L'art du titre mensonger : "monopoles magnétiques"...

[JacquesL]

Il dit qu'il trouvera des monopôles magnétiques...

http://www.physorg.com/news153074178.html

Making magnetic monopoles, and other exotica, in the lab
February 5th, 2009 in Physics / Physics


Physicist Shou-Cheng Zhang has proposed a way to physically realize the magnetic monopole. In a paper published online in the January 29 issue of Science Express, Zhang and post-doctoral collaborator Xiao-Liang Qi predict the existence of a real-world material that acts as a magic mirror, in which the never-before-observed monopole appears as the image of an ordinary electron. If his prediction is confirmed by experiments, this could mean the opening of condensed matter as a new venue for observing the exotica of high-energy physics.

Zhang is a condensed-matter theorist at the Stanford Institute for Materials and Energy Science (SIMES), a joint institute of SLAC National Accelerator Laboratory and Stanford University. He studies solids that exhibit unusual electromagnetic and quantum behaviors, with an eye towards their use in information storage. But due to his training as a particle physicist, Zhang always keeps the big picture in mind. That's why it was so easy for him to see that the material he was already working on could behave like what theorists call a magnetic monopole, an isolated north or south magnetic pole.

The monopole is thought of as electric charge's magnetic cousin, but unlike positive or negative charges, north or south poles always occur together in what's called a dipole. A lone north or south pole simply doesn't show up in the real world. Even if you take a bar magnet and cut it in half down the middle, you won't get a separate north and south pole, but two new dipole magnets instead. For symmetry-minded theorists, however, it's natural that there should be a magnetic equivalent of charge. String theories and grand unified theories rely on its existence, and its absence undermines the mathematical feng-shui of the otherwise elegant Maxwell's equations that govern the behavior of electricity and magnetism. What's more, the existence of a magnetic monopole would explain another mystery of physics: why charge is quantized; that is, why it only seems to come in tidy packets of about 1.602×10-19 coulombs, the charge of an electron or proton.

For decades, scientists have kept their eyes peeled for the elusive monopole, but perhaps they were looking in the wrong place. "They were literally hoping it would fall from sky," Zhang says. The notion isn't as far-fetched as it seems—our world is constantly bombarded by weird particles showering from far-off cosmic events, and magnetic monopoles could very well show up as part of that rain. Some enterprising physicists installed loops of superconducting material on their rooftops. If anything remotely like a magnetic monopole fell through, the loops, being sensitive to magnetic fluctuations, would register it.

But in more than 30 years of searching, no one's been able to conclusively detect this particle. Accelerator experiments have been no more successful, leading scientists believe existing monopoles must be far too heavy to create in even the Large Hadron Collider.

Interestingly, Zhang's magnetic monopole didn't fall from the heavens; instead, it was leading a quiet life on the other side of a mirror, but a mirror made of a very special type of alloy. What's more, says Zhang, the math to prove the effect is very clear. "You could give the last part of the mathematical derivation as a final exam in a junior or senior year undergraduate physics class."

To understand how a material can act like a magnetic monopole, it helps to examine first how an ordinary metal acts when a charge—an electron, say—is brought close to the surface. Because like charges repel, the electrons at the surface retreat to the interior, leaving the previously neutral surface positively charged. The resulting electric field looks exactly like that of a particle with positive charge the same distance below the surface—it's the positive mirror image of the electron. In fact, from an observer's point of view, it's impossible to tell the difference.

The concept of an image charge is something undergraduate physics students encounter in their very first electricity and magnetism class, along with the idea that the magnetic monopole doesn't exist. But Zhang's "mirror" alloy is no ordinary material. It's what's called a topological insulator, a strange breed of solid Zhang specializes in, in which "the laws of electrodynamics are dramatically altered," he says. In fact, if an electron was brought close to the surface of a topological insulator, Zhang's paper demonstrates, something truly eerie would happen. Instead of an ordinary positive charge, Zhang says, "You would get what looks like a magnetic monopole in the 'mirror.'"

To go back to the example of image charges, it's important to emphasize that there isn't actually half of a bar magnet somewhere inside this material. Instead, Zhang discovered, due to a peculiarity of the material called strong spin-orbit coupling, the nearby electron would induce a current in the surface that circulates constantly without dying out. This in turn—undergraduate physics majors, get out your pencils—would create a magnetic field that looks like that of a magnetic monopole. Experimentalists have tried to approximate this field before, for instance by arranging permanent magnets in certain ways. But to an outside observer, Zhang's material would be completely indistinguishable from the monopole particle that physicists were hoping to catch in their superconducting detectors.

"We like to find things that don't exist," says Zhang. His work on the monopole has further ramifications; this could be a way to physically realize a number of particles that, until now, have only existed as mathematical loopholes in high-energy physics theories. For instance, Zhang has shown that the electron and image monopole together would act like a so-called "anyon" located at the solid's surface. "The 'any,' in this case, is as in 'anything,'" Zhang explains—they are particles that only exist in two dimensions, whose properties straddle those of the two classes of three-dimensional particles, fermions and bosons.

Although Zhang works as a theorist, he has close ties to experimental physics. In 2007, his prediction of the quantum spin Hall effect in mercury telluride was confirmed experimentally, earning his work praise in Science as a runner-up breakthrough of that year. "As a theorist you're always motivated by the math, but it's a testament to our understanding that we can predict real-world materials," Zhang says. "Before, new materials were more or less found by accident." Now other SIMES researchers will be using the Stanford Synchrotron Radiation Lightsource at SLAC to closely study two specific materials, bismuth selenide and bismuth telluride, that Zhang has predicted will exhibit this strange mirror behavior. They hope to confirm the prediction experimentally some time this year.

"Exotic particles such as the magnetic monopole, dyon, anyon, and the axion have played fundamental roles in our theoretical understanding of quantum physics," Zhang writes in the paper. "Experimental observation of these exotic particles in table-top condensed matter systems could finally reveal their deep mysteries." Topological insulators could provide a new experimental outlet for high-energy physicists. "You don't have to look towards the cosmos," Zhang says. "I think we'll see more of the beautiful mathematical structures of high-energy physics become realized in condensed matter physics."

Provided by SLAC National Accelerator Laboratory, By Lauren Schenkman

Trouvez l'erreur.

Déjà expliquée par Clerk Maxwell en 1873.
Tout théoricien qu'il soit, Shou-Cheng Zhang semble tout ignorer du théorème des hérissons.

Il est impossible de peigner intégralement les poils d'une sphère.

Corollaire :
Il est impossible de recouvrir intégralement la surface d'une sphère d'un tourneur non nul, de sens uniforme.

En effet, il existe deux façons simples de presque peigner une sphère :
1. comme la surface latérale d'un cylindre, en long. Puis vous resserrez les ouvertures autour des pôles, qui restent impossibles à peigner, hérissés d'un poil soit saillant soit rentrant dans la sphère. Votre peignage est donc méridien.
2. comme la surface latérale d'un cylindre, selon trajectoire circulaire. Après serrage autour des pôles, votre peignage suit les parallèles. Sauf aux pôles où il ne sait dans quel sens se coucher.

3e solution moins simple, la combinaison des deux, le peignage loxodromique, à angle constant avec les parallèles (resp. les méridiens). Là encore, les pôles ne peuvent être peignés.

Vous prenez la solution n°2, et en prenez le rotationnel : les deux hémisphères sont couverts d'un tourneur opposé, sauf l'équateur qui n'est couvert que par le tourneur nul.
Une autre solution est priver la sphère du pôle Sud, de couvrir tout le restant d'un tourneur, disons horaire. Le pôle Sud est une singularité : tout parcours autour de lui montre une forte circulation, mais anti-horaire.


Le monopôle magnétique n'existe donc pas.
L'erreur historique de Dirac à ce sujet - qu'il a reconnue et dénoncée avant de mourir - était d'avoir gobé l'erreur mathématique standard, dénoncée par Clerk Maxwell en 1873 et Pierre Curie en 1894.

--
http://jacques.lavau.deonto-ethique.eu/SYNTAXE2_.pdf
http://jacques.lavau.deonto-ethique.eu/syntaxe3.pdf
http://jacques.lavau.deonto-ethique.eu/Syntaxe4.pdf

[JacquesL]

L'art du titre mensonger : "monopôles magnétiques"...

http://www.larecherche.fr/content/actualite-matiere/article?id=26506

Les premiers monopôles magnétiques



Des monopôles magnétiques – entités qui ne possèdent qu'un seul pôle, nord ou sud – ont été identifiés dans des solides cristallins. Leur existence avait été prédite par les théoriciens il y a près de quatre-vingts ans.

Depuis des décennies, les physiciens traquent d'hypothétiques entités dénommées « monopôles magnétiques », qui, contrairement aux aimants et autres objets magnétiques, ne possèdent qu'un seul pôle, nord ou sud. Deux équipes annoncent les avoir observées dans un milieu auquel les théoriciens ne s'attendaient pas : un solide cristallin.

Pôles antagonistes. En 1931, le physicien britannique Paul Dirac était le premier à postuler l'existence de ces monopôles. On savait que l'électricité et le magnétisme n'étaient que les deux facettes d'une même interaction fondamentale, la force électromagnétique. Or les particules électriquement chargées sont de signe soit positif, soit négatif. Dirac s'étonnait du fait que les particules et les objets magnétiques possèdent toujours, eux, deux pôles antagonistes. L'existence de monopôles lui permettait aussi d'expliquer pourquoi les charges électriques étaient quantifiées par une valeur élémentaire, notée « e ». D'autres travaux sont venus appuyer les suppositions de Dirac. Les théories de grande unification, par exemple, qui visent à intégrer les interactions faibles, fortes et électromagnétiques dans un même cadre conceptuel, prédisaient elles aussi l'existence de monopoles. Dès lors, les physiciens avaient entrepris leur traque systématique : dans les accélérateurs de particules, dans les fonds marins, dans l'espace, et même dans les roches lunaires. Sans succès.

Aimants microscopiques. C'est dans un milieu inattendu que des monopôles viennent finalement d'être observés : une famille des solides cristallins découverts en 1997, composés notamment d'oxygène, de titane et de dysprosium, sous forme ionique. En raison d'une propriété quantique fondamentale, appelée « spin », les ions se comportent comme de petits aimants (avec deux pôles) pointant dans une direction. À une température proche du zéro absolu, les pôles nord et sud se succèdent alternativement. Leur arrangement ressemble à celui dont le spin des atomes d'hydrogène est ordonné dans l'eau gelée. C'est pourquoi ces cristaux ont été baptisés « glaces de spin » par leurs découvreurs.

Deux équipes – l'une dirigée par Tom Fennell, de l'institut Laue-Langevin, à Grenoble (1) ; l'autre par Jonathan Morris, du centre Helmholtz de Berlin (2) – ont constaté qu'un certain type de monopôles se formait dans les glaces de spin. Leur apparition résulte d'une augmentation faible mais soudaine de la température. De façon apparemment aléatoire, le spin d'un ion bascule alors pour pointer dans un sens opposé. Son pôle nord se retrouve en face du pôle nord du spin voisin, créant une « charge magnétique » ponctuelle – autrement dit un monopôle. La même chose se produit avec le pôle Sud, générant un autre monopôle, de nature opposée. Le basculement des spins de proche en proche conduit au déplacement de ces deux monopôles au sein du cristal.

« Ces monopôles ne sont pas exactement ceux qu'avait prédits Dirac, explique Peter Holdsworth, de l'école normale supérieure de Lyon. Il ne s'agit pas de particules, mais plutôt de quasi-particules, dans le langage des physiciens. Cette découverte devrait néanmoins susciter une large activité de recherche pour en comprendre toutes les implications, sur le plan fondamental et appliqué. »

Franck Daninos

(1) D. Morris et al., Science Express,
doi:10.1126/science.1178868, 2009.
(2) T. Fennell et al., Science Express,
doi:10.1126/science.1177582, 2009.

Splendide, cet art du titre mensonger, pour attirer les gogos.

Les "monopôles" continuent d'aller par paires.
En revanche, cela continue de poser le problème des limites inférieures de la topologie et de la métrique de notre espace macroscopique familier. Le fameux théorème des hérissons (Il est impossible de peigner intégralement une boule de billard) qui rend impossible tout monopôle magnétique par la nature tornatorielle (et non pas vectorielle) du champ magnétique, est lié à notre espace-temps macroscopique. En quoi, à l'extrémité basse du domaine de validité du dit espace-temps, les "monopôles magnétiques" de l'article sont-ils séparés, au juste ?

Autre source :
http://en.wikipedia.org/wiki/Magnetic_monopole#.22Monopoles.22_in_condensed-matter_systems

While a magnetic monopole particle has never been conclusively observed, there are a number of phenomena in condensed-matter physics where a material, due to the collective behavior of its electrons and ions, can show emergent phenomena that resemble magnetic monopoles in some respect.[19][20][21][22][23]  These should not be confused with actual monopole particles. In particular, the divergence of the microscopic magnetic B-field is zero everywhere in these systems, which it would not be in the presence of a true magnetic monopole particle. The behavior of these quasiparticles  would only become indistinguishable from true magnetic monopoles — and they would truly deserve the name — if the so-called magnetic fluxtubes connecting these would-be monopoles became unobservable which also means that these flux tubes would have to be infinitely thin, obey the Dirac quantization rule, and thus deserve to be called Dirac strings.

In a paper published in the journal Science in September 2009, researchers Jonathan Morris and Alan Tennant from the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) along with Santiago Grigera from Instituto de Física de Líquidos y Sistemas Biológicos (IFLYSIB, CONICET) and other colleagues from Dresden University of Technology, University of St. Andrews and Oxford University described the observation of quasiparticles resembling magnetic monopoles. A single crystal of dysprosium titanate in a highly frustrated pyrochlore lattice (F d -3 m) was cooled to a temperature between 0.6 kelvin and 2.0 kelvin. Using observations of neutron scattering, the magnetic moments were shown to align in the spin ice into interwoven tubelike bundles resembling Dirac strings. At the defect formed by the end of each tube, the magnetic field looks like that of a monopole. Using an applied magnetic field to break the symmetry of the system, the researchers were able to control the density and orientation of these strings. A contribution to the heat capacity of the system from an effective gas of these quasiparticles was also described.[24][25]

# 
19# ^ Zhong, Fang; Naoto Nagosa, Mei S. Takahashi, Atsushi Asamitsu, Roland Mathieu, Takeshi Ogasawara, Hiroyuki Yamada, Masashi Kawasaki, Yoshinori Tokura, Kiyoyuki Terakura (October 3, 2003). "The Anomalous Hall Effect and Magnetic Monopoles in Momentum Space". Science 302 (5642): 92-95. doi:10.1126/science.1089408. ISSN 1095-9203. http://www.sciencemag.org/cgi/content/abstract/302/5642/92. Retrieved on 2 August 2007.
20# ^ Making magnetic monopoles, and other exotica, in the lab, Symmetry Breaking, 29 January 2009, accessed 31 January 2009
21# ^ Inducing a Magnetic Monopole with Topological Surface States, American Association for the Advancement of Science (AAAS) Science Express magazine, Xiao-Liang Qi, Rundong Li, Jiadong Zang, Shou-Cheng Zhang, 29 January 2009, accessed 31 January 2009
22# ^ Magnetic monopoles in spin ice, C. Castelnovo, R. Moessner and S. L. Sondhi, Nature 451, 42-45 (3 January 2008)
23# ^ Nature 461, 956-959 (15 October 2009); doi:10.1038/nature08500, Steven Bramwell et al
24# ^ "Magnetic Monopoles Detected In A Real Magnet For The First Time". Science Daily. 4 September 2009. http://www.sciencedaily.com/releases/2009/09/090903163725.htm. Retrieved 4 September 2009.
25# ^ D.J.P. Morris, D.A. Tennant, S.A. Grigera, B. Klemke, C. Castelnovo, R. Moessner, C. Czter-nasty, M. Meissner, K.C. Rule, J.-U. Hoffmann, K. Kiefer, S. Gerischer, D. Slobinsky, and R.S. Perry (3 September 2009). "Dirac Strings and Magnetic Monopoles in Spin Ice Dy2Ti2O7". Science, DOI: 10.1126/science.1178868. doi:10.1126/science.1178868.