Ludovic Berthier: even further from equilibrium

"I like to watch how things move in general, I find it fascinating. How they move, and why they move that way." With a long list of publications and projects supported by prestigious funding, Ludovic Berthier, a researcher at the Charles Coulomb Laboratory in Montpellier, still knows how to find simple words to explain a discipline that is anything but simple: statistical physics. This specialty focuses on understanding and predicting the behavior and evolution of physical systems, starting with the most basic building blocks: atoms, molecules, particles, etc. "More specifically, we try to understand why a material is a solid, for example, and what that implies in terms of its mechanical properties when it is heated, stretched, compressed, etc.," explains Ludovic Berthier.

Like a pile of oranges

When he began his PhD at the ENS in Lyon in the late 1990s, statistical physics was entering a new field with the study of disordered and non-equilibrium physical systems.This wasa topic that captivated the young scientist: "It was a pretty exciting time, with the community really buzzing about a whole host of new questions." He chose to focus his thesis on glassy systems, such as window glass and emulsions. "If you cool an ordinary material, the atoms will arrange themselves into a crystal lattice and equilibrate according to well-known steps in statistical physics. But if you subject window glass to the same treatment, the particles will remain disordered, a bit like a pile of oranges at the grocery store."

How can this behavior be explained? By the phenomenon of aging. Materials such as glass relax over very long periods of time and continue to evolve indefinitely toward a state of equilibrium that is therefore never reached. "The things that interest me are things that evolve, that are dynamic over time. Ordinary descriptions or traditional mathematical approaches therefore do not work. I don't ask myself what the property of this material is, because that property is evolving at the same time as I am describing it," continues Ludovic Berthier.

From atoms to grains of sand

His thesis was awarded the Saint Gobain Young Researchers Prize by the French Physics Society. He then went on to obtain a postdoctoral position at Oxford University and a prestigious Marie Sklodowska-Curie fellowship. In the theoretical physics department, he continues to explore new questions about disordered systems. "We laid the groundwork by studying highly simplified models to try to understand how particles move in a very dense and disordered system." In 2004, after three years in the bustling environment of Oxford, the physicist obtained a position at the CNRS; he settled in Montpellier and joined the L2C, where he still works today.

Keen to maintain strong international collaborations and expand his research areas to other materials, Ludovic Berthier moved closer to the University of Chicago, where he continued his work throughout 2007. " There were specialists in granular materials there. By working on grains of sand, I remained in disordered systems, but instead of studying atoms measuring one angstrom, I was looking at grains of sand measuring one millimeter. However, we observe similar states of matter with sand: if you shake sand on a plate, it crystallizes, and if there is wind, it forms a gas..."

Upon his return to France, Montpellier welcomed him with the CNRS bronze medal and funding from the Occitanie region. "At that time ,theoretical physics benefited from regional funding, which is no longer really the case today, with much more applied disciplines being preferred," observes the researcher. His excellent results enabled him to achieve his first major goal in 2012 with an ERC (European Research Council) grant. That same year, he returned to the ENS in Lyon, but this time to teach. He will be leaving this position at the start of the next academic year to teach at the ENS in Paris.

Monte Carlo swap and the Simons Foundation

His work caused a new stir in the community in 2017 with the "Monte Carlo swap," a numerical algorithm he developed to perform numerical simulations on computers by studying model materials. "Simulations allow us to simplify nature as much as possible while remaining close enough to the real system to be relevant." The challenge with disordered systems is to successfully simulate the very long timescales over which they evolve using faster algorithms."Thanks to the Monte Carlo swap, we can simulate timescales that are vastly longer than before, and the simulation can finally reproduce real-world experiments." 

At the same time, he participates in international scientific collaboration Cracking the Glass Problem which, thanks to theSimons Foundation in the US, provides him with significant funding that he still benefits from today. "In France, it is extremely difficult to obtain funding for physics, so being financially independent for eight years is a dream come true," says the researcher. "It ends in a year, after which it will be back to square one." For this new start, Ludovic Berthier has been able to count on a major asset since 2019: his position as a research associate at theUniversity of Cambridge, with which he now shares his time. "I challenged myself scientifically by going there. There is a high concentration of researchers there, and I renewed my ideas."

Silver medal

He is currently gathering these ideas in a draft ERC proposal focused on a new challenge: the physical study of the collective dynamics of biological systems. Following the example of 2021 Nobel Prize winner in Physics Giorgio Parisi, with whom Ludovic Berthier collaborates, and his studies on flocks of starlings, he would like to focus on biological tissues. "I can look at this tissue as a material; pull on it, deform it, watch it flow to try to understand the mechanical properties that emerge at the collective level from the cell, or how cells self-organize to restructure the material, for example when we injure ourselves and the tissue is torn. It's fascinating, it's going even further than equilibrium before."

The only thing dampening his enthusiasm is the difficulty of securing funding in a particularly competitive international environment. " The list of competitors is sometimes daunting: when you apply for an ERC grant, you may find yourself up against Nobel Prize winners and professors from Max Planck Institutes who have considerable resources at their disposal that we do not have to carry out their research. While waiting for this funding to arrive, Ludovic Berthier was delighted to receive the CNRS silver medal last March, fifteen years after his bronze medal. "This news made me really happy. I told myself that the work we are doing was being seen and recognized. Positive feedback from the institution is always very much appreciated."