Ludovic Berthier: Even Further from Balance

“I really enjoy observing how things move in general; I find it fascinating—how they move, and why they move that way.”With a list of publications as long as your arm and projects backed by prestigious grants, Ludovic Berthier, a researcher at the Charles Coulomb Laboratory in Montpellier, still manages to find simple words to explain a discipline that is anything but simple: statistical physics. It’s a field dedicated to understanding and predicting the behavior and evolution of physical systems, starting from the most basic building blocks: atoms, molecules, particles…“More specifically, we’re trying to understand why a material is a solid, for example—and what that implies for its mechanical properties when it’s heated, stretched, or compressed,” explains Ludovic Berthier.

Like a pile of oranges

As he began his Ph.D. in the late 1990s at the ENS in Lyon, a new field was emerging in statistical physics with the study of disordered and non-equilibrium physical systems. This topic captivated the young scientist:“It was a pretty exciting time; there was a real buzz within the community around a multitude of new questions.” He chose to focus his dissertation on glassy systems, such as window glass or emulsions. “If you cool an ordinary material, the atoms will arrange themselves into a crystal lattice and reach equilibrium according to well-known stages in statistical physics. But if you subject window glass to the same process, the particles will remain disordered, much like a pile of oranges at the grocery store.”

How can this behavior be explained? Through the phenomenon of aging. Materials such as glass undergo relaxation over very long periods of time and continue to evolve indefinitely toward a state of equilibrium that is, as a result, never reached.“The things that interest me are things that evolve, that are dynamic over time. Ordinary descriptions or traditional mathematical approaches therefore don’t work. I don’t ask myself what the property of this material is, since that property is changing even as I’m describing it,” continues Ludovic Berthier.

From the Atom to a Grain of Sand

His dissertation was awarded the Saint-Gobain Young Researchers Prize by the French Physical Society. Shortly thereafter, he secured a postdoctoral position at the University of Oxford as well as a prestigious Marie Sklodowska-Curie fellowship. In the Department of Theoretical Physics, he continued to explore emerging 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 spent in the vibrant environment of Oxford, the physicist secured a position at the CNRS; he settled in Montpellier and joined the L2C, where he still works today.

Eager to maintain strong international collaborations and expand his research into other materials, Ludovic Berthier established ties with the University of Chicago, where he continued his work throughout 2007. “There were specialists in granular materials there. By working with grains of sand, I was still dealing with disordered systems, but instead of studying atoms on the angstrom scale, I was looking at grains of sand on the millimeter scale. Yet, with sand, we observe states of matter that are similar: if you shake sand on a plate, it crystallizes, and if there’s 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 received regional funding; that’s no longer really the case today—much more applied disciplines are favored,” the researcher notes. His outstanding results enabled him to achieve a major milestone 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 leave that 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 made waves in the community again in 2017 with the “Monte Carlo swap,” a numerical algorithm he developed to perform computer simulations 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 therefore finally replicate real-world experiments.” 

At the same time, he is involved in international scientific collaboration Cracking the Glass Problem which, thanks to theSimons Foundation in the U.S., has provided him with significant funding that he continues to receive today.“For physicists in France, it’s extremely difficult to secure funding, so being financially independent for eight years is a dream come true,” the researcher says . “It ends in a year; after that, it’s back to square one.” For this new beginning, Ludovic Berthier has been able to count on a major asset since 2019: his position as a research associate atthe University of Cambridge, where he now divides his time.“I challenged myself scientifically by going there. There’s a high concentration of researchers there, and it helped me refresh my ideas.”

Silver Medal

He is currently bringing these ideas together as he writes a proposal for an ERC grant that takes on a new challenge: the physical study of the collective dynamics of biological systems. Following the example of Giorgio Parisi, the 2021 Nobel Laureate in Physics—with whom Ludovic Berthier collaborates—and his studies on flocks of starlings, Berthier 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 starting from the cell, or even how cells self-organize to restructure the material, for example when we get injured and the tissue is torn. It’s fascinating; it takes us even further beyond equilibrium than before.”

His enthusiasm is dampened only by the difficulty of securing funding in a particularly competitive international environment. “The list of competitors can sometimes be discouraging: when you apply for an ERC grant, you might be up against Nobel laureates and professors from the Max Planck Institutes, who have considerable resources at their disposal—resources we don’t have—to carry out their research. ” While waiting for this funding to come through, 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’re doing was being seen and recognized. Positive feedback from the institution is always very much appreciated.”