On March 2, 2021, at Stanislas, my friend Quentin and I came back to thank you, on behalf of many former students, for teaching us the "satisfaction mark." I've been using that little piece of punctuation for almost twenty years now and it still feels like belonging to some kind of guild, initiated into the sciences you still teach today. It's hard for me to put into words what that really means to others. So let me tell you why I love this mark instead, and maybe that will let everyone imagine what your teaching left behind in our lives.
To break free from Earth's gravitational pull, you need enough kinetic energy that the whole planet can no longer hold you back. By fighting that battle in front of us, teaching us how to wrestle with gravity itself, you made us understand what "escape velocity" truly means. What a magnificent expression! I still want to say it again: Earth's escape velocity! That astonishing speed doesn't depend on my mass. To really know it, you have to earn it, through nothing but the sheer force of thought, sitting at your own desk. Humanity calculated that speed long before any rocket ever reached it. It's roughly at that speed that engines can cut out and a craft keeps coasting through space, because Earth's grip has finally been beaten.
Right at the moment of the final blow, just as you were about to work out the numerical answer, you turned around and asked if we'd found the result of the calculation that was about to collapse. I'm terrible at mental arithmetic, but I spoke up a little too fast: "Eleven kilometers per second!" "And how did you get that?" you asked back, feigning astonishment like the fine actor you are. I lowered my head and answered: "Tintin, Destination Moon." The whole class laughed, you too and right there I knew I'd end up doing some science with my life.
In October 2020, defending my PhD in applied mathematics (or in artificial intelligence, if that expression means anything), I realized that every one of my scientific reflexes came from you: check the orders of magnitude, imagine what would happen if you removed something from the reasoning, ask whether your hypotheses can actually be tested… Do all the forces at play really matter or are some of them superfluous? What can safely be neglected? So, with an old classmate and childhood friend, Quentin, we went back to school to hand you a trophy we'd taken the liberty of inventing, shaped like the very punctuation mark you made us love so much: the satisfaction mark.
The satisfaction mark is a celebration of every mind's small victory over every calculation that dared to frighten us. Drawing it in chalk on a blackboard to close a proof is landing the final blow against the villains of our childhood heroes. Writing it on a sheet of paper is the joy of a vague idea turning into a scientific result, carrying the hope of a reality yet to come. It is the elegance of a triumphant will. And, finally, it is our respect and our gratitude toward an extraordinary teacher.
O professor, our professor, thank you for your energy, your passion for science and everything you passed on to us.
Warith Harchaoui, Paris, March 4, 2021
PS: my thanks to the Lycée Stanislas gazette for publishing this letter to my dear professor in L'Écho de Stan (page 30).