7 research outputs found
Molecular recognition through rational and combinatorial synthesis
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1996.Includes bibliographical references.by Edward Aurel Wintner.Ph.D
Emergent Semiclassical Time in Quantum Gravity. I. Mechanical Models
Strategies intended to resolve the problem of time in quantum gravity by
means of emergent or hidden timefunctions are considered in the arena of
relational particle toy models. In situations with `heavy' and `light' degrees
of freedom, two notions of emergent semiclassical WKB time emerge; these are
furthermore equivalent to two notions of emergent classical
`Leibniz--Mach--Barbour' time. I futhermore study the semiclassical approach,
in a geometric phase formalism, extended to include linear constraints, and
with particular care to make explicit those approximations and assumptions
used. I propose a new iterative scheme for this in the cosmologically-motivated
case with one heavy degree of freedom. I find that the usual semiclassical
quantum cosmology emergence of time comes hand in hand with the emergence of
other qualitatively significant terms, including back-reactions on the heavy
subsystem and second time derivatives. I illustrate my analysis by taking it
further for relational particle models with linearly-coupled harmonic
oscillator potentials. As these examples are exactly soluble by means outside
the semiclassical approach, they are additionally useful for testing the
justifiability of some of the approximations and assumptions habitually made in
the semiclassical approach to quantum cosmology. Finally, I contrast the
emergent semiclassical timefunction with its hidden dilational Euler time
counterpart.Comment: References Update
Quantized Surface Complementarity Diversity (QSCD): A Model Based on Small Molecule-Target Complementarity
Quantized Surface Complementarity Diversity (QSCD): A Model Based on Small Molecule−Target Complementarity
Using bomb radiocarbon to estimate age and growth of the white shark, Carcharodon carcharias, from the southwestern Indian Ocean
Historical biogeography of smoothhound sharks (genus Mustelus) of Southern Africa reveals multiple dispersal events from the Northern Hemisphere
Members of the smoothhound shark genus Mustelus display a widespread distribution pattern across ocean basins with a
high degree of sub-regional endemism. The patterns and processes that resulted in smoothhound biodiversity and presentday
distribution remain largely unknown. We infer the phylogenetic relationships of the genus Mustelus, based on sequence
data (3474 bp) from three mitochondrial genes (CR, NADH-2 and 12S-16SrRNA) and a nuclear gene (KBTBD2) from
seven species of Mustelus distributed across the eastern Atlantic- and Indo-Pacific oceans. Using the CR and KBTBD2
dataset, we infer the phylogeographic placement of Old World Mustelus, with particular reference to species from southern
Africa. Using a near-complete phylogeny of the genus including Old World and New World species of Mustelus and
publicly available sequences of the NADH-2 gene, we found supporting evidence indicating a major cladogenic event
separating placental and aplacental species. Biogeographical analyses further revealed that the radiation of Mustelus in the
southern African region was driven primarily by long-distance dispersal during the upper Miocene to lower Pleistocene. The
placement of the placental blackspotted smoothhound Mustelus punctulatus at the base of the placental non-spotted clade
suggests the secondary loss of black spots in the genus, and this was also supported by the ancestral state reconstruction.
The results furthermore suggest that the Southern Hemisphere species of the genus arose from multiple separate dispersal
events from the Northern Hemisphere which is in line with the earliest record of Mustelus in the Northern Hemisphere