-independent questions on the nature of nonperturbative quantum gravity and its observables. Based on results on the dynamical emergence of a 4D universe with de Sitter properties from first principles, early-universe cosmology may be our best bet in terms of quantum gravity phenomenology.

Quantum networks are the center of many of the recent advances in quantum science, not only leading to the discovery of new properties in the foundations of quantum theory but also allowing for novel communication and cryptography protocols. It is known that networks beyond that in the paradigmatic Bell’s theorem imply new and sometimes stronger forms of nonclassicality. We will review some recent experiments addressing non-classicality in different network structures.

Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, for instance a massive quantum system prepared in a quantum superposition of positions and sourcing a gravitational field, has attracted a lot of attention: experiments are working towards realising such a scenario in the laboratory, and measuring the gravitational field associated to a quantum source is expected to give some information about quantum aspects of gravity. However, there are still open questions concerning the precise conclusions that these experiments could draw on the nature of gravity, such as whether experiments in this regime will be able to test more than the Newtonian part of the gravitational field. In this talk, I will show that a static mass in a quantum state gives rise to effects that cannot be reproduced using the Newton potential nor with a known classical model of gravity. These effects can in principle be measured by performing an interference experiment, and are independent of graviton emission.
Identifying stronger quantum aspects of gravity than those reproducible with the Newton potential is crucial to prove the nonclassicality of the gravitational field and to plan a new generation of experiments testing quantum aspects of gravity in a broader sense than what proposed so far.

I will present a consistent theory of classical systems coupled to quantum ones via the path integral formulation. The dynamics is linear in the density matrix, completely positive and trace-preserving. We then apply the formalism to general relativity, since it’s reasonable to question whether spacetime should have a quantum nature given it’s status within quantum field theory. In the classical-quantum formalism, the measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical spacetime necessarily causes collapse of the wave-function. Any such classical quantum theory necessarily has an experimental signature which can be used to test the quantum nature of gravity. I’ll conclude with an update on the current status of the program.

Many presentations of quantum mechanics include a postulate that the state of a system undergoes an instantaneous change following a measurement. This is clearly incompatible with special and general relativity and raises questions concerning the description of measurement in quantum field theory (QFT). Attempts to extend measurement postulates to QFT by hand have produced pathologies, …

Christopher J. Fewster
University of YorkMeasurement in Quantum Field Theory: Problems and Solutions Read More »

Identical quantum particles exhibit only two types of statistics: bosonic and fermionic. Theoretically, this restriction is commonly established through the symmetrization postulate or (anti)commutation constraints imposed on the algebra of creation and annihilation operators. The physical motivation for these axioms remains poorly understood, leading to various generalizations by modifying the mathematical formalism in somewhat arbitrary …

Borivoje Dakić
University of ViennaReconstruction of Quantum Particle Statistics: Bosons, Fermions, and Transtatistics Read More »

We will discuss an interpretation of physics named “potentiality realism”. This view, which can be applied to classical as well as to quantum physics, regards potentialities (i.e. intrinsic, objective propensities for individual events to obtain) as elements of reality, thereby complementing the actual values taken by physical variables. This allows one to naturally reconcile realism …

Flavio Del Santo
University of Geneva & Constructor UniversityPotentiality realism: Classical and quantum indeterminism Read More »

Following some observations of Howard Stein, I argue that, contrary to contemporary standard philosophical views of physical theories, one cannot understand the structure and nature of our knowledge in physics without an analysis of the way that observers (and, more generally, measuring instruments and experimental arrangements) are modeled in theory. One upshot is that standard …

Erik Curiel
Munich Center for Mathematical PhilosophySchematizing the Observer and the Epistemic Content of Theories Read More »

After ten years in science, wandering between quantum gravity and quantum foundations, I have entered l’Institut National du Service Public (ex-ENA) to pursue a career as top executive in the French administration. In this unusual talk, I will offer my personal take on the following questions: – Is there a life after academia? – What …

Pierre Martin-Dussaud
Institut National du Service PublicCan French Administration be more sexy than Quantum Gravity? Read More »