Hermitian Dirac Hamiltonian in time dependent gravitational field

It is shown by a straightforward argument that the Hamiltonian generating the time evolution of the Dirac wave function in relativistic quantum mechanics is not hermitian with respect to the covariantly defined inner product whenever the background metric is time dependent. An alternative, hermitian, Hamiltonian is found and is shown to be directly related to the canonical field Hamiltonian used in quantum field theory.Comment: 9 pages, final version, to appear in Class. Quant. Gra

Canonical and gravitational stress-energy tensors

It is dealt with the question, under which circumstances the canonical Noether stress-energy tensor is equivalent to the gravitational (Hilbert) tensor for general matter fields under the influence of gravity. In the framework of general relativity, the full equivalence is established for matter fields that do not couple to the metric derivatives. Spinor fields are included into our analysis by reformulating general relativity in terms of tetrad fields, and the case of Poincare gauge theory, with an additional, independent Lorentz connection, is also investigated. Special attention is given to the flat limit, focusing on the expressions for the matter field energy (Hamiltonian). The Dirac-Maxwell system is investigated in detail, with special care given to the separation of free (kinetic) and interaction (or potential) energy. Moreover, the stress-energy tensor of the gravitational field itself is briefly discussed.Comment: final version, to appear in Int. J. Mod. Phys.

Non-uniqueness of the Dirac theory in a curved spacetime

We summarize a recent work on the subject title. The Dirac equation in a curved spacetime depends on a field of coefficients (essentially the Dirac matrices), for which a continuum of different choices are possible. We study the conditions under which a change of the coefficient fields leads to an equivalent Hamiltonian operator H, or to an equivalent energy operator E. In this paper, we focus on the standard version of the gravitational Dirac equation, but the non-uniqueness applies also to our alternative versions. We find that the changes which lead to an equivalent operator H, or respectively to an equivalent operator E, are determined by initial data, or respectively have to make some point-dependent antihermitian matrix vanish. Thus, the vast majority of the possible coefficient changes lead neither to an equivalent operator H, nor to an equivalent operator E, whence a lack of uniqueness. We show that even the Dirac energy spectrum is not unique.Comment: 13 pages (standard 12pt article format). Text of a talk given at the 1st Mediterranean Conference on Classical and Quantum Gravity, Kolymbari (Greece), Sept. 14-18, 200

Les croyances et perceptions de femmes atteintes d’hypertension artérielle (HTA) : une étude phénoménologique

Résumé Introduction L’hypertension artérielle (HTA) est le facteur de risque le plus commun pour le développement des maladies cardiovasculaires. La prévalence globale de l’HTA est aujourd’hui presque similaire entre les deux sexes, soit près de 20 % de la population canadienne. Alors que les lignes directrices sont sans distinction quant au genre, plusieurs particularités modulent la trajectoire de la maladie chez les femmes atteintes d’HTA. Objectif de l’étude L’étude vise à présenter l’expérience de femmes atteintes d’HTA. Méthode Guidés par une approche phénoménologique, des entretiens semi-structurés auprès de 15 femmes atteintes d’HTA ont été menés afin d’explorer leurs perceptions et leurs croyances. Principaux résultats Deux thèmes émergent des entretiens : la perspective salutogène de l’HTA et l’influence du soutien émotionnel dans la gestion de l’HTA. Ces résultats apportent une meilleure compréhension du vécu des femmes atteintes d’HTA, qui permettra aux infirmières de mieux les accompagner dans la gestion de cette maladie. Summary Introduction High blood pressure (HBP) is the most common risk factor for the development of cardiovascular disease. The overall prevalence of hypertension, nearly 20% of the Canadian population, is now almost equal between men and women. While guidelines do not distinguish for sex, there are several factors that modify the trajectory of women living with hypertension. Objective of the study The purpose of this study is to explore women's HBP experience. Method Using a phenomenological design, semi-structured interviews with 15 women living with hypertension were conducted to explore the perceptions and beliefs of these women. Discussion Two themes emerged from the participants’ discourses: the salutogenic perspective of HBP and the influence of emotional support in the management of HBP. These results lead to a better understanding of the experiences of women living with HBP, further enabling nurses to provide better support in the management of this disease

Shot Noise in Digital Holography

We discuss on noise in heterodyne holography in an off-axis configuration. We show that, for a weak signal, the noise is dominated by the shot noise on the reference beam. This noise corresponds to an equivalent noise on the signal beam of one photoelectron per pixel, for the whole sequence of images used to build the digital hologram

Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (<i>Porites</i>)

The "δ¹¹B-pH" technique was applied to modern and ancient corals <i>Porites</i> from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector – Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25&permil;, allowing a precision of about &plusmn;0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient K_<i>D</i> for different aragonite species. Modern coral δ¹¹B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50&plusmn;0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous <i>p</i>CO₂ to be 440&plusmn;25 ppmV at around 11.5 kyr BP for corals at Marquesas and ~500 ppmV near Tahiti where it was assumed that <i>p</i>CO₂ in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in <i>p</i>CO₂ between the ocean and the atmosphere at Marquesas (Δ<i>p</i>CO₂) indicates that the surface waters behave as a moderate CO₂ sink or source (−53 to 20 ppmV) during El Niño-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO₂ (21 to 92 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO₂ released with Δ<i>p</i>CO₂ of +185&plusmn;25 ppmV. This last finding provides further evidence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO₂ content