[PhilPhys] fondements et philosophie de la physique, J. Barbour 24/02 et V. Ogryzko 11/03

[GRINBAUM Alexei]

Séminaire du projet "Fondements de la physique en région parisienne <http://www-llb.cea.fr/Phocea/Vie_des_labos/Ast/ast_visu.php?id_ast=761> " 





24 février à 15h, ENS (Salle W, 45 rue d'Ulm, Paris 5e)
Julian Barbour (Oxford) "Shape Dynamics as the Physical Core of General Relativity"

Plan d'accès à la Salle W. Entrer dans le bâtiment principal, suivre le couloir de droite jusqu'à l'escalier B dans le coin sud-est ; monter au 3ème (= dernier) étage par l'escalier principal, puis suivre les flèches Toits du DMA : prendre le couloir, au bout du couloir tourner à droite et monter encore un étage. La salle W se trouve sur votre gauche.



11 mars à 11h au LARSIM (CEA-Orme des Merisiers, bât. 774, salle 50)
Vasily Ogryzko (INSERM, Institut Gustave Roussy) "Can one use the notion of ground state for the description of a living cell?"

 


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Résumés / Abstracts

24 février 2010, 15h, ENS, Salle W
Julian Barbour (Oxford) "Shape Dynamics as the Physical Core of General Relativity"
After an brief historical review of the concept of geometry developed by Riemann and employed by Einstein, I will discuss in more detail Weyl's desire to replace Riemannian geometry by one in which length is not absolute. This led Weyl in 1918 to introduce additional geometrical structure that he used to define transport of length. His great hope of thereby unifying gravity and electromagnetism foundered on physical objections by Einstein and the mathematical complexity of his theory. The basic idea was later subtly transformed by Weyl in 1929 into the precursor of the Yang-Mills gauge principle. Weyl's original aspiration of eliminating length was never forgotten and inspired numerous alternative attempts, including one by Dirac, to achieve the same goal. These too failed. I shall argue that in fact Weyl made a mistake in adding extra structure; he should simply have used less of the structure in Riemannian geometry, namely only its conformal part. This is determined solely by angles and is directly related to the observations that can actually be made. An analysis of the true physical degrees of freedom in general relativity in terms of three-dimensional conformal geometry then leads to the surprising conclusion that general relativity achieves everything that Weyl could have asked of it. In its core, lengths play no role. Everything is determined by shapes in the form of conformal geometry. Apart from its intrinsic interest and possible relevance to cosmology, this demonstration could have considerable implications for the programme of creating a quantum theory of gravity. For example, it suggests that in the quantum mechanics of the universe there will be a uniquely defined notion of simultaneity that is still compatible with lack of simultaneity in classical general relativity.

11 mars 2010, 11h, LARSIM
Vasily Ogryzko (INSERM, Institut Gustave Roussy) "Can one use the notion of ground state for the description of a living cell?"
According to a widely held opinion, Life corresponds to a physical state far from equilibrium. Thus, whereas such fundamental notion of equilibrium physics as 'ground state' is widely used to describe the properties of biological macromolecules or even macromolecular complexes, it is considered of no use for the description of a whole living cell. I would like to challenge this preconception, by discussing how the idea of a cell in a ground state is possible, and what could be the nature of the forces responsible for its stability. Strikingly, this line of enquiry leads to a novel justification of the self-organization principle, as the action of the forces responsible for the stability of the ground state amounts to "optimization without natural selection of replicators". Unlike the statistical-mechanical approaches to self-organization, our approach does not encounter the problem of 'tradeoff between stability and complexity' at the level of individual cell.

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