{"id":2192,"date":"2021-06-16T19:02:00","date_gmt":"2021-06-16T18:02:00","guid":{"rendered":"http:\/\/perso.ens-lyon.fr\/ghys\/?p=2192"},"modified":"2022-08-16T19:09:47","modified_gmt":"2022-08-16T18:09:47","slug":"aaa","status":"publish","type":"post","link":"https:\/\/perso.ens-lyon.fr\/ghys\/2021\/06\/16\/aaa\/","title":{"rendered":"The Sun is a kind of gigantic photon pinball machine"},"content":{"rendered":"\n

https:\/\/www.lemonde.fr\/sciences\/article\/2021\/06\/16\/le-soleil-est-une-sorte-de-gigantesque-flipper-a-photons_6084325_1650684.html<\/a><\/p>\n\n\n\n

Les grands prix des fondations de l\u2019Institut de France ont \u00e9t\u00e9 remis le 2 juin \u00e0 douze laur\u00e9ats : \u00e9crivains, artistes, ou scientifiques. Josselin Garnier, professeur de math\u00e9matiques \u00e0 l\u2019Ecole polytechnique, a remport\u00e9 le prix de la Fondation Simone et Cino Del Duca sur le th\u00e8me \u00ab Ph\u00e9nom\u00e8nes de diffusion : th\u00e9orie math\u00e9matique et applications \u00bb. Cette distinction permettra \u00e0 son \u00e9quipe d\u2019\u00e9tudier la propagation des ondes dans des milieux al\u00e9atoires. Les ph\u00e9nom\u00e8nes ondulatoires sont omnipr\u00e9sents : le son, la lumi\u00e8re, les ondes radio, la houle dans les oc\u00e9ans, les tsunamis, une corde de guitare, les tremblements de terre, sans oublier les fameuses vagues successives apport\u00e9es par la pand\u00e9mie. Les math\u00e9maticiens comme les physiciens s\u2019y int\u00e9ressent au moins depuis le XVIIe<\/sup> si\u00e8cle. Aujourd\u2019hui, notre compr\u00e9hension des ondes s\u2019est consid\u00e9rablement enrichie mais il reste encore beaucoup \u00e0 faire.<\/p>\n\n\n\n

Lors de la remise des prix, Josselin Garnier a donn\u00e9 un exemple pour illustrer son propos. La vitesse de la lumi\u00e8re dans le vide est de 300 000 kilom\u00e8tres par seconde et la distance entre le Soleil et la Terre est de 150 millions de kilom\u00e8tres, si bien que la lumi\u00e8re met environ huit minutes pour venir jusqu\u2019\u00e0 nous. Mais bien entendu, cette lumi\u00e8re est cr\u00e9\u00e9e quelque part \u00e0 l\u2019int\u00e9rieur du Soleil et il faut d\u2019abord que les photons qui portent la lumi\u00e8re trouvent un chemin qui les m\u00e8ne \u00e0 la surface. Cette recherche de la \u00ab sortie \u00bb n\u00e9cessite quant \u00e0 elle quelques dizaines\u2026 de milliers d\u2019ann\u00e9es.<\/p>\n\n\n\n

Encha\u00eenement fr\u00e9n\u00e9tique<\/strong><\/p>\n\n\n\n

L\u2019int\u00e9rieur du Soleil est un plasma incroyablement dense et chaud. Des noyaux d\u2019hydrog\u00e8ne fusionnent en permanence pour former de l\u2019h\u00e9lium. Des photons sont absorb\u00e9s et d\u2019autres sont cr\u00e9\u00e9s et repartent dans une direction al\u00e9atoire. C\u2019est cet encha\u00eenement fr\u00e9n\u00e9tique d\u2019absorptions et d\u2019\u00e9missions qui met quelques dizaines de milliers d\u2019ann\u00e9es pour produire un photon \u00e0 la surface, qui peut alors s\u2019\u00e9chapper et arriver chez nous huit minutes plus tard. On peut penser \u00e0 la bille d\u2019un flipper gigantesque qui percute toutes sortes de bumpers<\/em> avant de finir par sortir du plateau. On peut aussi penser \u00e0 la marche d\u2019un ivrogne qui fait des pas dans des directions al\u00e9atoires. Dans ces conditions, il faut bien s\u00fbr beaucoup de temps pour trouver la sortie. De grands progr\u00e8s ont \u00e9t\u00e9 r\u00e9alis\u00e9s ces derni\u00e8res ann\u00e9es dans la th\u00e9orie de la diffusion des ondes dans les milieux al\u00e9atoires, \u00e0 la fronti\u00e8re entre la th\u00e9orie des probabilit\u00e9s et l\u2019analyse math\u00e9matique.<\/p>\n\n\n\n

En 1946, le physicien L\u00e9on Brillouin affirmait que \u00ab toutes les ondes ont des comportements analogues \u00bb.<\/em> Des id\u00e9es et des m\u00e9thodes communes s\u2019appliquent en effet \u00e0 des ondes de natures tr\u00e8s diff\u00e9rentes et constituent aujourd\u2019hui un chapitre important de la physique math\u00e9matique. Josselin Garnier s\u2019int\u00e9resse principalement \u00e0 la propagation des ondes sismiques, pour mieux d\u00e9crire la structure interne de la Terre. La premi\u00e8re difficult\u00e9 provient du fait que l\u2019int\u00e9rieur du globe n\u2019est pas homog\u00e8ne et contient des irr\u00e9gularit\u00e9s r\u00e9parties de mani\u00e8re al\u00e9atoire : les ondes ont donc tendance \u00e0 se comporter un peu comme la lumi\u00e8re dans le Soleil. C\u2019est surtout l\u2019existence de la cro\u00fbte terrestre qui complique le probl\u00e8me, car les ondes de volume, qui traversent la Terre, interagissent avec les ondes de surface.<\/p>\n\n\n\n

Les recherches du laur\u00e9at se concentreront donc sur la propagation des ondes \u00e0 l\u2019int\u00e9rieur d\u2019un corps h\u00e9t\u00e9rog\u00e8ne et limit\u00e9 par une surface. Nul doute que, selon l\u2019adage de L\u00e9on Brillouin, les progr\u00e8s r\u00e9alis\u00e9s pour les ondes sismiques permettront de mieux comprendre la propagation des ondes dans des tissus biologiques, comme par exemple dans un corps humain, lui aussi irr\u00e9gulier et poss\u00e9dant \u00e9galement une surface. De belles applications concr\u00e8tes en perspective.<\/p>\n","protected":false},"excerpt":{"rendered":"

https:\/\/www.lemonde.fr\/sciences\/article\/2021\/06\/16\/le-soleil-est-une-sorte-de-gigantesque-flipper-a-photons_6084325_1650684.html Les grands prix des fondations de l\u2019Institut de France ont \u00e9t\u00e9 remis le 2 juin \u00e0 douze laur\u00e9ats : \u00e9crivains, artistes, ou scientifiques. Josselin Garnier, professeur de math\u00e9matiques \u00e0 l\u2019Ecole polytechnique, a remport\u00e9 le prix de la Fondation Simone et Cino Del Duca sur le th\u00e8me \u00ab Ph\u00e9nom\u00e8nes de diffusion : th\u00e9orie math\u00e9matique et applications \u00bb. Cette distinction permettra … Continuer la lecture de The Sun is a kind of gigantic photon pinball machine<\/span> →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","ub_ctt_via":"","_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"episode_type":"","audio_file":"","cover_image":"","cover_image_id":"","duration":"","filesize":"","date_recorded":"","explicit":"","block":"","itunes_episode_number":"","itunes_title":"","itunes_season_number":"","itunes_episode_type":"","filesize_raw":"","footnotes":""},"categories":[7],"tags":[],"class_list":["post-2192","post","type-post","status-publish","format-standard","hentry","category-non-classe-en"],"featured_image_src":null,"author_info":{"display_name":"ghys","author_link":"https:\/\/perso.ens-lyon.fr\/ghys\/author\/ghys\/"},"yoast_head":"\nThe Sun is a kind of gigantic photon pinball machine - \u00c9tienne Ghys<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/perso.ens-lyon.fr\/ghys\/2021\/06\/16\/aaa\/\" \/>\n<meta property=\"og:locale\" content=\"fr_FR\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Sun is a kind of gigantic photon pinball machine - \u00c9tienne Ghys\" \/>\n<meta property=\"og:description\" content=\"https:\/\/www.lemonde.fr\/sciences\/article\/2021\/06\/16\/le-soleil-est-une-sorte-de-gigantesque-flipper-a-photons_6084325_1650684.html Les grands prix des fondations de l\u2019Institut de France ont \u00e9t\u00e9 remis le 2 juin \u00e0 douze laur\u00e9ats : \u00e9crivains, artistes, ou scientifiques. 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