18 Février 2018 : Bon anniversaire Roland Omnès !
Je découvre par hasard aujourd'hui (avec un mois de retard) que le merveilleux professeur (aujourd'hui émérite) et chercheur reconnu encore actif avec qui j'ai eu le privilège de découvrir la mécanique quantique en première année de magistère de physique fondamentale d'Orsay vient de fêter cette année ses 2²+3²+5²+7² printemps !
J'espère avoir le temps un jour d'approfondir la lecture de ses dernières réflexions sur l'ambitieux programme de démontrer l'unicité de la réalité physique macroscopique par une approche quantique.
Pour évoquer ses qualités de chercheur et de professeur sans verser dans la flagornerie je laisse la parole à un autre physicien qui fait ci-dessous une recension d'un des livres d'Omnès les plus importants directement écrit en anglais.
The Interpretation of Quantum Mechanics. By Roland Omnès. Princeton University Press, Princeton, New Jersey, 1994,
This monograph is the first book-length treatment of the consistent histories approach to the interpretation of quantum mechanics which I initiated in 1984, and to which Omn`es (starting in 1987) and Gell-Mann and Hartle (starting in 1990) have made major contributions. While consistent historians do not agree on every detail, there is a common core of ideas which can be summarized as follows. A closed quantum system (the universe, if one is ambitious) is represented by a Hilbert space, and anything that can sensibly be said about it at a particular time is represented by some subspace of this Hilbert space; in other words, there are no hidden variables. A history consists of a sequence of subspaces E1, E2, . . . associated with times t1, t2, . . ., understood as events occurring, or properties which are true, at these times. Provided the history is a member of a consistent family of histories, it can be assigned a probability, and within a given consistent family these probabilities function in the same way as those of a classical stochastic theory (imagine a sequence of coin tosses): one and only one of them occurs, and the theory assigns a probability to each possibility. Inconsistent histories, those which do not belong to a consistent family, are meaningless. The unitary time evolution generated by the Schrödinger equation, without any stochastic or nonlinear modifications, is used both for determining the consistency conditions which define consistent families, and for calculating the probabilities of histories belonging to a particular family.
Measurements play no fundamental role in the consistent history interpretation; they simply correspond to sequences of events inside a closed system in which the measurement apparatus, along with everything else, is treated quantum mechanically. Thus a possible history for a closed system in which there is an apparatus for measuring the z component of the spin of a particle might include an initial state, a value of S z at a time shortly before the particle reaches the apparatus, and the position of a pointer on the apparatus at some later time. Using conditional probabilities one can show, under suitable conditions, that the particle earlier had the property indicated later by the pointer position. In this and various other ways the consistent history approach replaces the smoky dragons which inhabit textbook and other treatments of “measurement” with precise mathematical and logical rules yielding results which are often much closer to the intuition of experimental physicists than to what one finds in the literature on quantum foundations...
While it does have defects, I nevertheless admire this book as a bold attempt to give substance to a vision common to consistent historians, namely that our current scientific understanding of the physical world, macroscopic phenomena as well as microscopic, can be linked to a firm foundation of quantum mechanical principles by appropriate and precise rules of sound reasoning. There is no need to add hidden variables to the Hilbert space, or tinker with the Schrödinger equation, or restrict ourselves to talk about “measurements” in order to construct a coherent interpretation of quantum mechanics which overcomes the well-known conceptual problems which have given such trouble to those who have been attempting to understand the subject for the past seventy years. We are indebted to Omnès for showing how much of this program can actually be carried out within the consistent histories framework. Some imperfections are inevitable in a pioneering effort, and those portions of the book which I find problematical are nonetheless useful as indications of what needs to be better understood, or more clearly explained, or perhaps both.
Robert B. Griffiths (Carnegie-Mellon)(Submitted on 16 May 1995)
Pour goûter toute la saveur de la pensée d'Omnès exposée en termes non techniques, il vaut sûrement mieux le lire quand il écrit dans sa langue maternelle. Etudiant, je me souviens avoir beaucoup apprécié Alors l'un devint deux en particulier son chapitre sobrement intitulé conclusions. (Mon indécrottable romantisme péri-germanique m'inciterait à proposer comme autres titres : Tractatus logo-physicus ou bien De quantum natura pour évoquer le caractère philosophique et analytique mais aussi l'élégance et la simplicité de l'exposition).
Voici ce qu'annonce la quatrième de couverture :
Que sont donc les mathématiques ? Après Platon et Russell ou Gödel, cette grande question suscite toujours plus de débats et de controverses et elle attend encore sa réponse. Il s’agirait d’un pur jeu formel pour les uns, de l’image d’une réalité immatérielle pour les autres... Et voici que les acquis récents de la physique des particules invitent à explorer une troisième voie. Car les lois qui gouvernent les quarks et autres constituants de la matière, la manière dont elles furent découvertes et les modes de pensée par lesquels il fallut passer sont autant de révélateurs d’une parenté intimement profonde entre physique et mathématique. Pourtant, si l’on pénètre davantage les rapports complexes qui s’exercent entre la physis – la réalité empirique – et le logos – la réalité des formes –, force est de constater aussi qu’un irréductible hiatus les sépare. Alors, l’un devient deux...
What is mathematics? After Plato and Russell or Gödel, this fundamental question is still the subject of more debate and controversy and is still waiting for an answer. It would be a pure formal game for some, the image of an immaterial reality for others... And now recent achievements in particle physics invite us to explore a third way. Because the laws that govern the quarks and other constituents of matter, the way they were discovered and the ways of thinking through which they had to pass are all revealing of a profound relationship between physics and mathematics. However, if we penetrate more deeply into the complex relationships between the physis - the empirical reality - and the logos - the reality of forms - we must also note that they are separated by an irreducible hiatus. So, one becomes two...
(Translated with the help of www.DeepL.com/Translator)
Dans ma thèse j'ai choisi la citation suivante :
Proposition 32
La connaissance de la physis ne peut provenir que d'une forme d'expérience. La disjonction de nature entre physis et logos se laisse voir dans la philosophie traditionnelle comme l'impossibilité de construire la science par pure induction
Knowledge of physis can only come from a form of experiment. The natural disjunction between physis and logos can be seen in traditional philosophy as the impossibility of building science by pure induction.
