Black holes and planckian discreteness
by Alejandro Perez
I will argue that the (quantum information) insights provided by general relativity and quantum field theory about key features of black holes make a strong case for the discreteness of geometry at the Planck scale of the type predicted by loop quantum gravity.
Quantum Gravity: the history of its main ideas
by Carlo Rovelli
The problem of understanding the quantum aspects of gravity was noticed by Einstein in 1916 (a decade before quantum theory). A tiny number of possible solutions to this problem are known today. These have emerged from a slow process of technical results, conceptual clarifications and novel ideas, and have survived decades of coherence checks and empirical testing. I review the key steps of this beautiful historical discovery path, which has given us remarkable insights about possible ways of thinking about quantum spacetime.
Causality in the quantum world
by Cyril Branciard
Causality is one of those fundamental notions, for which our classical intuitions get deeply challenged as soon as we enter the quantum domain. In these lectures I will present a few research directions related to quantum causality: how Bell’s “local causality assumption”, or more generally classical causal models, fail to explain quantum correlations; how one may try to understand these in terms of quantum causal models instead; and how one can envisage and exploit quantum processes with indefinite causal structures.
Quantum Gravity and Quantum Information
by Eugenio Bianchi
This is an introduction to problems at the interface of quantum gravity and quantum information. We will focus on the quantum geometry of space in loop quantum gravity, and discuss the entanglement entropy of observables in a region of space. The objective is to clarify the distinction between entanglement of observables, vs entanglement of non-observable quantities. We will use the quantum polyhedron and intertwiner space as a model system.
- E. Bianchi and E.R. Livine, “Loop Quantum Gravity and Quantum Information”) https://arxiv.org/abs/2302.05922
- E. Bianchi, P. Dona, and S. Speziale, “Polyhedra in Loop Quantum Gravity” https://arxiv.org/abs/1009.3402
Causal Set Quantum Gravity, Time and Consciousness
by Fay Dowker
I will present the causal set approach to the problem of quantum gravity from the perspective of its attitude to the question of the nature of time. I will present the foundations of the theory. I will explain how the foundational concepts of the theory allow one to evade the conclusion that many have drawn from Relativity that the universe must be thought of as a Block Universe in which the future already exists because they allow the introduction of the novel concept of the process of the birth of atoms of spacetime. I will explain how a clear separation of the concept of event and the concept of the process of the occurrence of an event, in the setting of causal set theory, allow one to connect the physical passage of time with objective conscious experience.
Quantum reference frames: a relational perspective on nonclassical spacetime
by Flaminia Giacomini
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, namely quantum systems sourcing a gravitational field and interacting gravitationally, has attracted a lot of attention, thanks to the possibility of realising these scenarios in the laboratory in the near future. When a quantum system sources a gravitational field, spacetime is fundamentally nonclassical. In this situation, also the notion of a reference frame needs to be generalised to account for quantum features of spacetime, thus realising a quantum reference frame (QRF). In this lecture, I will review how one can make sense of superpositions of coordinate transformations, namely quantum reference frame transformations, and argue that QRFs can contribute to formulate physics on nonclassical spacetime.
Gravitational quantum physics in the lab
by Markus Aspelmeyer
I will report on the current state of the art in quantum measurement and control techniques to investigate on gravity at small scales and, in the long run, on the phenomenology of the gravity-quantum interface in table-top experiments. Topics will include principles of quantum optomechanics and precisions tests of gravity sourced by microscopic masses.
It from qubit underground: how quantum theory and spacetime constrain each other
by Markus Müller
It is by now a widespread belief that spacetime emerges somehow from quantum theory. However, this idea is typically analyzed at a rather high level, presupposing e.g. holographic duality or a particular approach to quantum gravity. There is, however, a complementary approach that I will introduce in this course: we can study the relation between the structures of spacetime and quantum theory at a more basic and foundational level. Similarly as a study of the relation between the structures of biological bodies and their geological environments can give us hints on the workings of evolution, the hope is that this modest approach may give us some valuable hints of relevance in the search for a theory of quantum gravity. At the very least, it provides us with fascinating insights into the structural architecture of our world, and some potential applications in quantum information processing.
I will first briefly trace the historical roots of the underlying idea, dating back to physicists such as Bill Wootters and Carl Friedrich von Weizsäcker. Then I will present some results that can be seen as first steps within a modern version of this research program: the dimensionality of the qubit Bloch ball can be derived from relativity of simultaneity; the quantum (2,2,2)-Bell correlations can be exactly characterized via local rotational symmetry; and a semi-device-independent randomness generator can be constructed via spacetime symmetries, in a way that its security does not rely on quantum mechanics. I will show how internal degrees of freedom of particles could respond to spatial rotations in more general ways that quantum physics allows, and present some hints that the ubiquitous notion of linearity (say, of tangent space) in physics can ultimately be grounded in the linearity of probabilities.
The search for a table top quantum gravity signature
by Marios Christodoulou
We will briefly review the main protocols discussed the past 5 years for future `table top’ experiments hoped to give empirical evidence for quantum gravity. We will then go over the controversy and debate on the relevance of such experiments and discuss open theoretical questions.