1,004 research outputs found
Algorithmic Randomness for Infinite Time Register Machines
A concept of randomness for infinite time register machines (ITRMs),
resembling Martin-L\"of-randomness, is defined and studied. In particular, we
show that for this notion of randomness, computability from mutually random
reals implies computability and that an analogue of van Lambalgen's theorem
holds
Generalized Effective Reducibility
We introduce two notions of effective reducibility for set-theoretical
statements, based on computability with Ordinal Turing Machines (OTMs), one of
which resembles Turing reducibility while the other is modelled after Weihrauch
reducibility. We give sample applications by showing that certain (algebraic)
constructions are not effective in the OTM-sense and considerung the effective
equivalence of various versions of the axiom of choice
Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)
Felsic veins (plagiogranites) are distributed throughout the whole oceanic crust section and offer insight into late-magmatic/high temperature hydrothermal processes within the oceanic crust. Despite constituting only 0.5% of the oceanic crust section drilled in IODP Site 735B, they carry a significant budget of incompatible elements, which they redistribute within the crust. Such melts are saturated in accessory minerals, such as zircon, titanite and apatite, and often zircon is the only remaining phase that preserves magmatic composition and records processes of felsic melt formation and evolution. In this study, we analysed zircon from four depths in IODP Site 735B; they come from the oxide gabbro (depth approximately 250 m below sea floor) and plagiogranite (depths c. 500, 860, 940 m below sea floor). All zircons have similar εHf composition of c. 15 units indicating an isotopically homogenous source for the mafic magmas forming IODP Site 735B gabbro. Zircons from oxide gabbro are scarce and variable in composition consistent with their crystallization from melts formed by both fractionation of mafic magmas and hydrous remelting of gabbro cumulate. On the other hand, zircon from plagiogranite is abundant and each sample is characterized by compositional trends consistent with crystallization of zircon in an evolving melt. However, the trends are different between the plagiogranite at 500 m bsf and the deeper sections, which are interpreted as the record of plagiogranite formation by two processes: remelting of gabbro cumulate at 500 m bsf and fractionation at deeper sections. Zircon from both oxide gabbro and plagiogranite has δ18O from 3.5 to 6.0‰. Values of δ18O are best explained by redistribution of δ18O in a thermal gradient and not by remelting of hydrothermally altered crust. Tentatively, it is suggested that fractionation could be an older episode contemporaneous with gabbro crystallization and remelting could be a younger one, triggered by deformation and uplift of the crustal pile.1017/S/ING/15-
Project Cerberus: Flyby Mission to Pluto
The goal of the Cerberus Project was to design a feasible and cost-effective unmanned flyby mission to Pluto. The requirements in the request for proposal for an unmanned probe to Pluto are presented and were met. The design stresses proven technology that will avoid show stoppers which could halt mission progress. Cerberus also utilizes the latest advances in the spacecraft industry to meet the stringent demands of the mission. The topics covered include: (1) mission management, planning, and costing; (2) structures; (3) power and propulsion; (4) attitude, articulation, and control; (5) command, control, and communication; and (6) scientific instrumentation
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Cerium Compounds as Scintillators
Stoichiometric Ce-materials with negligible Ce-Ce interactions should have superior scintillator properties. We present two materials: CeF{sub 3} and Ce{sub x}La{sub 1-x}P{sub 5}O{sub 14}. While cerium trifluoride is a known scintillator, pentaphosphate is of a limited usefulness, except as a remarkable model material. We show that quenching in fluoride is responsible for loss of 50% of the light output and is the cause of the, so-called, ultra fast component (2 ns). Light output of fluoride (about 50% of BGO) could be significantly improved. Deeper understanding of Ce-systems is needed to fully exploit their potentials. 10 figs., 6 refs
Determination of Omega_b From Big Bang Nucleosynthesis in the Presence of Regions of Antimatter
Production of regions of antimatter in the early universe is predicted in
many baryogenesis models. Small scale antimatter regions would annihilate
during or soon after nucleosynthesis, affecting the abundances of the light
elements. In this paper we study how the acceptable range in Omega_b changes in
the presence of antimatter regions, as compared to the standard big bang
nucleosynthesis. It turns out that it is possible to produce at the same time
both a low 4He value (Y_p < 0.240) and a low D/H value (D/H < 4e-5), but
overproduction of 7Li is unavoidable at large Omega_b.Comment: 9 pages, PRD version, ref. 6 correcte
Lithium-6: A Probe of the Early Universe
I consider the synthesis of 6Li due to the decay of relic particles, such as
gravitinos or moduli, after the epoch of Big Bang Nucleosynthesis. The
synthesized 6Li/H ratio may be compared to 6Li/H in metal-poor stars which, in
the absence of stellar depletion of 6Li, yields significantly stronger
constraints on relic particle densities than the usual consideration of
overproduction of 3He. Production of 6Li during such an era of non-thermal
nucleosynthesis may also be regarded as a possible explanation for the
relatively high 6Li/H ratios observed in metal-poor halo stars.Comment: final version, Physical Review Letters, additional figure giving
limits on relic decaying particle
Excitons in a Photosynthetic Light-Harvesting System: A Combined Molecular Dynamics/Quantum Chemistry and Polaron Model Study
The dynamics of pigment-pigment and pigment-protein interactions in
light-harvesting complexes is studied with a novel approach which combines
molecular dynamics (MD) simulations with quantum chemistry (QC) calculations.
The MD simulations of an LH-II complex, solvated and embedded in a lipid
bilayer at physiological conditions (with total system size of 87,055 atoms)
revealed a pathway of a water molecule into the B800 binding site, as well as
increased dimerization within the B850 BChl ring, as compared to the
dimerization found for the crystal structure. The fluctuations of pigment (B850
BChl) excitation energies, as a function of time, were determined via ab initio
QC calculations based on the geometries that emerged from the MD simulations.
From the results of these calculations we constructed a time-dependent
Hamiltonian of the B850 exciton system from which we determined the linear
absorption spectrum. Finally, a polaron model is introduced to describe quantum
mechanically both the excitonic and vibrational (phonon) degrees of freedom.
The exciton-phonon coupling that enters into the polaron model, and the
corresponding phonon spectral function are derived from the MD/QC simulations.
It is demonstrated that, in the framework of the polaron model, the absorption
spectrum of the B850 excitons can be calculated from the autocorrelation
function of the excitation energies of individual BChls, which is readily
available from the combined MD/QC simulations. The obtained result is in good
agreement with the experimentally measured absorption spectrum.Comment: REVTeX3.1, 23 pages, 13 (EPS) figures included. A high quality PDF
file of the paper is available at
http://www.ks.uiuc.edu/Publications/Papers/PDF/DAMJ2001/DAMJ2001.pd
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