10,880 research outputs found
The art of being human : a project for general philosophy of science
Throughout the medieval and modern periods, in various sacred and secular guises, the unification of all forms of knowledge under the rubric of ‘science’ has been taken as the prerogative of humanity as a species. However, as our sense of species privilege has been called increasingly into question, so too has the very salience of ‘humanity’ and ‘science’ as general categories, let alone ones that might bear some essential relationship to each other. After showing how the ascendant Stanford School in the philosophy of science has contributed to this joint demystification of ‘humanity’ and ‘science’, I proceed on a more positive note to a conceptual framework for making sense of science as the art of being human. My understanding of ‘science’ is indebted to the red thread that runs from Christian theology through the Scientific Revolution and Enlightenment to the Humboldtian revival of the university as the site for the synthesis of knowledge as the culmination of self-development. Especially salient to this idea is science‘s epistemic capacity to manage modality (i.e. to determine the conditions under which possibilities can be actualised) and its political capacity to organize humanity into projects of universal concern. However, the challenge facing such an ideal in the twentyfirst century is that the predicate ‘human’ may be projected in three quite distinct ways, governed by what I call ‘ecological’, ‘biomedical’ and ‘cybernetic’ interests. Which one of these future humanities would claim today’s humans as proper ancestors and could these futures co-habit the same world thus become two important questions that general philosophy of science will need to address in the coming years
Dynamics of swimming bacteria at complex interfaces
Flagellated bacteria exploiting helical propulsion are known to swim along
circular trajectories near surfaces. Fluid dynamics predicts this circular
motion to be clockwise (CW) above a rigid surface (when viewed from inside the
fluid) and counter-clockwise (CCW) below a free surface. Recent experimental
investigations showed that complex physicochemical processes at the nearby
surface could lead to a change in the direction of rotation, both at solid
surfaces absorbing slip-inducing polymers and interfaces covered with
surfactants. Motivated by these results, we use a far-field hydrodynamic model
to predict the kinematics of swimming near three types of interfaces: clean
fluid-fluid interface, slipping rigid wall, and a fluid interface covered by
incompressible surfactants. Representing the helical swimmer by a superposition
of hydrodynamic singularities, we first show that in all cases the surfaces
reorient the swimmer parallel to the surface and attract it, both of which are
a consequence of the Stokes dipole component of the swimmer flow field. We then
show that circular motion is induced by a higher-order singularity, namely a
rotlet dipole, and that its rotation direction (CW vs. CCW) is strongly
affected by the boundary conditions at the interface and the bacteria shape.
Our results suggest thus that the hydrodynamics of complex interfaces provide a
mechanism to selectively stir bacteria
Shearing or Compressing a Soft Glass in 2D: Time-concentration superposition
We report surface shear rheological measurements on dense insoluble
monolayers of micron sized colloidal spheres at the oil/water interface and of
the protein -lactoglobulin at the air/water surface. As expected, the
elastic modulus shows a changing character in the response, from a viscous
liquid towards an elastic solid as the concentration is increased, and a change
from elastic to viscous as the shear frequency is increased. Surprisingly,
above a critical packing fraction, the complex elastic modulus curves measured
at different concentrations can be superposed to form a master curve, by
rescaling the frequency and the magnitude of the modulus. This provides a
powerful tool for the extrapolation of the material response function outside
the experimentally accessible frequency range. The results are discussed in
relation to recent experiments on bulk systems, and indicate that these two
dimensional monolayers should be regarded as being close to a soft glass state.Comment: to appear in PR
Neutrino-Neutrino Scattering and Matter-Enhanced Neutrino Flavor Transformation in Supernovae
We examine matter-enhanced neutrino flavor transformation
() in the region above the neutrino
sphere in Type II supernovae. Our treatment explicitly includes contributions
to the neutrino-propagation Hamiltonian from neutrino-neutrino forward
scattering. A proper inclusion of these contributions shows that they have a
completely negligible effect on the range of - vacuum
mass-squared difference, , and vacuum mixing angle, , or
equivalently , required for enhanced supernova shock re-heating.
When neutrino background effects are included, we find that -process
nucleosynthesis from neutrino-heated supernova ejecta remains a sensitive probe
of the mixing between a light and a with a
cosmologically significant mass. Neutrino-neutrino scattering contributions are
found to have a generally small effect on the
parameter region probed by -process nucleosynthesis. We point out that the
nonlinear effects of the neutrino background extend the range of sensitivity of
-process nucleosynthesis to smaller values of .Comment: 38 pages, tex, DOE/ER/40561-150-INT94-00-6
Domestication as innovation : the entanglement of techniques, technology and chance in the domestication of cereal crops
The origins of agriculture involved pathways of domestication in which human behaviours and plant genetic adaptations were entangled. These changes resulted in consequences that were unintended at the start of the process. This paper highlights some of the key innovations in human behaviours, such as soil preparation, harvesting and threshing, and how these were coupled with genetic ‘innovations’ within plant populations. We identify a number of ‘traps’ for early cultivators, including the needs for extra labour expenditure on crop-processing and soil fertility maintenance, but also linked gains in terms of potential crop yields. Compilations of quantitative data across a few different crops for the traits of nonshattering and seed size are discussed in terms of the apparently slow process of domestication, and parallels and differences between different regional pathways are identified. We highlight the need to bridge the gap between a Neolithic archaeobotanical focus on domestication and a focus of later periods on crop-processing activities and labour organization. In addition, archaeobotanical data provide a basis for rethinking previous assumptions about how plant genetic data should be related to the origins of agriculture and we contrast two alternative hypotheses: gradual evolution with low selection pressure versus metastable equilibrium that prolonged the persistence of ‘semi-domesticated’ populations. Our revised understanding of the innovations involved in plant domestication highlight the need for new approaches to collecting, modelling and integrating genetic data and archaeobotanical evidence
Vision loss associated with the use and removal of intraocular silicone oil
Patrick D Williams1, Christopher G Fuller1, Ingrid U Scott2, Dwain G Fuller1, Harry W Flynn Jr31Texas Retina Associates, Dallas, TX, USA; 2Departments of Ophthalmology and Health Evaluation Sciences, Penn State College of Medicine, Hershey, PA, USA; 3Department of Ophthalmology, Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Miami, FL, USAPurpose: To describe vision loss associated with the use or removal of silicone oil retinal tamponade.Methods: Records were reviewed of all patients with a decrease in visual acuity of at least 3 Snellen lines from best acuity with 5000 centistoke silicone oil in place or after removal of silicone oil at a single retina-only practice between 1996 and 2006.Results: Nine patients (6 men, 3 women) with a mean age of 48 years (range, 16–61) met study inclusion criteria. Seven patients lost at least three Snellen lines of vision while the silicone oil was in place. Four patients had late modest improvements in acuity when compared to their final recorded Snellen vision before silicone oil removal, however no patients exhibited visual improvement when comparing their final recorded visual acuities after oil removal with best recorded acuities under oil tamponade. Loss of the foveal depression was a consistent feature on optical coherence tomography.Conclusions: Vision loss is a possible complication of silicone oil use and removal. Late visual improvement may occur in some patients. Further research is warranted to elucidate the mechanism(s) of vision loss associated with the use or removal of silicone oil.Keywords: retinal tamponade, visual acuity, snellen vision, silicone oi
A novel method of supplying nutrients permits predictable shoot growth and root: shoot ratios of pre-transplant bedding plants
BACKGROUND AND AIMS: Growth of bedding plants, in small peat plugs, relies on nutrients in the irrigation solution. The object of the study was to find a way of modifying the nutrient supply so that good-quality seedlings can be grown rapidly and yet have the high root : shoot ratios essential for efficient transplanting.
METHODS: A new procedure was devised in which the concentrations of nutrients in the irrigation solution were modified during growth according to changing plant demand, instead of maintaining the same concentrations throughout growth. The new procedure depends on published algorithms for the dependence of growth rate and optimal plant nutrient concentrations on shoot dry weight Ws (g m–2), and on measuring evapotranspiration rates and shoot dry weights at weekly intervals. Pansy, Viola tricola ‘Universal plus yellow’ and petunia, Petunia hybrida ‘Multiflora light salmon vein’ were grown in four independent experiments with the expected optimum nutrient concentration and fractions of the optimum. Root and shoot weights were measured during growth.
KEY RESULTS: For each level of nutrient supply Ws increased with time (t) in days, according to the equation {Delta}Ws/{Delta}t=K2Ws/(100+Ws) in which the growth rate coefficient (K2) remained approximately constant throughout growth. The value of K2 for the optimum treatment was defined by incoming radiation and temperature. The value of K2 for each sub-optimum treatment relative to that for the optimum treatment was logarithmically related to the sub-optimal nutrient supply. Provided the aerial environment was optimal, Rsb/Ro{approx}Wo/Wsb where R is the root : shoot ratio, W is the shoot dry weight, and sb and o indicate sub-optimum and optimum nutrient supplies, respectively. Sub-optimal nutrient concentrations also depressed shoot growth without appreciably affecting root growth when the aerial environment was non-limiting.
CONCLUSION: The new procedure can predict the effects of nutrient supply, incoming radiation and temperature on the time course of shoot growth and the root : shoot ratio for a range of growing conditions
Tests of star formation metrics in the low metallicity galaxy NGC 5253 using ALMA observations of H30 line emission
We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of
H30 (231.90 GHz) emission from the low metallicity dwarf galaxy NGC
5253 to measure the star formation rate (SFR) within the galaxy and to test the
reliability of SFRs derived from other commonly-used metrics. The H30
emission, which originates mainly from the central starburst, yields a
photoionizing photon production rate of (1.90.3)10 s
and an SFR of 0.0870.013 M yr based on conversions that
account for the low metallicity of the galaxy and for stellar rotation. Among
the other star formation metrics we examined, the SFR calculated from the total
infrared flux was statistically equivalent to the values from the H30
data. The SFR based on previously-published versions of the H flux that
were extinction corrected using Pa and Pa lines were lower than
but also statistically similar to the H30 value. The mid-infrared (22
m) flux density and the composite star formation tracer based on H
and mid-infrared emission give SFRs that were significantly higher because the
dust emission appears unusually hot compared to typical spiral galaxies.
Conversely, the 70 and 160 m flux densities yielded SFR lower than the
H30 value, although the SFRs from the 70 m and H30 data
were within 1-2 of each other. While further analysis on a broader
range of galaxies are needed, these results are instructive of the best and
worst methods to use when measuring SFR in low metallicity dwarf galaxies like
NGC 5253.Comment: 14 pages, 5 figures, accepted for publication in MNRA
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
Bulk Viscosity, Decaying Dark Matter, and the Cosmic Acceleration
We discuss a cosmology in which cold dark-matter particles decay into
relativistic particles. We argue that such decays could lead naturally to a
bulk viscosity in the cosmic fluid. For decay lifetimes comparable to the
present hubble age, this bulk viscosity enters the cosmic energy equation as an
effective negative pressure. We investigate whether this negative pressure is
of sufficient magnitude to account fo the observed cosmic acceleration. We show
that a single decaying species in a flat, dark-matter dominated cosmology
without a cosmological constant cannot reproduce the observed
magnitude-redshift relation from Type Ia supernovae. However, a delayed bulk
viscosity, possibly due to a cascade of decaying particles may be able to
account for a significant fraction of the apparent cosmic acceleration.
Possible candidate nonrelativistic particles for this scenario include sterile
neutrinos or gauge-mediated decaying supersymmetric particles.Comment: 7 pages, 4 figure
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