Tribute to Ludwik Celnikier

Ludwik Celnikier passed away on Saturday 14 May 2022. He had turned 82 the previous Sunday.

After studying at Imperial College London, he began his research career in the field of nuclear physics at CERN. There he met Evry Schatzman, who attracted him to astrophysics. At the end of 1972, Ludwik joined the Observatoire de (Paris in) Meudon.

A great physicist, his scientific culture encompassed an immense part of physics, for which astrophysics was an ideal field of application (almost a playground). A free spirit who eschewed fashion and convention, he tackled a wide range of research topics, from scintillation and scattering to density fluctuations and turbulence in solar wind plasma, the detection of turbulence in clear skies, a model of X-ray bursts, and relativistic jets. Some of these works played a founding role in their field.

But Ludwik was above all a formidable teacher. He knew better than anyone how to explain the physics of astrophysical phenomena with the help of simple mathematics, and he trained a large number of undergraduate students of all levels, at the University as well as in engineering schools. Many of them have become researchers. For, even more than his excellence in making physics understood, it was his ability to make people love it that made him an exceptional teacher. He gave of himself without counting to his students (a one-hour lecture lasted three hours in his reference frame), and they in turn are grateful for having benefited from his teaching for several years. They will not soon forget him.

In addition to the countless course handouts - studded with quotations and cartoons - that he wrote and distributed to his students at all levels, Ludwik published some fifteen articles in international physics pedagogical journals, and contributed to various encyclopaedias (notably Flammarion Astronomy). It is a pleasure to (re)read his "Variations sur le néant" (Bulletin de la Société Française de Physique, 1991), in which Ludwik paints a picture of the entire cosmology of the moment on the basis of simple equations.

Between 1986 and 2006, he published four monographs. In his two "Basics of...", intended for students and researchers, he details with great originality and his usual pedagogy the physics of all cosmic structures, and that of space flight. His other two books, "History of Western Astronomy" and "History of Nuclear Astrophysics" (Find a hotter place!), are references testifying to Ludwik’s immense culture, in which he rooted his approach to physics. In the preface to the History of Western Astronomy, Jean-Claude Pecker writes "... if the author is, by profession, an astronomer, his culture is so vast and diverse that the sky, the Universe rather, is also for him the occasion to show himself, indeed... universal.

With his impeccable English and French, and his great sense of humour, Ludwik knew how to enthrall his audience, both in writing and in speaking. His books and articles combine the pleasure of the language with the depth of his ideas. With "Basics of Cosmic Structures", he managed to write a 400-page book, incredibly clear ... without a single figure or diagram!

At the same time, working tirelessly for the transmission and cross-fertilisation of ideas in physics, he organised or co-organised several international colloquia in fields ranging from planetary sciences to cosmology, in France and in Vietnam, and published their proceedings.

At the same time, Ludwik was also involved in the transmission of information to the general public. In the 1980s, he organised or co-organised summer schools for secondary school teachers. In the mid-1990s, he was invited to the Haute-Maurienne Astronomy Festival for a one-hour conference, where he spoke for five hours over two days, to an audience that wanted more. Ludwik gave more to them by becoming for almost 20 years a pillar of the Festival, giving courses, conferences and contributing to its organisation.

There too, he left his mark, a mixture of scientific rigour, culture, clarity, passion, non-conformism, smiling irony, sometimes mocking, but always marked by kindness, and in the end giving rise to solid friendships. If he was reserved on a personal level, his fellow researchers or teachers, his students or pupils, all those with whom he interacted to organise schools, colloquia or festivals, can testify that interactions with him were anything but gloomy.

In short, Ludwik Celnikier was quite a character for some, a great guy for others, a free spirit in any case. He is one of those people you like to know they exist, even if you rarely meet them. He left unexpectedly. We will miss him enormously. And of course we think of his wife Laurence.

Ludwik was buried on May 19, 2022 in the Trivaux cemetery in Meudon.

Philippe Zarka, with contributions from Jean Tran Than Van, Arturo Ortega-Molina, Jean-Louis Masnou, Bernard Leroy, Pierre Léna, Catherine Lacombe, Marie-Pierre Issartel, Pierre Huart, Christian Gouiffes, Eric Gérard, Sidonie Foadey, Yves Delaye, Patrick Canu, Jean-Louis Bougeret, Jean Aboudarham

Testimonies

I met Lud in my first days at CERN in 1970. This was not difficult since he frequently occupied pride of place in the main canteen, the meeting place where so many scientific discussions took place. His healthy cynicism, irony, original views and likeable good company singled him out as a special character even in the early CERN’s rich field of personalities. One sensed that bubble chamber physics was insufficient for him, he was looking elsewhere. Nonetheless his move to astrophysics seemed a courageous transition and it was relief that we found him at Meudon working in his new field. As others have written he went on to write books and give talks on astrophysics in two languages, both admirably mastered. It was a pleasure to work with him again in other activities such as the “Frontiers of Life” Conference in Blois where I saw just how far his knowledge of other sciences had grown together with his human network. Of course this was only one of so very many conferences where Ludwig was an essential component of the organisation.

Ludwig was original, his own man and a friend. We shall miss him,

Chris and Terry Jones, Geneva

At the bottom of the page, one will find many pedagogical papers published by Ludwik between 1978 and 1995 in the American & European Journals of Physics. They are examples of Ludwik’s approach and style.