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In the planning - Autumn 2021 Series
(Updated: July 6, 2021)

Session Speakers Title

Completed - Spring 2021 Series
(Updated: July 6, 2021)

Session Speakers Title
08.04.2021, 16:00
CEST (Brussels time)

There's plenty of room at the bottom

Roger Penrose
Penrose: How the Large and Small Interrelate in General Relativity and Quantum Mechanics

Despite much theorizing on how quantum theory might modify general relativity at extremely tiny scales, thereby resolving the space-time singularity problem, there are strong reasons for believing that this cannot provide an overall solution. On the other hand, conformal geometry, relating large to small-a remote expanding future to an initiating big bang-leads to important thermodynamical insights and confirmed observations of previously unexpected effects. On the quantum side, the most troublesome and fundamental issue is the measurement problem-or collapse of the wave-function-and here I argue that it is in the large effects of tiny gravitational fields where we must find our answers.

Watch the recording on YouTube.

06.05.2021, 16:00
CEST (Brussels time)

Towards exact theories of nature

Robert Wald &
Michael Kiessling
Wald: Point Particles and Self-Force in Electromagnetism

Point charges are a very useful idealization in electromagnetism. Although they are commonly treated as though they are fundamental, they have infinite self-energy and mathematical inconsistencies arise if one tries to describe their self-consistent motion. On the other hand, the self-consistent motion of distributions of continuous charged matter is entirely well defined in classical Maxwell theory. I will describe how to take a mathematically rigorous point particle limit of continuous charged matter wherein the charge and mass scale to zero in proportion to the size of the body. Lorentz force motion is obtained in this limit and self-force arises as a perturbative correction. It is shown how to obtain self-consistent motion that respects causality and does not admit spurious run-away solutions.

Kiessling: Revisiting the 1920s --- with the benefit of hindsight [slides]

The 1927 Solvay Conference is perhaps the most renowned physics meeting of the 20th century. An eminently distinguished group of physicists had been invited to discuss the apparent breakthroughs that had happened in the previous two years, in the hope perhaps to come to a common understanding of what had been accomplished. Instead, as one participant (Langevin) later wrote, at the 1927 Solvay conference the confusion of ideas reached its peak. Given the fact that nowadays professional physicists typically disagree with each other about what quantum theory really says about nature, it is fair to say that the confusion has not been cleared up yet. In this talk I will argue that by the end of 1927 we may well have been on the way to a meaningful and accurate quantum mechanics of electrons, nuclei, and photons that does not suffer from the infamous measurement problem of what came to be known as orthodox (standard textbook) quantum mechanics. For this it would have only been necessary that Luis de Broglie, Max Born, and Erwin Schrödinger would have listened to each other more carefully and with an open mind.

Watch the recording on YouTube.

03.06.2021, 16:00
CEST (Brussels time)

Beyond quantum gravity

Francesca Vidotto &
Stefano Liberati
Vidotto: If you want to build a universe from scratch you must first invent a quantum state of the geometry

The application of the covariant LQG techniques to the universe allows to compute cosmological observable in the deep quantum regime of the early universe. In particular, it is possible to define a cosmological quantum state and study its property. This approach differs from the standard procedure in cosmology, where a primordial vacuum state needs to be assumed to provide the initial condition of the successive evolution. In this talk I will present how such a quantum state can be constructed, providing a spinfoam analogue of the Hartle-Hawking wave function of the universe. I will emphasize the conceptual steps that need to be taken in defining the appropriate observable and the relevant degrees of freedom. I will present the first results obtained with improved numerical techniques.

Liberati: Hearts of Darkness: probing of black holes inside out

Black holes are the purest expression of gravity and at the same time the places where our best theory of gravitation, Einstein General Relativity, meets its demise in the form of singularities. We know, however, that any successful theory of quantum gravity should be able to resolve these uncharted regions, but can they do so without showing any modification outside the event horizon? can real black holes be undistinguishable from the one predicted by general relativity? The recent direct observation of these tantalising objects represents an unprecedented possibility to answer these questions. In this talk, we shall explore on general grounds what alternative objects we can expect from a quantum gravity induced regularisation of singularities and discuss what observations can or will tell us about their nature.

Watch the recording on YouTube.

01.07.2021, 16:00
CEST (Brussels time)

Future of quantum foundations

Siddhant Das &
Sandro Donadi
Das: Can we fix quantum arrival times before 2026?

I will discuss the problem of predicting the arrival (or detection) time of a quantum particle on a detector surface that remains unresolved despite the time-honoured empirical successes of quantum mechanics. Given that arrival-time or time-of-flight (TOF) measurements are the quintessence of standard experimental techniques of atomic and particle physics, this is particularly striking. Drawing upon the early history of this issue, I will discuss many (disparate) theoretical predictions for the TOF distribution of a particle suggested in the literature. These are informed by various semi-classical heuristics, principles, extensions, or (for want of a better word) "interpretations" of quantum theory. Next, I will describe a so-called "back-wall" experiment---an arrival-time experiment developed in collaboration with my late (and truly great) advisor Prof. Detlef Dürr that can reliably distinguish the aforementioned proposals. A key feature of our set-up is that the particle is escaping a potential barrier (the back-wall) in the direction of a distant detector, as opposed to moving completely freely. Without the back-wall, most proposals become nearly indistinguishable, which renders usual experimental tests inconclusive. I will also describe a concrete ion-trap implementation of the back-wall experiment doable with present-day technology. Time permitting, I will mention a version of this experiment featuring a spin-1/2 particle as an electron whose de Broglie-Bohm analysis predicts, thanks to quantum backflow, intriguing spin-dependent arrival-time distributions hitherto unknown, demanding experimental inspection.

Donadi: Collapse Models: State of the Art and Future Perspectives

Collapse models solve the measurement problem by modifying the Schrödinger equation, adding new terms which describe spontaneous localization in space of the wavefunction. Because of these additional terms, collapse models make different predictions compared to Quantum Mechanics, so they can be experimentally tested. In this talk, we will give an introduction to the most important collapse models and to the state of the art of the bounds on their phenomenological parameters, coming from different experiments. We then also discuss possible improvements of these bounds as well as theoretical developments of the models.

Watch the recording on YouTube.

Moderated by: Dr. Paula Reichert


Laws of Nature Series is a new initiative to promote the exchange of physical, philosophical, and mathematical ideas in the field of the foundations of quantum physics.

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The Laws of Nature Series comprises a Spring and Autumn Series each comprising monthly online colloquia held in the form of themed sessions on Zoom. The session theme can be found in the first column of the schedules. The chosen format for each session is one or two 45+15 minutes talks followed by a 30 minutes moderated panel discussion.


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