Qiss

The laws of Physics are time-reversible—yet we can only go towards the future. This apparent contradiction is known as the “arrow of time problem”. Its resolution is that the future is the direction of increasing entropy. But entropy can only increase towards the future… if it was low in the past, and past low entropy is a very strong hypothesis to make, because low entropy states are so improbable!
Some authors suggest that we can do away with this “past hypothesis” within reversible dynamical laws featuring expansion à la GR. We prove that this is indeed the case in principle, within a toy model. It consists in just graphs upon which particles circulate and interact according to local reversible rules. Some rules locally shrink or expand the graph. We prove that almost all states expand and that entropy always increases as a consequence.
Interestingly, the toy universe can easily be run backwards, until “before the Big Bang”. We can therefore tell the story of “the beginnings of times” in this model. It’s enlightening… and mind-bending at the same time.
Joint work with Gilles Dowek and Amélia Durbec

We propose a special relativistic framework for quantum mechanics. It is based on introducing a Hilbert space for events. Events are taken as primitive notions (as customary in relativity), whereas quantum systems (e.g. fields and particles) are emergent in the form of joint probability amplitudes for position and time of events. Textbook relativistic quantum mechanics and quantum field theory can be recovered by dividing the event Hilbert spaces into space and time (a foliation) and then conditioning the event states onto the time part. Our theory satisfies the full Poincare’ symmetry as a `geometric’ unitary transformation, and possesses relativistic observables for space (location of an event) and time (position in time of an event).

Bell’s theorem and Bell inequality violations are one of the most iconic results in quantum physics. While certainly further from experimental realisation, experiments involving quantum control of observers—the so-called Extended Wigner’s friend scenarios—pose an even stronger challenge to our understanding of quantum theory. In this talk, after a review of the implications of Bell and Local Friendliness no-go theorems on the interpretations of quantum mechanics, I will present a recent result about the failure of causal reasoning in dealing with the predictions of quantum mechanics in extended Wigner’s friend scenarios: not just classical causal reasoning, but causal reasoning using generalised probabilistic theories fails to faithfully account for the predictions of QM.

Understanding the properties of a generic state given by the superposition of different spacetimes configuration is one of the major questions that quantum gravity aim to address. This has major implication for the physics of the early universe, where the seeds of cosmic structures are originated from quantum fluctuations of the geometry. Predicting the strength of correlations of geometrical observables in this regime gives direct access to the initial conditions of our universe, from which all matter structures later evolved.
Exploiting techniques from covariant Loop Quantum Gravity, I present an operational recipe to construct physical states of the quantum geometry, to define a cosmological interpretation for them, and compute analytically and numerically the quantities that characterize them, i.e. correlations between the volumes of spacially-separated region and the corresponding entanglement entropy. I review the results obtained and I discuss the future perspective of this research program.