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Séminaires 2015-2016

Le mercredi 6 avril  2016 15h45 - Amphi Schrödinger
Michel Koenig (École Polytechnique, CNRS, Paris).
Les Lasers de Puissance: un nouvel outil pour l’astrophysique.

Le développement des lasers de puissance, motivé principalement par la quête de la fusion nucléaire contrôlée par confinement inertiel, a aussi permis l’émergence d’une nouvelle discipline : l’astrophysique de laboratoire. En utilisant ces instruments, nous pouvons aujourd’hui recréer en laboratoire des conditions de hautes températures et densités typiques d’objets astrophysiques. Au cours de ce séminaire, je décrirai comment il est possible de mettre en place des expériences opportunément conçues pour étudier en laboratoire l’intérieur des planètes en me focalisant sur celles de type terrestre. J’établirai de façon simple comment avec des lois d’échelle appropriées, on peut étudier des phénomènes "violents" typiques de notre univers tels que les chocs radiatifs, les jets d’étoiles jeunes ou les phénomènes liés au processus d’accrétion dans les variables cataclysmiques. Ainsi, grâce à ces lasers de puissance tel que le LULI2000 (Ecole Polytechnique) ou le LMJ (CEA Bordeaux) on dispose d’un outil d’étude complémentaire aux observations astronomiques, très utile notamment pour les questions concernant la dynamique des phénomènes astrophysiques, souvent trop lente et hors de notre portée.
Voir la video ici en HD ou ici.
Pour le son uniquement ici.

Le mercredi 2 mars  2016 15h45 - Amphi Schrödinger
Jan Plefka (Humboldt-University Berlin, Allmagne).
The world as a hologram: News from string theory.

We are all familiar with holograms: Two dimensional optical structures which - when suitably lit - create the „illusion“ of a three-dimensional object. In fact the light waves emerging from holograms are identical to the ones one would preceive from the three-dimensional object - a distant observer cannot distinguish the two. Recent research in fundamental physics has revealed that the gravitational force of nature might in fact be a holographic illusion in this sense. It is replacable by a lower dimensional structure, known as gauge field theory. The latter is the theoretical framework to describe the non-gravitational forces in nature. Our lecture will begin with reviewing the basic concepts of gravitation, quantum mechanics and quantum fields. Then the holographic concept and its relation to superstrings will be presented. Finally, current insights on how to exploit this duality to answer questions in gauge field theory, which had not been accessible so far, will be presented.
Voir la video ici en HD ou ici.
Pour le son uniquement ici.

Le mercredi 3 février  2016 15h45 - Amphi Schrödinger
Friedrich-Karl Thielemann (Université Basel, Suisse).
Making the heaviest elements in the Universe.

The origin of the elements and the history of their creation in the Universe is the essential stepping stone in order to provide the necessary prerequisites for any bio-chemistry leading to the appearance of life.
The recent high precision measurements of the cosmic microwave background established clearly that only H, He and Li are originating in the Big Bang. Thus, stars had to do the job for all of the heavier elements. Already early concepts from the 1950s led to the conclusion, that the production of elements heavier than Fe and Ni requires free neutrons.
The mechanisms of two processes (a slow and a rapid neutron capture process) have been identified, but only the first (the s-process) is understood from stellar evolution. Recent observations of low metallicity stars as well as radioactive debris in deep-sea sediments indicate that the second (the r-process) is originating from very rare events.
The seminar will discuss options and possible solutions to this puzzle: 1. the neutrino wind in core collapse supernovae, 2. jet ejecta from magneto-rotationally driven (MHD) supernovae, and 3. neutron star mergers.
Voir la video ici en HD ou ici.
Pour le son uniquement ici.

Le mercredi 18 novembre  2015 15h45 - Amphi Schrödinger
Peter Read, Trinity College, University of Oxford, UK.
Jets, vortices and macroturbulence in the laboratory and on Jupiter and Saturn.

The banded organization of clouds associated with intense zonal (east-west) jet streams and large-scale oval vortices on Jupiter and Saturn have long fascinated astronomers and atmospheric dynamicists for many years. The current view is that these features are a manifestation of strongly anisotropic energy transfers within a highly turbulent fluid on a rapidly rotating, spherical planet that is energised at relatively small scales, either by free convection or baroclinic instabilities, but the details are still not well understood.
In this talk we will explore insights from a laboratory experiment on the Coriolis platform in Grenoble that investigated a plausible physical analogue of Jupiter’s atmospheric circulation, energized mainly by free thermal convection with strong background rotation. Weak, eddy-driven jets were obtained through anisotropic energy exchanges, though (for reasons to be discussed) it was not possible to match Jupiter's parameter regime very closely in the laboratory.
We will compare the dynamics and energetics of our laboratory experiment with new measurements of energy exchanges, spectra and structure functions in Jupiter’s atmosphere from analysis of Cassini spacecraft images, which indicate some new directions for models of Jupiter’s atmospheric circulation."
Voir la video ici

Le mercredi 4 novembre  2015 15h45 - Amphi Schrödinger
Miguel Aubouy, LETI-CEA.
Pourquoi sommes-nous si bien préparés à ne jamais découvrir ?

On constate une récurrence dans l’histoire des découvertes : la curiosité combat l’obsession. C'est précisément de cette lutte dont je vais parler de diverses manières, et ses conséquences dans nos vies.

A propos de l'auteur :
Miguel Aubouy a eu une première vie professionnelle consacrée à la recherche en physique fondamentale. Puis une deuxième vie consacrée à l’innovation technologique. Depuis 2014, il écrit des livres, il aide les jeunes entrepreneurs, il intervient dans les entreprises et il donne des conférences sur les sujets qui le passionnent.
Ses recherches portent sur les trois cercles de l'innovation : la découverte scientifique, l’invention technique et la création artistique. "Qu’y a-t-il à l’intersection de ces trois cercles, c’est ce que j’aimerais éclairer. Depuis tant de temps nous fabriquons des objets qui nous ressemblent, et ils continuent de nous échapper."
Voir la video ici

Le mercredi 21 octobre 2015 15h45 - Amphi Schrödinger
Robert Ecke, LANL, USA.
Physics in the 21st Century: Is it the Century of Complexity?

At the beginning of the 20th century, much of what we know about classical physics including mechanics, waves, and thermodynamics had been established and yet there were nagging concerns as reflected by Lord Kelvin's address in 1900 to the Royal Society entitled "Nineteenth-Century Clouds over the Dynamical Theory of Heat and Light". These clouds were the Michelson-Morely experiment and black body radiation which led soon after to the development of special and general relativity and quantum mechanics. By the end of the 20th century, physics had made tremendous progress in understanding the world around us based upon these two revolutionary ideas. Having laid the foundations for how the world works over many orders of magnitude in spatial and temporal scales, the challenges in physics seem ever more governed by how basic principles combine to form complex phenomena. In the words of Stephen Hawking: "We have already discovered the basic laws that govern matter ... There is no limit to the complexity that we can build using those basic laws.” I will reflect upon the future of physics from this perspective while remembering the words of Neils Bohr - "Prediction is very difficult, especially if it's about the future."
Voir la video ici en HD ou ici.
Bande son ici en mp3 ou ici en aac .

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