50 research outputs found
Frequency shift of hyperfine transitions due to blackbody radiation
We have performed calculations of the size of the frequency shift induced by
a static electric field on the clock transition frequencies of the hyperfine
splitting in Yb+, Rb, Cs, Ba+, and Hg+. The calculations are used to find the
frequency shifts due to blackbody radiation which are needed for accurate
frequency measurements and improvements of the limits on variation of the fine
structure constant, alpha. Our result for Cs (delta nu E^2=-2.26 times
10^{-10}Hz/(V/m)^2) is in good agreement with early measurements and ab initio
calculations. We present arguments against recent claims that the actual value
might be smaller. The difference (approx 10%) is due to the contribution of the
continuum spectrum in the sum over intermediate states.Comment: Added discussion of Cs results and reference
Limits on a Dynamically Varying Fine Structure Constant
We show that using the modified form of the Dirac Hamiltonian as suggested by
Bekenstein, does not affect the analysis in [1-4] of QSO data pertaining to a
measurement of alpha variation. We obtain the present time limit on
Bekenstein's parameter, tan^{2}chi =(0.2 \pm 0.7)\times10^{-6}, from the
measurement of the hydrogen 2p fine structure using value of alpha obtained
from different experiments.Comment: references added, typos correcte
Reaction-diffusion and reaction-subdiffusion equations on arbitrarily evolving domains
Reaction-diffusion equations are widely used as the governing evolution
equations for modeling many physical, chemical, and biological processes. Here
we derive reaction-diffusion equations to model transport with reactions on a
one-dimensional domain that is evolving. The model equations, which have been
derived from generalized continuous time random walks, can incorporate
complexities such as subdiffusive transport and inhomogeneous domain stretching
and shrinking. A method for constructing analytic expressions for short time
moments of the position of the particles is developed and moments calculated
from this approach are shown to compare favourably with results from random
walk simulations and numerical integration of the reaction transport equation.
The results show the important role played by the initial condition. In
particular, it strongly affects the time dependence of the moments in the short
time regime by introducing additional drift and diffusion terms. We also
discuss how our reaction transport equation could be applied to study the
spreading of a population on an evolving interface.Comment: 38 pages, 10 figure
A New Option for a Search for Alpha Variation: Narrow Transitions with Enhanced Sensitivity
We consider several transitions between narrow lines that have an enhanced
sensitivity to a possible variation of the fine structure constant, alpha. This
enhancement may allow a search to be performed with an effective suppression of
the systematic sources of uncertainty that are unavoidable in conventional
high-resolution spectroscopic measurements. In the future this may provide the
strongest laboratory constraints on alpha variation
Coulomb energy contribution to the excitation energy in Th and enhanced effect of variation
We calculated the contribution of Coulomb energy to the spacing between the
ground and first excited state of Th nucleus as a function of the
deformation parameter . We show that despite the fact that the odd
particle is a neutron, the change in Coulomb energy between these two states
can reach several hundreds KeV.This means that the effect of the variation of
the fine structure constant may be enhanced
times in the 7.6 eV "nuclear clock" transition
between the ground and first excited states in the Th nucleus.Comment: 6 pages,2 figure
Placenta previa percreta left in situ - management by delayed hysterectomy: a case report
<p>Abstract</p> <p>Introduction</p> <p>Placenta percreta is an obstetric emergency often associated with massive hemorrhage and emergency hysterectomy.</p> <p>Case presentation</p> <p>We present the case of a 30-year-old African woman, gravida 7, para 5, with placenta percreta managed by an alternative approach: the placenta was left <it>in situ</it>, methotrexate was administered, and a delayed hysterectomy was successfully performed.</p> <p>Conclusions</p> <p>Further studies are needed to develop the most appropriate management option for the most severe cases of abnormal placentation. Delayed hysterectomy may be a reasonable strategy in the most severe cases.</p
Cold atom Clocks and Applications
This paper describes advances in microwave frequency standards using
laser-cooled atoms at BNM-SYRTE. First, recent improvements of the Cs
and Rb atomic fountains are described. Thanks to the routine use of a
cryogenic sapphire oscillator as an ultra-stable local frequency reference, a
fountain frequency instability of where
is the measurement time in seconds is measured. The second advance is a
powerful method to control the frequency shift due to cold collisions. These
two advances lead to a frequency stability of at 7\times 10^{-16}^{87}^{133}$Cs fountains.
Finally we give an update on the cold atom space clock PHARAO developed in
collaboration with CNES. This clock is one of the main instruments of the
ACES/ESA mission which is scheduled to fly on board the International Space
Station in 2008, enabling a new generation of relativity tests.Comment: 30 pages, 11 figure
Varying constants, Gravitation and Cosmology
Fundamental constants are a cornerstone of our physical laws. Any constant
varying in space and/or time would reflect the existence of an almost massless
field that couples to matter. This will induce a violation of the universality
of free fall. It is thus of utmost importance for our understanding of gravity
and of the domain of validity of general relativity to test for their
constancy. We thus detail the relations between the constants, the tests of the
local position invariance and of the universality of free fall. We then review
the main experimental and observational constraints that have been obtained
from atomic clocks, the Oklo phenomenon, Solar system observations, meteorites
dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic
microwave background and big bang nucleosynthesis. At each step we describe the
basics of each system, its dependence with respect to the constants, the known
systematic effects and the most recent constraints that have been obtained. We
then describe the main theoretical frameworks in which the low-energy constants
may actually be varying and we focus on the unification mechanisms and the
relations between the variation of different constants. To finish, we discuss
the more speculative possibility of understanding their numerical values and
the apparent fine-tuning that they confront us with.Comment: 145 pages, 10 figures, Review for Living Reviews in Relativit