Constantin Piron - Phyisicien


Nombreux scientifiques, collègues, assistants, étudiants, ont côtoyé Piron, voici leurs témoignages.

Merci à :

Diederik Aerts,  Bob Coecke,  Massimiliano Sassoli  de Bianchi,  Werner Aimrein,  Michael Reinhardt, Benoit Boulat, Charles P. Enz, Nicolas Macris

Dear IQSA members,

I have the sad news that Constantin Piron has died on May 9, 2012. He was one of the great scientists in the foundations of quantum physics and quantum axiomatics. The deep sadness I personally feel as one of his students can find some light in the knowledge that the fruits of his mind remain alive. And not only alive, they carry the full potentiality to be developed further. Because, as we all know, quantum physics, which was the focus of reflection during all of his life, remains a great challenge as ever before. Waiting for young fresh brilliant minds to be explored further. I am certain that aspects of the foundations lead by Constantin Piron will play an important role in this future endeavor.

Prof. Diederik Aerts – Center Leo Apostel, Brussels Free University

It is with sadness that I report that Constantin Piron, famous for many things including the Jauch-Piron NoGo-theorem and his reconstruction theorem, and of course his beautiful book Foundations of Quantum Physics, has died at the age of 80. While usually associated with quantum logic, he strongly emphasized an operational underpinning of axiomatic concepts, and boldly went against any dogma, including quantum logic. He was probably the most colorful, entertaining, and especially the boldest scientist I ever have met, in many ways.

Prof. Bob Coecke – Oxford

I was at the École de Physique, in Geneva, in the years 1989-1991, and got to interact on a daily basis with Constantin Piron, when I started a doctoral thesis with him. I have many souvenirs of Constantin’s colorful personality and original mind.

When he hired me, I still remember very well when he told me, in an extremely serious tone, that the job of a theoretical physicist did not entail particular risks in terms of accidents at work, with the exception maybe of that of losing one’s reason!

For a while, he convinced me to share the office with his wife Armelle, as he didn’t want me to stay with the other graduate students (who were located in another building), otherwise they would have influenced and corrupted my young mind with their misconceptions about quantum mechanics and relativity theory.

Discussing with him was always extremely challenging, as he had original ideas on almost every topic. Just to give an example, I remember that when I mentioned him my allergy to pollen, after looking at me in a fatherly way, he smiled and said: you know, maybe you should just not believe in your allergy, go to a field full of the pollen you pretend to be allergic to, take many deep breaths, and everything could be over after that…

Constantin’s mood was extremely unpredictable, like a quantum phenomenon. For example, when I had to go to his office, I never knew if he would welcome me gracefully, as he sometimes used to do, or if he would scold me for my disturbance. And I also remember that all the people I met, who have interacted with him in depth, remained in a sort of love-hate superposition state, without ever managing to disentangle the dilemma.

At that time, I was the assistant for his famous course in quantum mechanics. His approach to quantum mechanics was an operational and realistic one, and the (counterfactual) Einstein-Podolsky-Rosen reality criterion played an important role in it. To stress its importance, Constantin loved to remind his students, particularly when he was writing at the blackboard, to not confuse a broken chalk with a breakable chalk! Good chalks are breakable, he used to say with conviction, and one must take care not to break them unnecessarily.

The time that he really surprised me, was when he once took a 10 francs bill from his wallet and torn it in half, in front of a class of stunned students. He would then move the two halves along different trajectories, to finally bring them back again together, in his wallet, to show that the “10 francs entity” is able to behave in a way that is very similar to a quantum entity, as in a sense the 10 francs can be simultaneously present in different locations, without being actually present in any of them.

I never completed my thesis with Constantin, as at some point I decided to work on a different subject, with Philippe Martin, a friend of him and also a student of Josef Maria Jauch. When I left Geneva, Constantin was palpably not very happy with my decision, as it meant that our scientific collaboration kind of failed. But for sure, my interaction with him deeply marked my future way of doing and understanding physics.

In a sense, I remained connected with him “at a distance,” as many decades later I came into contact with one of his most brilliant students, Diederik Aerts, who took over the legacy of the Geneva school and brought it to a new phase of important developments and discoveries. With Diederik I am today collaborating on many of the themes who were at the heart of Constantin’s reflections, and I like to believe that, seeing the results we have obtained, in the ambit of what is today called the Geneva-Brussels school of quantum mechanics, he would be very happy with it.

P.S.: in the photo, professors Werner Amrein (left, who was one the examiners of my PhD thesis), Constantin Piron, at the center, and Josef Maria Jauch, here in the seventies, in Varenna (Italy).

Massimiliano Sassoli de Bianchi – Center Leo Apostel – Brussels Free University

En Italie avec Aimreim et Auch

Je connaissais Constantin depuis ma venue à Genève en 1965 afin d’y étudier pour mon doctorat sous la direction de Josef Jauch. Constantin était partiellement impliqué dans mon travail de thèse, et tout au long des nombreuses années qui suivirent nous étions assez souvent ensemble, surtout pour des discussions scientifiques, pour l’organisation de notre enseignement, pour les repas à midi – auxquels il tenait beaucoup, et lors de participations à des conférences.

Avant sa nomination comme professeur, et à nouveau depuis ma propre retraite, nous partagions un bureau à l’Ecole de Physique. J’ai toujours apprécié son savoir et son sens de l’humour assez original.

Une vie terrestre s’est achevée, et cela me rend toujours plutôt pensif. C’est réconfortant de savoir que sa tombe se trouve à un endroit calme et d’une grande beauté.

Prof. Werner Amrein – Université de Genève

Au début des années 1960, quelques souvenirs d’étudiants que se remémore un ami de Constantin…

Dr Michael Reinhardt – médecin pédiatre

J’ai toujours trouvé curieux que Constantin Piron n’est pas trouvé plus d’échos pour célébrer sa vie, du moins celle du scientifique et du penseur accompli. C’est donc avec plaisir que je suis tombé par hasard sur ce site internet. Je fut son étudiant de thèse et je travaillai à Genève de 1984 à 1989. La vie avec Piron ne fut pas toujours facile mais je n’ai jamais regetté cette période car elle me permit l’ouverture que j’attendais alors, de savoir qu’il est possible de rechercher les questions que l’on a en soit, que des mondes peuvent s’ouvrir et être explorés. Constantin Piron fut sans doute le seul penseur que j’ai rencontré dans ma vie professionnelle et cette expérience fut très enrichissante. Je n’eus pas l’occasion d’appréhender s’il était aussi un poète et qu’il cachait un cœur sensible. Je crois que pour lui son attachement à sa quête de la vérité était preuve suffisante de son amour du monde et des gens même si de prime abord ses manières pouvaient choquer.

Voici les remarques que j’avais envoyées aux Professeurs Coecke et Aerts, lorsque j’appris le décès de Constantin Piron:

Dear Bob Coecke,

By the way I am one of Piron’s Ph.D. student. Maybe the last official one and certainly battling Terje Aaberge for the qualification of most obscure. I wrote a few lines to Diederick Aerts to tell him the mixed feelings I had towards Piron. I paste them below. Prior to that I remember a « Pironnerie » that you may find funny:

In June 1986 there was to be a conference entitled: « Geometry and Physics » in Paris and the Saturday before that the « Séminaire Bourbaki » was taking place. Piron told me to attend both. I did that for the first lecture of the Bourbaki and then left as I could not get really what they were talking about. I met Piron in the corridor who was joking with the secretaries. We went for a drink in a café nearby and came back to the Bourbaki’s lectures which were in recess. Piron told me: « why don’t we seat there in the first row ». I was reluctant but then I obliged (or obeyed). These were the kind of old school benches and desks where when seated the guy in the middle is trapped if others on his side don’t leave beforehand, unless he manages to scramble underneath the desk; not very discreet to say the least. When the crowd had gathered back and I was then a prisoner, Piron whispered to my ear: « usually the first row is reserved to people who belongs to the Bourbaki committee ». Boy! There I was in the first row not understanding a word from what the speaker was telling, my screw Piron next to me, probably grasping at the most one or two more, but delighted by his teasing of the establishment. Typical Piron.

Benoit Boulat


Dear Professor Aerts,

This weekend I went to Switzerland for a family reunion and I was traveling by train from Geneva airport to Montreux. I was reminded of my days when I was doing my Ph.D. in Geneva under the direction of Constantin Piron. At that time I was living in Lausanne, commuting daily. Whenever I was taking a regional train stopping at intermediate stations I was half hoping, half dreading that Piron would show up since he was also commuting, but from one of these intermediate station between Lausanne and Geneva. A mixture of fear and hope depicts well my relationship with Piron. Coming from a family that had very little scientific tradition I was not ready to defend my point of view in front of Piron; I must add that, as viewed from today, I don’t think Piron was ready to guide me in defending my point of view. Perhaps you were for something in that situation, since Piron mentioned several times to me, « that when someone shows that what you have done is not correct, c’est dur, c’est très dur ». He was referring to yours showing that some of his « axioms » for quantum mechanics were not valid for composite system.

In any case I remember that the first time I met Piron, I told him something like: « of course experimentalists are sometimes successful at verifying theories, because they do everything possible in order to do so. » I thought Piron got interested by that remark but he never engaged in discussing it again after he decided to take me as a Ph.D. student. Instead the subject of my thesis ended up to develop a mathematical formalism that would allow to describe a physical system without precluding of its classical or quantum nature. This was done using some version of the deformation theory of algebraic structures, an approach advocated then by Moshe Flato, André Lichnerowitch and coworkers and Piron was mingling with that crowd during that period. Was it too early after the completion of your Ph.D. thesis and he wanted to stay away from too foundational questions that were risking to damage further his own constructions? Maybe so. Had I had a stronger scientific character I would have insisted on developing something around my remark above. Had Piron been a better master he would have helped me to do so.

In my opinion you are the most successful of Piron’s student, in the sense that the pupil has surpassed the master in a way that has enlarged and developed the master’s point of view. Gisin has certainly been successful, but certainly more for the experiments he directed than the theories he proposed. Others I don’t know them well. In the nineties Piron apparently went again through a new creative period when he interacted with the Moore, Coecke, … (as viewed from 2019, Coecke has become very successful, probably surpassing his mentor Aerts)

Piron and I were not really meant to work together I suppose. Yet I learned a lot from him and although my attraction to quantum physics predated my encounter with him, after I gained access to some of his thinking, the work of his student (you) and the considerable amount of work that has been done on and around this subject, I have lived in a world in which to this day I keep a lot of interest into. I am indebted tremendously to Piron for this and certainly for some time, I considered him to be my master. I was always embarrassed however by the way he was defending his point of view. I was never able to understand how one could be so squarish and bullish in doing so. There were notions in the Piron(-Aerts) approach of QM that I could not fathom like saying (I believe this is mentioned in your lecture at the AVCP course on QM in the Valais, in the Swiss alps, and I am quoting from bad memory): « we do not have to justify why we consider this particular system that is defined by the type of questions we ask about it. » Then of course you can claim that you are a « realist », since you have evacuated all the linguistic apparatus which has helped you to circumscribe the system you want to study. Once again I did not pursue my own penchant, attraction or point of view but much later, reading the (later) Wittgenstein helped me gaining a better understanding about this problematic.

If one were sensitive, life with Piron was not always agreeable. Like when I said finally « tu » to him after he insisted for several years that I do so and from that point on he started to say « vous » to me. Or like after the successful completion of my Ph.D. I asked for a letter of reference from him; handing it to me he said (getting back to « tu », then): »tu comprends, je ne peux pas en écrire une trop bonne, car sinon les gens vont me demander: ‘pourquoi tu ne le gardes pas?’ . »

The third anecdote I want to convey describes well the mixed feeling I had towards Piron. The feeling that even if he was performing apparently badly, I would try to take the opportunity to grow from it. Piron was giving a course he entitled: « axiomatique quantique » (he claimed later he never gave such a course saying: « pourquoi tu parles comme Gisin? ») and a retired person was following it. After the lectures we usually went to the cafeteria for a drink and once the retiree asked a question and said looking at me: « Monsieur qu’est-ce que vous en pensez? » I could not even open the mouth that Piron had said: « il ne pense pas ». I was so baffled that I kept silent. I thought I could say: « je ne suis pas Cartésien, ce n’est donc pas grave » but this was more a sophism that something I was really feeling. This interjection by Piron was written in me like with hot iron though. About a decade later as I was at Stanford Unversity for an interview, I was strolling through the Rodin garden. I caught view of the « gates of hell » and realized who was sitting right at the top of them, contemplating them? None but the famous « thinker » by the famous sculptor! . There was the illumination! There was my answer to Piron! I contacted Piron again to ask him to write a letter of reference on my behalf for the Stanford position (!! I did not get the position ) but mostly to tell him about my revelation. And there was Piron telling something like: « c’est dommage que tu n’en ai pas parlé plus tot, … » and citing some reference about « le maître » by his favorite philosopher « Thomas d’Aquin » as if he had done that to educate me.

From then on we had a quieter correspondence for a while. He sent me his latest papers and I asked him a few more question about the Gödel theorem for which I was interested in. We deviated to the Prologue to the Gospel of John and the sentence where it is said: « in the beginning was the Word, and the Word was with God and the Word was God »; I told Piron that this situation was « in the Beginning » and that this should be our goal for the Word in the End. Until then we had to live with an imperfect Word to express and describe things. This form of « The Word was God » Piron began to explain was the result of a bad translation; that in reality one should say « … and God was the Word » . I understood then the misunderstanding there was between Piron and myself and I told him he was wrong. The pupil had finally realized he had to trace his own path. We never exchanged any e-mails any more.

In the office of Stuckelberg he moved into when I was a Ph.D. student after the latter died, there was a reproduction of M.C. Escher’s Belvedere, ( Piron had placed or kept on one of the wall which acted on me as a parabola. Piron was the man with the feathered beret on the first floor of the building, looking away peacefully to the mountainous landscape. Although it was not the best vantage point in the building it was the place that offered the most freedom. Away from the fight to get to the top floor where a lady was waiting but where one risked to loose one’s purpose either in the fight or the courtship. Away from the jails in the basement where there was no freedom of thinking and away from the bench where the lazies were sitting but where no inspiring views were available. And away from the base of the stair case where learning started and one was not sure yet whether truth or seduction was our mentor.


Benoit Boulat


Benoit Boulat

Ces quelques mots du Prof. Charles P.Enz reçus suite au décès de Constantin:

Pour Constantin Piron, mon collègue direct à Genève et mon ami, cadet de 7 ans, dont les idées me dépassaient souvent, en témoignage d’estime et d’attachement.

Prof Charles P. Enz, 1299 Crans-près-Céligny

Ton père était certainement un homme passionné, qui entre autre, a marqué plusieurs générations de physiciens. Comme tu le sais, j’ai eu l’occasion d’en faire l’expérience, bien que de façon limitée. Je garde bien la copie de son cours d’électrodynamique qu’il m’avait envoyée, après notre première rencontre chez Payot, alors que j’étais encore étudiant en première année au Poly. Je me rappelle bien aussi, comment il m’a « collé sur l’hypothèse ergodique » devant un auditoire rempli à l’université de Genève, lors d’une conférence, vers la fin de ma thèse ! Ton père par ses remarques inattendues, provoquait souvent la réflexion scientifique.

(Extrait d’une lettre du 8 juin 2012 adressée à sa fille Françoise)

Nicolas Macris physicien EPFL