Physique classique | EPFL Graph Search

La physique classique désigne d'une manière générale l'ensemble des théories physiques antérieures à l'avènement de théories plus récentes, plus complètes, ou dotées d'un domaine d'application plus vaste. Lorsqu'une théorie physique qui a cours actuellement est considérée comme moderne, et si son introduction a représenté un majeur, les théories précédentes (ou les théories nouvelles basées sur le paradigme antérieur) seront souvent considérées comme relevant de la physique « classique ». En cela, la définition d'une théorie classique dépend du contexte. Les concepts de la physique classique sont souvent mis en œuvre lorsque les théories modernes sont d'une complexité superflue dans une situation donnée. Le plus souvent, le terme de physique « classique » fait référence à l'ensemble des théories physiques développées jusqu'à la fin du tandis que l'on parle de physique moderne pour les théories postérieures à 1900. Vue d'ensemble La notion de théorie classique possède a

Gravitational causality and the self-stress of photons

Giulia Isabella

We study causality in gravitational systems beyond the classical limit. Using on-shell methods, we consider the 1-loop corrections from charged particles to the photon energy-momentum tensor - the self-stress - that controls the quantum interaction between two on-shell photons and one off-shell graviton. The self-stress determines in turn the phase shift and time delay in the scattering of photons against a spectator particle of any spin in the eikonal regime. We show that the sign of the beta-function associated to the running gauge coupling is related to the sign of time delay at small impact parameter. Our results show that, at first post-Minkowskian order, asymptotic causality, where the time delay experienced by any particle must be positive, is respected quantum mechanically. Contrasted with asymptotic causality, we explore a local notion of causality, where the time delay is longer than the one of gravitons, which is seemingly violated by quantum effects.

2022

Couplage d'une loi d'adhésion à une loi de contact avec frottement pour l'étude de la décohésion dans les matériaux composites

Christian Talon

In this thesis, an original law of adhesion is developed and then coupled to the classical law of unilateral contact with threshold friction, in order to study the phenomenon of fibre/matrix debonding in composite materials. This tribological law is developed within the framework of standard generalized materials adapted to interfaces. Thus, the law is derived from a free energy potential ψc and a dissipation potential Φc. The adhesive interface is interpreted as a bundle of links connecting the two contact surfaces. Each link is assumed to have a plastic behavior with softening and damage, caused by stretching, to represent the loss of adhesion. An internal variable ga measuring an irreversible adhesive gap is introduced and associated by energetic duality to an adhesive stress pa. The ranges of ga and pa are limited by two characteristic constants of adhesion, gM and pM respectively. pM is the maximum adhesive tension below which the links remain elastic. Above this limit, debonding and damage of the links occur as ga increases. Once ga reaches the value gM, total rupture of the adhesive bond occurs. This adhesion law is then mounted in parallel to the contact and friction laws into a unique law. The law is regularized using either the approximate penalty method or the exact augmented Lagrangian method, in option. The regularized laws are implemented in a node-to-node contact element of the finite element code TACT. The adhesive force is computed by means of a predictor-corrector with projection algorithm for integrating the evolution of ga, and the relevant Jacobian tensors required for the resolution of non-linearities by the iterative scheme of Newton are calculated. This is accomplished for the penalty and the augmented Lagrangian regularizations. A traction test of a glass/epoxy interface in the normal direction is designed and used to experimentally determine the parameters pM and gM of such an interface (pM directly and gM indirectly). Numerical simulations of adhesion between a rigid punch and an elastic half-space enable the comparison of the proposed model of adhesion to the classical theories of adhesion. A numerical simulation of the standard pull-out test of a fibre embedded in a matrix is then performed, and the numerical shear distribution along the interface is compared to the analytical one existing for this experiment. The results are in good agreement with existing ones. Finally, a crack propagation in a fibrous composite is simulated.

EPFL2002

Large atoms and quantum dots

Claude Plocek

A functional integration approach – whose main ingredient is the Hubbard-Stratonovich transformation – for the quantum nonrelativistic many-fermion problem is investigated. With this method, the ground state energy correponds to a systematic expansion in powers of a small parameter related to the number of fermions. It is a functional of a potential determined by a self-consistent equation. The semiclassical Hartree energy is obtained at lowest order of the expansion, the exchange energy at first order, and the correlation energy at second order. This approach is applied to large neutral atoms, for which the correlation energy is computed. This approach is also applied to many-electron quantum dots with harmonic confinement. The self-consistent equation is solved as a function of a small parameter depending on the confinement strength. The Hartree and exchange energies are computed in powers of this parameter, and the correlation energy is computed at lowest order. The energy oscillations, arising from the Hartree energy, are also evaluated; they are related to the periodic orbits of the classical dynamics of the self-consistent potential.

EPFL2006

Couplage d'une loi d'adhésion à une loi de contact avec frottement pour l'étude de la décohésion dans les matériaux composites

Christian Talon

EPFL2002