vendredi 31 octobre 2014

Faites moi peur (ou Vous voulez me rassurez) ...

//Le blogueur fête aujourd'hui Halloween à sa manière en convoquant quelques physiciens qui n'hésitent pas à parler du scénario cauchemardesque (nightmare scenario) de la physique des hautes énergies, à savoir pas de phénomènes au delà de ceux prévus par le Modèle Standard observables au LHC. Le but est évidemment de se rassurer en montrant que ses mêmes physiciens réfléchissent sur ce qui pourrait faire avancer leur discipline.

... Monsieur Shifman
String theory appeared as an extension of the dual resonance model of hadrons in the early 1970, and by mid-1980 it raised expectations for the advent of “the theory of everything” to Olympic heights. Now we see that these heights are unsustainable. Perhaps this was the greatest mistake of the string-theory practitioners. They cornered themselves by promising to give answers to each and every question that arises in the realm of fundamental physics, including the hierarchy problem, the incredible smallness of the cosmological constant, and the diversity of the mixing angles. I think by now the “theory-of-everything-doers” are in disarray, and a less formal branch of string theory is in crisis [a more formal branch evolved to become a part of mathematics or (in certain occasions) mathematical physics]. 
At the same time, leaving aside the extreme and unsupported hype of the previous decades, we should say that string theory, as a qualitative extension of field theory, exhibits a very rich mathematical structure and provides us with a new, and in a sense superior, understanding of mathematical physics and quantum field theory. It would be a shame not to explore this structure. And, sure enough, it was explored by serious string theorists. 
The lessons we learned are quite illuminating. First and foremost we learned that physics does not end in four dimensions: in certain instances it is advantageous to look at four dimensional physics from a higher-dimensional perspective... A significant number of advances in field theory, including miracles in N = 4 super-Yang-Mills... came from the string-theory side...
... since the 1980s Polyakov was insisting that QCD had to be reducible to a string theory in 4+1 dimensions. He followed this road... arriving at the conclusion that confinement in QCD could be described as a problem in quantum gravity. This paradigm culminated in Maldacena’s observation (in the late 1990’s) that dynamics of N=4 super-Yang- Mills in four dimensions (viewed as a boundary of a multidimensional bulk) at large N can be read off from the solution of a string theory in the bulk... 
Unfortunately (a usual story when fashion permeates physics), people in search of quick and easy paths to Olympus tend to overdo themselves. For instance, much effort is being invested in holographic description in condensed matter dynamics (at strong coupling). People pick up a supergravity solution in higher dimensions and try to find out whether or not it corresponds to any sensible physical problem which may or may not arise in a condensed matter system. To my mind, this strategy, known as the “solution in search of a problem” is again a dead end. Attempts to replace deep insights into relevant dynamics with guesses very rarely lead to success.
(Submitted on 31 Oct 2012 (v1), last revised 22 Nov 2012 (this version, v3))

... Monsieur White
In his overview talk[1] at Strings 2013, David Gross discussed the “nightmare scenario” in which the Standard Model Higgs boson is discovered at the LHC but no other new short-distance physics, in particular no signal for SUSY, is seen. He called it the “extreme pessimistic scenario” but also said it was looking more and more likely and (if it is established) then, he acknowledged
“We got it wrong.” “How did we misread the signals?” “What to do?”.
He said that if it comes about definitively the field, and string theorists in particular, will suffer badly. He said that it will be essential for theorists who entered the field most recently to figure out where previous generations went wrong and also to determine what experimenters should now look for.
In the following, I will argue that a root cause has been the exaggeration of the significance of the discovery of asymptotic freedom that has led to the historically profound mistake of trying to go forward by simply formulating new short-distance theories, supersymmetric or otherwise, while simultaneously ignoring both deep infra- red problems and fundamental long-distance physics.
In his recent “Welcome” speech[2] at the Perimeter Institute, Neil Turok expressed similar concerns to those expressed by Gross. He said that
“All the {beyond the Standard Model} theories have failed ... Theoretical physics is at a crossroads right now ... {there is} a very deep crisis.”
He argued that nature has turned out to be simpler than all the models - grand unified, super-symmetric, super-string, loop quantum gravity, etc, and that string theorists, especially, are now utterly confused - with no predictions at all. The models have failed, in his opinion, because they have no new, simplifying, underlying principle. They have complicated the physics by adding extra parameters, without introducing any simplifying concepts.

(Submitted on 5 Jun 2014)

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