Morton E. Gurtin passed away on April 20, 2022. As his former student and, later, his friend, it is with great sorrow that I accept the honor to remember to everyone his stature as a mathematician, a mentor, and a person.

His scientific achievements are monumental. The topics on which he mainly focused are all related to either foundational or applied issues in rational continuum mechanics. Most of the many research articles and books he authored are mandatory references in the field.

Right after his college degree in mechanical engineering, he worked for a short period at Douglas Aircraft as a mechanical engineer. I remember him recalling that one of his first tasks was to tame washing machines that were literally walking across the lab due to undamped vibrations of the engines. He did not stay long in the industry: he went back to school and grew up scientifically at Brown University, where he got his doctoral degree in Applied Mathematics under the supervision of Eli Sternberg, and where he started his academic career, which he later mainly performed as a Professor of Mathematics at Carnegie-Mellon University, in Pittsburgh.

Those were the years, the early sixties of last century, when the work started by Clifford Truesdell, which led to a rigorous mathematical foundation of continuum mechanics, was at his full maturity. In that scientific environment he started his career, probably attracted by the beauty and rigor of the deductive approach to mechanics of the Truesdell school.

Indeed, even though his work lies in the realm of applied mathematics, he was deeply a true pure mathematician at his heart, often advising me, when trapped in some dead end of my research work, to trust mathematics and let me be guided by it.

He mainly worked in specific areas of continuum mechanics, of which I will briefly mention those I have been exposed to.

He laid down the foundations of the mechanics and thermodynamics of phase transitions in solids, in a period in which only a few ad-hoc models were available, but the demand for rigorous mathematical models was huge by the applied analysis community, the tools for non-convex variational problems being discovered just then. His research led him to realize that the basic equations governing the evolution of phase boundaries could not be derived by the classical laws of continuum physics, because new physics is involved in all phase transition processes in diverse contexts. One of his main achievements was to understand that all these novel evolution equations could be unified by the common concept of configurational forces. This represented a huge advance in the understanding of these phenomena, providing also a basic tool to help formulate correct evolution equations. As such, this represents perhaps one of the last, great achievements of the rational approach to continuum mechanics. In his later years, his interest shifted to plasticity, a field of utmost importance in applications, but to which the ancient Roman phrase ‘hic sunt leones’ very much applied. Here again, a plethora of ad-hoc and sparse models were being developed to explain phenomena that the classical theory used by engineers could not explain. And here again, his rational approach provided a solid foundation, now regarded as classical, on which models for new phenomena could be formulated.

Finally, I would like to recall his contributions to population dynamics, specifically on the McKendrick–von Foerster equation for age-structured populations, which he was able to generalize by taking into account logistic terms.

As mentioned before, his books provide clean, elegant and sound foundational tools for every student or scholar approaching or performing research in continuum mechanics: they are simply unavoidable to everybody working in that field. I just mention those I have first studied and used many times in my life, either to learn about a subject or to dissipate some conundrum in my research.

M.E.G., The Linear Theory of Elasticity. In: Truesdell, C. (ed.) Linear Theories of Elasticity and Thermoelasticity, Springer, Berlin, Heidelberg (1973).

M.E.G., An Introduction to Continuum Mechanics, Academic Press, New York (1982).

M.E.G., Thermomechanics of Evolving Phase Boundaries in the Plane, Clarendon Press, Oxford (1993).

M.E.G., Configurational Forces as Basic Concepts of Continuum Physics, Springer, New York (2000).

M.E.G., Eliot Fried, Lallit Anand, The Mechanics and Thermodynamics of Continua, Cambridge University Press (2010).

As a mentor, he taught me many things, even though his teaching style was often relying on a student’s skill to interpret his often sad or blank looks when results were presented on the blackboard, before realizing that some major logical flaw was hidden in the argument. Seriously now, I was often surprised by his ability to grasp immediately the implications of what one tried to explain, as well as by his strong professional and scientific rigor: many times he refused to publish pieces of work that were not completely understood or were trivial, just the opposite of the infamous 'publish or perish' attitude that was spreading in those years. The depth of his digging into hard problems, as well as his refusal to be satisfied unless he had really understood the roots of the problems, he shared with some of his best friends and collaborators, among those I had the chance to become acquainted with are Paolo Podio-Guidugli, Antonio di Carlo and Eliot Fried, who all enriched me professionally and as a person. A final remark, that tells much about his attitude towards life and research, is that he was a marathon runner: the same tenacity, deep involvement and dedication he put in both science and running, his two lifelong passions that I have witnessed working with him.

Today we are entering an era in which, again, new, original and unexpected approaches are needed to deal with complex phenomena, of which only ad-hoc models are available. I think that Morton Gurtin’s legacy, made of rigor, intellectual curiosity and hard work, can help us find the path to develop sound paradigms and solid theories in many different directions.