Café

Introduction

• Luca Fabbri (-)

No Time No Problem

• Francesca Vidotto (IMAPP Nijmegen)

Exactly half of a century ago Wheeler and de Witt wrote the iconic first equation of quantum gravity. Puzzlingly, the equation does not feature any time variable. Where is time gone? How can we describe the evolution of physical systems in absence of a preferred time variable? How can time emerge in the passage from the quantum to the classical world? I will review the problem, the strategies adopted to overcome it and some insights from ongoing research in quantum gravity.

Do Virtual Particles Really Exist?

• Marco Guagnelli (INFN Pavia)

When physicists tried to unify Quantum Mechanics with Relativity, it soon became clear that the resulting theory could not conceivably be a single particle one. Creation and destruction of particles had to be taken into account. The theory we end up with is a Relativistic Quantum Field Theory: particles are just quanta of the respective fields. The outstanding success of Feynman's Path Integral formalism brought into some kind of "physical reality" the so called "virtual particles". Virtual particles are not observable asymptotic states, but can exist as long as they don't violate Heisenberg uncertainty principle. Their existence is used, didactically, to explain a lot of phenomena; for example, the running of the electron charge, or coupling constant, with energy, via the screening obtained by vacuum polarization of the cloud of e+ e- virtual couples that surround the "true" electron. I'd like to discuss what happens to this kind of reasoning when a QFT is treated in a non-perturbative way.

Introduction

• Luca Fabbri (-)

What is the Problem With the Cosmological Constant?

• Philippe Brax (IPhT CEA Saclay)

I will discuss the different aspects of the cosmological constant problem, both theoretically from the point of field theory and phenomenologically with an increasing number of observations suggesting that some form of dark energy is required. Several attempts to address this issue will be reviewed, although none has proved to be successful so far.

A Physically Realist Ontology for Quantum Mechanics

• Alexia Auffèves (Institut NÉEL Grenoble)

I will present a possible way to make usual quantum mechanics fully compatible with physical realism, defined as the statement that the goal of physics is to study entities of the natural world, existing independently from any particular observer’s perception, and obeying universal and intelligible rules. Rather than elaborating on the quantum formalism itself, a new quantum ontology is proposed, where physical properties are attributed jointly to the system, and to the context in which it is embedded. In combination with a quantization principle, this non-classical definition of physical reality sheds new light on counter-intuitive features of quantum mechanics such as the origin of probabilities, non-locality, and the quantum-classical boundary.