"I have a dream" / Pourquoi croire à une symétrie déjà vraie (mathématiquement cohérente) qui n'est pas encore réelle (physiquement manifeste)?

Un quantum d'obstination (dernier billet de cette rubrique)

(Groupe de) Supersymétrie : de (forcément) pas fausse à (probablement) vrai?

Supersymmetry was (and is) a beautiful mathematical idea. It extends the symmetry of special relativity. Special relativity postulates the invariance of physical law under motion with a constantvelocity. It thereby allows us to transform between objects with different speeds and to predict the properties of moving particles from those of stationary ones. Supersymmetry postulates the invariance of physical law under certain kinds of motion in a quantum-mechanical extension of space-time, superspace. Superspace has four extra quantum-mechanical dimensions, quite different from the familiar four dimensions of space and time. When a particle “moves” in the extra quantum dimensions it just acquires a tiny amount of angular momentum, or spin. So you wouldn’t want to live in the new suburbs of superspace: it’s very cramped, and makes you dizzy. But the mathematics of supersymmetry promised (and still promises) to help us unify fundamental physics...  

The problem with applying supersymmetry is that it’s too good for this world. We simply don’t find particles of the sort it predicts. We don’t, for example, see particles with the same charge and mass as electrons, but a different amount of spin. 

Symmetry principles that might help to unify fundamental physics are hard to come by, however, so theoretical physicists won’t give up on them easily. Based on previous experience with other forms of symmetry, we’ve developed a fallback strategy, called spontaneous symmetry breaking. In this approach we postulate that the fundamental equations of physics have the symmetry, but the stable solutions of these equations do not... 

While straight supersymmetry is a (wrong) statement about the properties of the world, spontaneously broken supersymmetry is a research program. For it is a statement about equations that describe the world, and only indirectly about the world itself. Carrying forward this research program involves model building – the creative activity of proposing candidate equations and analyzing their consequences... 

Building models with spontaneously broken supersymmetry that are consistent with everything else we know about physics is a difficult business. Even if you manage to get the symmetry to break, the extra particles are still there (just heavier) and cause various mischief...  

To get oriented and make a definite  calculation, we started by doing the crudest thing, that is to ignore the whole problem of breaking supersymmetry, which allowed us to use very simple (but manifestly unrealistic) models. 

The result was amazing, at least to me. The supersymmetric versions of the gauge symmetry models, although they were vastly different from the originals, gave very nearly the same answer for the couplings.  

That was the turning point. We put aside the “not wrong” complicated models with spontaneous supersymmetry breaking, and wrote a short paper [5] that, taken literally (with unbroken supersymmetry), was wrong. But it presented a result that was so straightforward and successful that it made the idea of putting gauge symmetry and supersymmetry unification together seem (maybe) right. We put off the problem of how supersymmetry gets broken. And while there are some good ideas about it, there is still no generally accepted solution... 

We all eagerly await operation of the Large Hadron Collider (LHC) at CERN, where, if these ideas are correct, the new particles of supersymmetry – or, you might say, the new dimensions of superspace – must make their appearance... 

This little episode, it seems to me, is 179 degrees or so out of phase from Popper’s idea that we make progress by falsifying theories. Rather in many cases, including some of the most important, we suddenly decide our theories might be true, by realizing that we should strategically ignore glaring problems. It was a similar turning point when David Gross and I decided to propose QCD based on asymptotic freedom, putting off the problem of quark confinement. But that’s another story ...

F. Wilczek, From "Not Wrong" to (Maybe) Right 24/03/2004

Grande symétrie (algébrique non commutative) : de (nécessairement) valide à (peut-être) réelle?

La lecture de ce texte du célèbre physicien Wilczek, belle défense épistémologique de la supersymétrie, par un des pères de la théorie moderne des interactions fortes, a fini de me convaincre qu'il fallait par ailleurs défendre l'application des idées de la géométrie non commutative en physique des particules car les outils et les idées qu'elle apporte commencent à montrer leur utilité. Cette tâche spécifique cadrant mal avec la ligne éditoriale tracée initialement pour ce blog, lequel vise à comparer des développements scientifiques entre eux mais pas à défendre systématiquement un cadre théorique donné, le blogueur décide aujourd'hui de mettre fin à la rubrique un quantum d'obstination en la promouvant au rang de blog autonome intitulé : quantumostinato.

Du rêve à la réalité

Hommage en passant à un homme dont le grand rêve d'hier sera célébré demain et dont la fin tragique n'a pas empêché que son rêve se transforme pas à pas en réalité aujourd'hui grâce aux hommes de bonne volonté.