1,544 research outputs found
On the Existence of Spacetime Structure
I examine the debate between substantivalists and relationalists about the
ontological character of spacetime and conclude it is not well posed. I argue
that the so-called Hole Argument does not bear on the debate, because it
provides no clear criterion to distinguish the positions. I propose two such
precise criteria and construct separate arguments based on each to yield
contrary conclusions, one supportive of something like relationalism and the
other of something like substantivalism. The lesson is that one must fix an
investigative context in order to make such criteria precise, but different
investigative contexts yield inconsistent results. I examine questions of
existence about spacetime structures other than the spacetime manifold itself
to argue that it is more fruitful to focus on pragmatic issues of physicality,
a notion that lends itself to several different explications, all of
philosophical interest, none privileged a priori over any of the others. I
conclude by suggesting an extension of the lessons of my arguments to the
broader debate between realists and instrumentalists.Comment: 42 pages, 2 figures, forthcoming (2015) in British Journal for
Philosophy of Scienc
Fourier transform spectroscopy of a spin-orbit coupled Bose gas
We describe a Fourier transform spectroscopy technique for directly measuring
band structures, and apply it to a spin-1 spin-orbit coupled Bose-Einstein
condensate. In our technique, we suddenly change the Hamiltonian of the system
by adding a spin-orbit coupling interaction and measure populations in
different spin states during the subsequent unitary evolution. We then
reconstruct the spin and momentum resolved spectrum from the peak frequencies
of the Fourier transformed populations. In addition, by periodically modulating
the Hamiltonian, we tune the spin-orbit coupling strength and use our
spectroscopy technique to probe the resulting dispersion relation. The
frequency resolution of our method is limited only by the coherent evolution
timescale of the Hamiltonian and can otherwise be applied to any system, for
example, to measure the band structure of atoms in optical lattice potentials
A simple algorithm for optimization and model fitting: AGA (asexual genetic algorithm)
Context. Mathematical optimization can be used as a computational tool to
obtain the optimal solution to a given problem in a systematic and efficient
way. For example, in twice-differentiable functions and problems with no
constraints, the optimization consists of finding the points where the gradient
of the objective function is zero and using the Hessian matrix to classify the
type of each point. Sometimes, however it is impossible to compute these
derivatives and other type of techniques must be employed such as the steepest
descent/ascent method and more sophisticated methods such as those based on the
evolutionary algorithms. Aims. We present a simple algorithm based on the idea
of genetic algorithms (GA) for optimization. We refer to this algorithm as AGA
(Asexual Genetic Algorithm) and apply it to two kinds of problems: the
maximization of a function where classical methods fail and model fitting in
astronomy. For the latter case, we minimize the chi-square function to estimate
the parameters in two examples: the orbits of exoplanets by taking a set of
radial velocity data, and the spectral energy distribution (SED) observed
towards a YSO (Young Stellar Object). Methods. The algorithm AGA may also be
called genetic, although it differs from standard genetic algorithms in two
main aspects: a) the initial population is not encoded, and b) the new
generations are constructed by asexual reproduction. Results. Applying our
algorithm in optimizing some complicated functions, we find the global maxima
within a few iterations. For model fitting to the orbits of exoplanets and the
SED of a YSO, we estimate the parameters and their associated errors.Comment: 10 pages, 8 figures, Astronomy and Astrophysics (in press
Synthesis of N-Heterocycles from Donor Acceptor Cyclopropanes and Progress towards Flinderole A, B, and C
The first chapter of this thesis describes two projects, one explores the novel reactivity of quaternary donor acceptor cyclopropanes and the second one involves progress toward the total synthesis of the flinderoles A, B, and C. The first project involves the Lewis acid catalyzed nucleophilic ring opening of quaternary donor acceptor cyclopropanes with indoline. It was found that the ring opening reaction worked well with either Sc(OTf)3 or Yb(OTf)3 as the Lewis acids. The ring opened products were also able to be converted into pyrroloindoles via a manganese (III) oxidative radical cyclization reaction. Cyclopropanes bearing alkynyl, vinyl, and aryl substituents were well tolerated as well as indolines bearing substitution at the 3-position. The second project involves the application of the ring opening/cyclization reaction to synthesize the pyrroloindole scaffold of the flinderoles. The chapter also describes our efforts to complete the synthesis of the natural products, and despite many alternative routes, we were not able to access the flinderoles.
The second chapter describes the Lewis acid catalyzed annulation reactions of donor acceptor cyclopropanes with vinyl azide and 2H-azirine. Surprisingly, the reaction with either the vinyl azide or 2H-azirine gave the same azabicyclic product. The reaction was also limited to cyclopropanes bearing trifluoroethyl esters instead of the common methyl esters. The reaction scope with respect to the cyclopropanes tolerated aryl, heteroaryl, vinyl, alkynyl and quaternary substituents on the cyclopropane. In both reactions, the azabicycle was obtained as a single diastereomer, which was confirmed by x-ray crystallography
Commuting times and the mobilisation of skills in emergent cities
Labour mobility within a large city or metropolitan area is a necessary condition for the optimal exploitation of agglomeration economies. We propose a method to establish which municipalities should be considered part of a metropolitan area based on labour market integration. In order to aggregate geographically proximate urban municipalities, we develop a network-based model that makes industry productivity (manifesting in formal employment creation) dependent on firms’ ability to find, within city limits, the diversity of skills they need to move to new complex industries. In this way, we uncover the natural city scale at which firms optimally exploit the range of skills available to them within a broad catchment area. Considering Colombian cities, we find that commuting times between 45 and 75 min, corresponding to between 43 and 62 distinct cities or integrated labour markets, allow firms to maximise formal employment creation (between 2008 and 2013). This result supports the development of passenger transport limiting commuting times within cities, connecting small- and mid-size cities to nearby large cities, and coordinating transportation investments across traditional administrative boundaries
Failure to Recover from Proactive Semantic Interference Differentiates Amnestic Mild Cognitive Impairment and PreMCI from Normal Aging after Adjusting for Initial Learning Ability
Background: There is increasing evidence that the failure to recover from proactive semantic interference (frPSI) may be an early cognitive marker of preclinical Alzheimer’s disease (AD). However, it is unclear whether frPSI effects reflect deficiencies in an individual’s initial learning capacity versus the actual inability to learn new semantically related targets. Objective: The current study was designed to adjust for learning capacity and then to examine the extent to which frPSI, proactive semantic interference (PSI) and retroactive semantic interference (RSI) effects could differentiate between older adults who were cognitively normal (CN), and those diagnosed with either Pre-Mild Cognitive Impairment (PreMCI) or amnestic MCI (aMCI). Methods: We employed the LASSI-L cognitive stress test to examine frPSI, PSI and RSI effects while simultaneously controlling for the participant’s initial learning capacity among 50 CN, 35 aMCI, and 16 PreMCI participants who received an extensive diagnostic work-up. Results: aMCI and PreMCI participants showed greater frPSI deficits (50% and 43.8% respectively) compared to only 14% of CNparticipants. PSI effects were observed for aMCI but not PreMCI participants relative to their CN counterparts. RSI failed to differentiate between any of the study groups. Conclusion: By using participants as their own controls and adjusting for overall learning and memory, it is clear that frPSI deficits occur with much greater frequency in individuals at higher risk for Alzheimer’s disease (AD), and likely reflect a failure of brain compensatory mechanisms.Fil: Curiel, Rosie E.. University of Miami; Estados UnidosFil: Crocco, Elizabeth A.. University of Miami; Estados UnidosFil: Raffo, Arlene. University of Miami; Estados UnidosFil: Guinjoan, Salvador MartÃn. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Houssay; Argentina. Fundación para la Lucha contra las Enfermedades Neurológicas de la Infancia; ArgentinaFil: Nemeroff, Charles B.. University of Miami; Estados UnidosFil: Penate, Ailyn. Mount Sinai Medical Center; Estados Unidos. University of Miami; Estados UnidosFil: Piña, Daema. University of Miami; Estados UnidosFil: Loewenstein, David A.. Mount Sinai Medical Center; Estados Unidos. University of Miami; Estados Unido
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