« La signification fondamentale des méthodes d’approximation en physique théorique » : Vladimir Fock épistémologue

Vladimir Fock (1898-1974) est un scientifique russo-soviétique connu pour diverses contributions à la physique moderne. Il en fut aussi un interprète, développant une position critique face à l’orthodoxie incarnée par Niels Bohr en mécanique quantique et Albert Einstein en relativité générale. Fock ayant adhéré au matérialisme dialectique, l’historiographie sur ses contributions aux débats d’interprétation met généralement l’accent sur sa défense d’une position réaliste, en faveur de l’objectivité du monde extérieur. Le présent article complète cette observation en s’appuyant sur un texte du physicien soviétique jusqu’alors peu connu : « La signification fondamentale des méthodes d’approximation en physique théorique ». En effet, il révèle une forme d’antiréductionnisme essentielle à la compréhension du discours de Fock sur l’interprétation des théories de la physique moderne.Vladimir Fock (1898-1974) was a Russian-Soviet scientist renowned for his several contributions to modern physics. He was also an interpreter and articulated a critical stance towards the orthodoxy embodied by Niels Bohr in quantum mechanics and Albert Einstein in general relativity. As Fock embraced dialectical materialism, historiography on his contributions to the interpretive debates generally emphasizes his defense of a realistic position, in favor of the objectivity of the outside world. The present article complements this view by drawing on a text by the Soviet physicist so far little known : “The fundamental significance of approximate methods in theoretical physics”. Indeed, it reveals a form of antireductionism essential to the understanding of Fock’s discourse on the interpretation of the theories of modern physics

Distribution of the organic matter in the channel-levees systems of the Congo mud-rich deep-sea fan (West Africa). Implication for deep offshore petroleum source rocks and global carbon cycle

International audienceThe Corinth Rift is superimposed on the Hellenic nappe stack that formed at the expense of the Apulian continental crust above the subducting African slab. Extension started in the Pliocene and the major steep normal faults that control the geometry of the present-day rift were born very recently, some 600 kyr ago only. They root into a shallow-dipping zone of microseismicity recorded near the base of the upper crust. The significance of this seismogenic zone is debated. Considering the northward dip of the zone of microseismicity, the depth of microearthquakes and their focal mechanisms, we observe a strong similarity with the northern Cycladic detachments in terms of expected pressure, temperature conditions and kinematics. We herein show (1) that the formation of the Corinth Rift can be considered a part of a continuum of extension that started some 30–35 Ma in the Aegean and that was recently localised in a more restricted area, (2) that the present-day structure and kinematics of the Corinth Rift can be explained with a series of decollements relayed by steeper ramps that altogether formed a mechanically weak, crustal-scale detachment, and (3) that the deformation, fluid behaviour and metamorphic features seen in the northern Cycladic metamorphic core complexes can be good analogues of the processes at work below the Corinth Rift

ETUDE DES SÉDIMENTS COTIERS DU CAP CORSE: RECONSTRUCTION PALÉOENVIRONNEMENTALE ET SUIVI DE LA CONTAMINATION EN ÉLÉMENTS TRACES MÉTALLIQUES AU COURS DE LA PÉRIODE HISTORIQUE

Les sédiments sont d’excellentes archives de la contamination de notre environnement. Depuis l’Antiquité, les activités humaines génèrent des aérosols anthropiques enrichis en métaux lourds (e.g. Pb, Zn, Cu, Cd, Hg) ou associés (As, Sb). Ces polluants métalliques se mélangent aux aérosols naturels (altération des roches) et leurs retombées sont incorporées dans les sols et les sédiments où ils sont préservés (Boyd, 2004). Sur des sédiments datés, une approche géochimique permet de quantifier l’apport anthropique en éléments métalliques par rapport aux sources naturelles et de reconstituer l’historique de la pollution. Les côtes méditerranéennes sont caractérisées par d’intenses échanges commerciaux, et ce depuis l’Antiquité. Nous proposons une reconstruction paléoenvironnementale ainsi qu’un suivi de la contamination anthropique au cours de la période historique pour différents sites du Cap Corse (France ; Figure 1). Cette région de Méditerranée se caractérise par une riche activité économique au cours de la période historique. Depuis le VIème siècle, la Corse a été colonisée successivement par les Grecs, les Carthaginois, les Etrusques puis par les Romains. En milieu continental, des témoins archéologiques (sépulture, oppidum, chapelle) attestent d’une occupation de cette zone sur une période assez longue. Cependant de nombreuses interrogations subsistent quant à l’importance des établissements, leurs périodes d’occupation, leurs activités économiques et leurs rapports commerciaux avec les autres cités du Cap Corse et des côtes méditerranéennes (de La Brière, 2010). Des carottes de 1 m à 1,5 m ont été prélevées à la tarière dans différents sites du Cap Corse. Les concentrations en éléments majeurs et en éléments traces métalliques ont été mesurées par spectrométrie de masse (ICP-MS), activation neutronique (INAA) et par XRF core scanner. Les datations 14C réalisées sur des macrorestes de matières végétales et charbons montrent que ces carottes couvrent toute la période historique, à l’exception de celle prélevée à Méria qui ne couvre qu’environ 300 ans. Cette carotte a également la particularité de contenir d’importantes concentrations en métaux lourds tels que le Sb (2000 ppm) et l’As (300 ppm). Cette contamination est d’origine locale et liée à la présence d’une ancienne mine d’antimoine à 2 km en amont de Méria. Afin d’obtenir un enregistrement sur les 2000 dernières années, un carottage plus long sera réalisé lors d’une nouvelle campagne de terrain. D’autres carottages seront également réalisés entre la mine et la côte actuelle afin d’observer la variation spatiale dans le but de quantifier l’impact environnemental des activités humaines locales et régionales

The Insula and Its Epilepsies

Insular seizures are great mimickers of seizures originating elsewhere in the brain. The insula is a highly connected brain structure. Seizures may only become clinically evident after ictal activity propagates out of the insula with semiology that reflects the propagation pattern. Insular seizures with perisylvian spread, for example, manifest first as throat constriction, followed next by perioral and hemisensory symptoms, and then by unilateral motor symptoms. On the other hand, insular seizures may spread instead to the temporal and frontal lobes and present like seizures originating from these regions. Due to the location of the insula deep in the brain, interictal and ictal scalp electroencephalogram (EEG) changes can be variable and misleading. Magnetic reso- nance imaging, magnetic resonance spectroscopy, magnetoencephalography, positron emission tomography, and single-photon computed tomography imaging may assist in establishing a diagnosis of insular epilepsy. Intracranial EEG recordings from within the insula, using stereo-EEG or depth electrode techniques, can prove insular seizure origin. Seizure onset, most commonly seen as low-voltage, fast gamma activity, however, can be highly localized and easily missed if the insula is only sparsely sampled. Moreover, seizure spread to the contralateral insula and other brain regions may occur rapidly. Extensive sampling of the insula with multiple electrode trajectories is necessary to avoid these pitfalls. Understanding the functional organization of the insula is helpful when interpreting the semiology produced by insular seizures. Electrical stimulation mapping around the central sulcus of the insula results in paresthesias, while stimulation of the posterior insula typically produces painful sensations. Visceral sensations are the next most common result of insular stimulation. Treatment of insular epilepsy is evolving, but poses challenges. Surgical resections of the insula are effective but risk significant morbidity if not carefully planned. Neurostimulation is an emerging option for treatment, especially for seizures with onset in the posterior insula. The close association of the insula with marked autonomic changes has led to interest in the role of the insula in sudden unexpected death in epilepsy and warrants additional study with larger patient cohorts

Elementary Excitation Modes in a Granular Glass above Jamming

The dynamics of granular media in the jammed, glassy region is described in terms of "modes", by applying a Principal Component Analysis (PCA) to the covariance matrix of the position of individual grains. We first demonstrate that this description is justified and gives sensible results in a regime of time/densities such that a metastable state can be observed on long enough timescale to define the reference configuration. For small enough times/system sizes, or at high enough packing fractions, the spectral properties of the covariance matrix reveals large, collective fluctuation modes that cannot be explained by a Random Matrix benchmark where these correlations are discarded. We then present a first attempt to find a link between the softest modes of the covariance matrix during a certain "quiet" time interval and the spatial structure of the rearrangement event that ends this quiet period. The motion during these cracks is indeed well explained by the soft modes of the dynamics before the crack, but the number of cracks preceded by a "quiet" period strongly reduces when the system unjams, questioning the relevance of a description in terms of modes close to the jamming transition, at least for frictional grains.Comment: 11 pages, 10 figure

Towards a monolithic optical cavity for atom detection and manipulation

We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs substrate. We experimentally determined the propagation losses in the waveguide at 780 nm, the wavelength of Rb atoms. We have also made a numerical and analytical estimate of the losses induced by the presence of the gap which would allow the interaction of cold atoms with the cavity field. We found that the intrinsic finesse of the gapped cavity can be on the order of F ~ 30, which, when one takes into account the losses due to mirror transmission, corresponds to a cooperativity parameter for our system C ~ 1

Modelling the signal delivered by a population of first-order neurons in a moth olfactory system

A statistical model of the population of first-order olfactory receptor neurons (ORNs) is proposed and analysed. It describes the relationship between stimulus intensity (odour concentration) and coding variables such as rate and latency of the population of several thousand sex-pheromone sensitive ORNs in male moths. Although these neurons likely express the same olfactory receptor, they exhibit, at any concentration, a relatively large heterogeneity of responses in both peak firing frequency and latency of the first action potential fired after stimulus onset. The stochastic model is defined by a multivariate distribution of six model parameters that describe the dependence of the peak firing rate and the latency on the stimulus dose. These six parameters and their mutual linear correlations were estimated from experiments in single ORNs and included in the multidimensional model distribution. The model is utilized to reconstruct the peak firing rate and latency of the message sent to the brain by the whole ORN population at different stimulus intensities and to establish their main qualitative and quantitative properties. Finally, these properties are shown to be in agreement with those found previously in a vertebrate ORN population