The most common numeral system is decimal, based on our ten fingers, but countless other systems exist and have been used throughout history. They use a different "base" or "radix". Traces of these alternative systems are still present today: when we buy a dozen eggs, use base 24 and 60 for timekeeping, or hear a French speaker say ``four-twenty four'' to mean eighty-four, a vestige of a vigesimal (base-20) system. We use our fingers to count in the same way that we may use our feet to measure lengths. We rely on these anthropocentric standards to communicate, but we expect natural phenomena to be independent of the language we speak, the metrics we choose, and even the kind of numbers we use. Both numbers and units should be informed by nature, not human conventions. To illustrate this radical idea, I will explore examples from the capillary dynamics of droplets and bubbles, and from explosions—two classical subjects of scaling. Starting with a single power law, I will show that its eventual breakdown is actually a necessity if the associated phenomenon is to be independent of our human imprint. No power law can extend indefinitely.

In complex or disordered media, standing waves can undergo a strange phenomenon that has intrigued physicists and mathematicians for over 60 years, known as “wave localization.” It consists in a concentration of the wave energy in a very restricted sub-region of the complete domain. This localization has major consequences not only on the vibration but also the propagation properties of the system, in particular electronic transport in disordered alloys. It has been demonstrated experimentally in mechanics, acoustics, electromagnetics and quantum physics. In this talk, we will present a theory that highlights the existence of an underlying and universal structure, the localization landscape, obtained by solving a problem associated with the wave equation [1]. In quantum systems, this landscape makes it possible to define an “effective potential” that predicts the localization subregions, the energies of localized modes, the density of states and the long-range decay of wave functions, regardless of the dimension, both in continuous and discrete settings. Finally, we will provide an overview of the applications of this theory to mechanics, semiconductor physics, as well as for molecular and cold atom systems. [1] M. Filoche & S. Mayboroda, PNAS (2012) LF9:14761-14766.

The citation for the 2014 Nobel Prize in Physics was for “the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”, thus suggesting that visible LEDs for lighting lead to global energy savings. As is well known among economists (but perhaps less among physicists, engineers and policy makers), the various rebound effects can severely affect the overall energy-consumption gains (if any) when a more energy-efficient technology comes to market. It is an open question to assess whether the III-N LED technology actually leads to energy savings. I will discuss the notion of rebound effect (notably direct and indirect rebound effects) in general, for semiconductor technologies and for the case of the III-N LED technology, notably in light of the surprisingly scarce literature on the topic. I will then discuss what I think this implies when discussing the impacts of our research practices and mostly the potential (social, economic and environmental) impacts of the technologies developed in academic laboratories.

Flocks of birds, bacteria colonies, sheep herds, crowds of people ... are just some common examples of large groups of living entities behaving in a remarkable collective fashion. From a physicists viewpoint, these are systems composed of many interacting units, each somehow able to convert energy from their environment to stay 'alive' and perform some task. Such dynamics lifts the constraints imposed by equilibrium, opening many challenges and eventually the possibility for the rich phenomena displayed in the natural world. However, we lack a general theoretical framework to describe it. Much understanding has been gained from the investigation of model systems (both theoretical and experimental), usually dealing with 'active units' consisting in self-propelled particles with simple physical interactions, such as excluded volume. Interestingly, the mere competition between self-propulsion and crowding, is already enough to trigger a novel non-equilibrium phase transition. I will review our current understanding of this phenomenon, and I will then delve into a complementary perspective on active systems, as made of entities that interact in a non-reciprocal way. This will lead us to consider 'active' extensions of paradigmatic models in statistical mechanics, such as the Ising model, to decipher what such new ingredient brings into the classical picture of phase transitions.

The steady-state theory, which held that the density of the universe remains constant due to the continuous creation of matter during expansion, died during the 1960s. This death is commonly attributed to the discovery of the Cosmic Microwave Background (CMB) in 1965, which was interpreted as evidence that the universe had evolved from a superdense state—contradicting the core assumption of the steady-state model. Historian Stephen Brush argued that the theory's supporters, who embraced a Popperian view of science, abandoned it in light of the CMB's apparent falsification. However, this narrative has been challenged by other historians, notably Helge Kragh, who downplay both Popper’s influence and the CMB's role in the theory’s downfall. This talk reopens the cold case of the steady-state theory's death. Did the discovery of the CMB really shoot the fatal blow? Was Popper an (unwilling) accomplice? Is the real murderer still unidentified? Drawing on new testimonies from both theorists and observers, I propose an alternative explanation: it was not the CMB, nor Popper, that "killed" the steady-state model, but its declining fecundity during the 1960s. Using new tools of history and philosophy of science, I argue that while Popper played a significant role in the theory’s development during the 1950s, he had little influence on its fate in the 1960s. Likewise, the CMB was not seen as a straightforward falsification of the steady-state theory, nor as direct confirmation of its rival, the big bang model. Rather, it was perceived as a fertile phenomenon that opened new avenues for connecting nuclear and particle physics, general relativity, and radio astronomy.