1,202 research outputs found
Entwined Paths, Difference Equations and the Dirac Equation
Entwined space-time paths are bound pairs of trajectories which are traversed
in opposite directions with respect to macroscopic time. In this paper we show
that ensembles of entwined paths on a discrete space-time lattice are simply
described by coupled difference equations which are discrete versions of the
Dirac equation. There is no analytic continuation, explicit or forced, involved
in this description. The entwined paths are `self-quantizing'. We also show
that simple classical stochastic processes that generate the difference
equations as ensemble averages are stable numerically and converge at a rate
governed by the details of the stochastic process. This result establishes the
Dirac equation in one dimension as a phenomenological equation describing an
underlying classical stochastic process in the same sense that the Diffusion
and Telegraph equations are phenomenological descriptions of stochastic
processes.Comment: 15 pages, 5 figures Replacement 11/02 contains minor editorial
change
Is level of neighbourhood green space associated with physical activity in green space?
Background
There is accumulating evidence that greater availability of green space in a neighbourhood is associated with health benefits for the local population. One mechanism proposed for this association is that green space provides a venue for, and therefore encourages, physical activity. It has also been suggested that socio-economic health inequalities may be narrower in greener areas because of the equalised opportunity for physical activity green spaces provide. However, research exploring associations between the availability of green space and physical activity has produced mixed results. Limits to the assessment of the type and amount of physical activity which occurs specifically in green space may account for these mixed findings. This observational study was therefore concerned with the extent to which green space is a venue for physical activity and whether this could account for narrower socio-economic health inequalities in greener neighbourhoods.<p></p>
Method
Secondary analysis of cross sectional data on 3679 adults (16+) living in urban areas across Scotland matched with a neighbourhood level measure of green space availability. Associations between green space availability and both total physical activity, and activity specifically within green space, were explored using logistic regression models. Interactions between socio-economic position and physical activity were assessed. All models adjusted for age, sex and household income.<p></p>
Results
The availability of green space in a neighbourhood was not associated with total physical activity or that specifically in green space. There was no evidence that income-related inequalities in physical activity within green space were narrower in greener areas of Scotland.<p></p>
Conclusion
Physical activity may not be the main mechanism explaining the association between green space and health in Scotland. The direct effect of perceiving a natural environment on physiological and psychological health may offer an alternative explanation.<p></p>
A Study of Giant Pulses from PSR J1824-2452A
We have searched for microsecond bursts of emission from millisecond pulsars
in the globular cluster M28 using the Parkes radio telescope. We detected a
total of 27 giant pulses from the known emitter PSR J1824-2452A. At wavelengths
around 20 cm the giant pulses are scatter-broadened to widths of around 2
microseconds and follow power-law statistics. The pulses occur in two narrow
phase-windows which correlate in phase with X-ray emission and trail the peaks
of the integrated radio pulse-components. Notably, the integrated radio
emission at these phase windows has a steeper spectral index than other
emission. The giant pulses exhibit a high degree of polarization, with many
being 100% elliptically polarized. Their position angles appear random.
Although the integrated emission of PSR J1824-2452A is relatively stable for
the frequencies and bandwidths observed, the intensities of individual giant
pulses vary considerably across our bands. Two pulses were detected at both
2700 and 3500 MHz. The narrower of the two pulses is 20 ns wide at 3500 MHz. At
2700 MHz this pulse has an inferred brightness temperature at maximum of 5 x
10^37 K. Our observations suggest the giant pulses of PSR J1824-2452A are
generated in the same part of the magnetosphere as X-ray emission through a
different emission process to that of ordinary pulses.Comment: Accepted by Ap
Challenges in the development of the orbiter atmosphere revitalization subsystem
The space shuttle orbiter atmospheric revitalization subsystem provides thermal and contaminant control as well as total- and oxygen partial-pressure control of the environment within the orbiter crew cabin. Challenges that occurred during the development of this subsystem for the space shuttle orbiter are described. The design of the rotating hardware elements of the system (pumps, fans, etc.) required significant development to meet the requirements of long service life, maintainability, and high cycle-fatigue life. As a result, a stringent development program, particularly in the areas of bearing life and heat dissipation, was required. Another area requiring significant development was cabin humidity control and condensate collection
Quantum-classical transition in Scale Relativity
The theory of scale relativity provides a new insight into the origin of
fundamental laws in physics. Its application to microphysics allows us to
recover quantum mechanics as mechanics on a non-differentiable (fractal)
spacetime. The Schrodinger and Klein-Gordon equations are demonstrated as
geodesic equations in this framework. A development of the intrinsic properties
of this theory, using the mathematical tool of Hamilton's bi-quaternions, leads
us to a derivation of the Dirac equation within the scale-relativity paradigm.
The complex form of the wavefunction in the Schrodinger and Klein-Gordon
equations follows from the non-differentiability of the geometry, since it
involves a breaking of the invariance under the reflection symmetry on the
(proper) time differential element (ds - ds). This mechanism is generalized
for obtaining the bi-quaternionic nature of the Dirac spinor by adding a
further symmetry breaking due to non-differentiability, namely the differential
coordinate reflection symmetry (dx^mu - dx^mu) and by requiring invariance
under parity and time inversion. The Pauli equation is recovered as a
non-relativistic-motion approximation of the Dirac equation.Comment: 28 pages, no figur
The Scintillation Velocity of the Relativistic Binary Pulsar PSR J1141-6545
We report a dramatic orbital modulation in the scintillation timescale of the
relativistic binary pulsar J1141--6545 that both confirms the validity of the
scintillation speed methodology and enables us to derive important physical
parameters. We have determined the space velocity, the orbital inclination and
even the longitude of periastron of the binary system, which we find to be in
good agreement with that obtained from pulse timing measurements. Our data
permit two equally-significant physical interpretations of the system. The
system is either an edge-on binary with a high space velocity ( km
s) or is more face-on with a much slower velocity ( km
s). We favor the former, as it is more consistent with pulse timing and
the distribution of known neutron star masses. Under this assumption, the
runaway velocity of 115 km s is much greater than is expected if pulsars
do not receive a natal kick at birth. The derived inclination of the binary
system is (76\pm 2.5^{\circ}) degrees, implying a companion mass of 1.01 (\pm
)~0.02 M(_{\odot}) and a pulsar mass of 1.29 (\pm)~0.02 M(_{\odot}). Our
derived physical parameters indicate that this pulsar should prove to be an
excellent laboratory for tests of gravitational wave emission.Comment: Minor text and figure changes and corrections following referee's
Comments. 14 pages, 3 figures, accepted for publication in Ap
Paternal effects in a wild-type zebrafish implicate a role of sperm-derived small RNAs
While the importance of maternal effects has long been appreciated, a growing body
of evidence now points to the paternal environment having an important influence
on offspring phenotype. Indeed, research on rodent models suggests that paternal
stress leaves an imprint on the behaviour and physiology of offspring via nonge netic information carried in the spermatozoa; however, fish have been understudied
with regard to these sperm-mediated effects. Here, we investigated whether the ze brafish was subjected to heritable influences of paternal stress by exposing males to
stressors (conspecific-derived alarm cue, chasing and bright light) before mating and
assessing the behavioural and endocrine responses of their offspring, including their
behavioural response to conspecific-derived alarm cue. We found that after males are
exposed to stress, their larval offspring show weakened responses to stressors. Small
RNA sequencing subsequently revealed that the levels of several small noncoding
RNAs, including microRNAs, PIWI-interacting RNAs and tRNA-derived small RNAs,
were altered in the spermatozoa of stressed fathers, suggesting that stress-induced
alterations to the spermatozoal RNA landscape may contribute to shaping offspring
phenotype. The work demonstrates that paternal stress should not be overlooked as
a source of phenotypic variation and that spermatozoal small RNAs may be important
intergenerational messengers in fish
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