vendredi 12 avril 2013

Qu'est-ce que la physique du boson de Higgs doit vraiment à la physique de la matière condensée ?

En attendant le séminaire sur le sujet ...
L'écriture de ce billet a débuté dans l'attente d'un séminaire (alors à venir) de P.W. Anderson où le célèbre physicien théoricien de la matière condensée dev(r)ait défendre son point de vue sur le mécanisme dit de Anderson-Higgs ou mécanisme (A?)BEHHGK déjà évoqué dans un précédent billet) !

Un physicien des particules verse son tribu à un physicien de la matière condensée
Commençons notre enquête par ce que Peter Higgs, le désormais fameux physicien des particules nobélisable, doit à Philp W. Anderson, physicien théoricien de la matière condensée anciennement nobélisé (1977). Il suffit de lire l'article désormais célèbre (et en accès libre!) de Higgs :
In a recent note ... it was shown that the Goldstone theorem ..., that Lorentz-covariaint field theories in which spontaneous breakdown of symmetry under an internal Lie group occurs contain zero-mass particles, fails if and only if the conserved currents associated with the internal group are coupled to gauge fields. The purpose of the present note is to report that, as a consequence of this coupling, the spin-one quanta of some of the gauge fields acquire mass; the longitudinal degrees of freedom of these particles (which would be absent if their mass were zero) go over into the Goldstone bosons when the coupling tends to zero. This phenomenon is just the relativistic analog of the plasmon phenomenon to which Anderson [3] has drawn attention.
         [3] P. W. Anderson, Phys. Rev. 130, 439 (1963)
Peter Higgs Broken symmetries and the masses of gauge bosons 1964
Le phénomène dont parle Higgs dans la dernière phrase (que nous avons souligné par les caractères gras) est aujourd'hui décrit par le terme générique de "brisure spontanée d'une symétrie de jauge". 

Un physicien de la matière condensée rend hommage à un physicien des particules 
Passons maintenant à Anderson qui a beaucoup œuvré pour promouvoir l'utilisation du concept précédent tout en reconnaissant clairement ce qu'il doit au physicien Yoichiro Nambu, récemment nobélisé (2008) pour le succès de ses idées en physique subatomique. L'extrait suivant est explicite :
... he visited us at Bell Labs in '59 and his understanding of the deep meaning of what we were doing had an enormous effect on my own thinking about broken symmetry. I think    I even learned the words from him, if not the idea ... 
 I give Nambu Sensei full credit for opening all of our minds to the consequences of allowing Nature to tell us what field theory can do, rather than confining Nature by our own preconceptions. The intellectual thread runs unbroken from his work to the final triumphs of the Standard Model.
P.W Anderson More and Different 2011
Un physicien (peut) a(voir) deux amours : la physique des particules et de la matière condensée
Il est intéressant de s'attarder sur Nambu, qui contrairement aux deux physiciens précédents a vraiment une double culture théorique : celle de la matière condensée (on disait autrefois physique du solide) et celle des particules. Il expose ainsi les circonstances qui ont conduit à cet état de fait :
I will begin with a short story about my background. I studied physics at the university of Tokyo. I was attracted to particle physics because of three famous names, Nishina, Tomonaga and Yukawa, who were the founders of particle physics in Japan. But these people were at different institutions than mine. On the other hand, condensed matter physics was pretty good at Tokyo. I got into particle physics only when I came back to tokyo after the war. In hindsight, though, I must say that my early exposure to condensed matter physics has been quite beneficial to me.
Y. Nambu Spontaneous symmetry breaking in particle physics: a case of cross-fertilization 2008

Dans le texte de sa conférence Nobel (présentée par son collaborateur Giovani Jona-Lasinio) le physicien japonais expose autant sa passion pour la phénoménologie des particules et leurs interactions que pour le formalisme mathématique rigoureux qui la modélise, il décrit aussi tout ce qu'il doit à la théorie BCS de la supraconductivité :
I will now recall the chain of events which led me to the idea of SSB and its application to particle physics. one day in 1956 R. Schrieffer gave us a seminar on what would come to be called the BCS theory [5] of superconductivity. I was impressed by the boldness of their ansatz for the state vector, but at the same time I became worried about the fact that it did not appear to respect gauge invariance. Soon thereafter Bogoliubov [6] and Valatin [7] independently introduced the concept of quasiparticles as fermionic excitations in the BCS medium. The quasiparticles did not carry a definite charge as they were a superposition of electron and hole, with their proportion depending on the momentum. How can one then trust the BCS theory for discussing the electromagnetic properties like the Meissner effect? It actually took two years for me to resolve the problem to my satisfaction. There were a number of people who also addressed the same problem, but I wanted to understand it in my own way. Essentially it is the presence of a massless collective mode, now known by the generic name of Nambu-Goldstone boson, that saves charge conservation or gauge invariance.
Y. Nambu ibid. 
De l'enquête rapide à l'histoire objective
Pour finir voici deux très brefs extraits d'un article intéressant dans lequel trois acteurs actuels et reconnus de la physique des particules et bons connaisseurs de celle du boson de Higgs tentent d'en brosser un historique bien informé :
The existence of the Higgs boson was first postulated in 1964 [1], following earlier theoretical work that introduced spontaneous symmetry breaking into condensed-matter [2] and particle physics [3–5]...   
B. W. Lee also carries much of the responsibility for calling the Higgs boson the Higgs boson, mentioning repeatedly ‘Higgs scalar fields’ in a review talk at the International Conference on High-Energy Physics in 1972 [29].
J. Ellis, M.K. Gaillard et D.V. Nanopoulos A Historical profile of the Higgs boson 29/01/12

Ce que certains physiciens (des particules?) reprochent à un fameux physicien de la matière condensée
//Dernière mise à jour (17/04/13) après le séminaire mentionné en préambule 
Si la physique porte sur l'étude d'êtres inanimés : particules individuelles ou condensées, l'histoire des sciences et le récit de sa médiatisation mettent en jeux des êtres très animés ! Certains blogs mettent cela bien en lumière. A ce sujet on recommande la lecture du dernier billet de Peter Woit, lequel a assisté au séminaire d'Anderson évoqué en introduction ! On peut aussi lire par curiosité les commentaires qui accompagnent son billet afin d'y découvrir d'autres enjeux ... voici l'un d'eux :
Well, it is true that Anderson spoke against the SSC and this ultimately greatly contributed to killing the project. However, this does not change the fact he did understand the mechanism before others. I think Jester once put this quite nicely in
“However, the name of Higgs somehow stuck, probably because it’s cute, or maybe because we all hate Anderson for cutting the throat of the SSC.”
Scientists are not immune to hatred, nationalism and others “isms” but, as best as we can, we should try to be. As I see, it is clear Anderson nailed it first, so everything else he might or might have not done should not be relevant….
I know that in the end this will be taken into account and the Nobel committee is certainly not made by neutral people without bias. However, this is what we should strive for. We should speak in favor of Anderson despite “hating his guts”.
Bernhard, commentaire sur le blog Not Even Wrong,  April 17, 2013 at 5:58 am

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