391 research outputs found
Preliminary space mission design under uncertainty
This paper proposes a way to model uncertainties and to introduce them explicitly in the design process of a preliminary space mission. Traditionally, a system margin approach is used in order to take them into account. In this paper, Evidence Theory is proposed to crystallise the inherent uncertainties. The design process is then formulated as an Optimisation Under Uncertainties (OUU). Three techniques are proposed to solve the OUU problem: (a) an evolutionary multi-objective approach, (b) a step technique consisting of maximising the belief for different levels of performance, and (c) a clustering method that
firstly identifes feasible regions. The three methods are applied to the BepiColombo mission and their
effectiveness at solving the OUU problem are compared
Geometry of effective Hamiltonians
We give a complete geometrical description of the effective Hamiltonians
common in nuclear shell model calculations. By recasting the theory in a
manifestly geometric form, we reinterpret and clarify several points. Some of
these results are hitherto unknown or unpublished. In particular, commuting
observables and symmetries are discussed in detail. Simple and explicit proofs
are given, and numerical algorithms are proposed, that improve and stabilize
common methods used today.Comment: 1 figur
Mission design for LISA Pathfinder
Here we describe the mission design for SMART-2/LISA Pathfinder. The best
trade-off between the requirements of a low-disturbance environment and
communications distance is found to be a free-insertion Lissajous orbit around
the first co-linear Lagrange point of the Sun-Earth system L1, 1.5x 10^6 km
from Earth. In order to transfer SMART-2/LISA Pathfinder from a low Earth
orbit, where it will be placed by a small launcher, the spacecraft carries out
a number of apogee-raise manoeuvres, which ultimatively place it to a parabolic
escape trajectory towards L1. The challenges of the design of a small mission
are met, fulfilling the very demanding technology demonstration requirements
without creating excessive requirements on the launch system or the ground
segment.Comment: 7 pages, 6 figures, 5th International LISA Symposium, see
http://www.landisoft.de/Markus-Landgra
Determining a quantum state by means of a single apparatus
The unknown state \hrho of a quantum system S is determined by letting it
interact with an auxiliary system A, the initial state of which is known. A
one-to-one mapping can thus be realized between the density matrix \hrho and
the probabilities of occurrence of the eigenvalues of a single and factorized
observable of S+A, so that \hrho can be determined by repeated measurements
using a single apparatus. If S and A are spins, it suffices to measure
simultaneously their -components after a controlled interaction. The most
robust setups are determined in this case, for an initially pure or a
completely disordered state of A. They involve an Ising or anisotropic
Heisenberg coupling and an external field.Comment: 5 pages revte
Critical view of WKB decay widths
A detailed comparison of the expressions for the decay widths obtained within
the semiclassical WKB approximation using different approaches to the tunneling
problem is performed. The differences between the available improved formulae
for tunneling near the top and the bottom of the barrier are investigated.
Though the simple WKB method gives the right order of magnitude of the decay
widths, a small number of parameters are often fitted. The need to perform the
fitting procedure remaining consistently within the WKB framework is emphasized
in the context of the fission model based calculations. Calculations for the
decay widths of some recently found super heavy nuclei using microscopic
alpha-nucleus potentials are presented to demonstrate the importance of a
consistent WKB calculation. The half-lives are found to be sensitive to the
density dependence of the nucleon-nucleon interaction and the implementation of
the Bohr-Sommerfeld quantization condition inherent in the WKB approach.Comment: 18 pages, Late
Longitudinal study of urban malaria in a cohort of Ugandan children: description of study site, census and recruitment
BACKGROUND: Studies of malaria in well-defined cohorts offer important data about the epidemiology of this complex disease, but few have been done in urban African populations. To generate a sampling frame for a longitudinal study of malaria incidence and treatment in Kampala, Uganda, a census, mapping and survey project was conducted. METHODS: All households in a geographically defined area were enumerated and mapped. Probability sampling was used to recruit a representative sample of children and collect baseline descriptive data for future longitudinal studies. RESULTS: 16,172 residents living in 4931 households in a densely-populated community (18,824 persons/km(2)) were enumerated. A total of 582 households were approached with at least one child less than 10 years of age in order to recruit 601 children living in 322 households. At enrollment, 19% were parasitaemic, 24% were anaemic, 43% used bednets, and 6% used insecticide-treated nets. Low G6PD activity (OR = 0.33, P = 0.009) and bednet use (OR = 0.64, P = 0.045) were associated with a decreased risk of parasitaemia. Increasing age (OR = 0.62 for each year, P < 0.001) and bednet use (OR = 0.58, P = 0.02) were associated with a decreased risk of anaemia CONCLUSION: Detailed surveys of target populations in urban Africa can provide valuable descriptive data and provide a sampling frame for recruitment of representative cohorts for longitudinal studies. Plans to use a multi-disciplinary approach to improve the understanding of the distribution and determinants of malaria incidence and response to therapy in this population are discussed
How to determine a quantum state by measurements: The Pauli problem for a particle with arbitrary potential
The problem of reconstructing a pure quantum state ¿¿> from measurable quantities is considered for a particle moving in a one-dimensional potential V(x). Suppose that the position probability distribution ¿¿(x,t)¿2 has been measured at time t, and let it have M nodes. It is shown that after measuring the time evolved distribution at a short-time interval ¿t later, ¿¿(x,t+¿t)¿2, the set of wave functions compatible with these distributions is given by a smooth manifold M in Hilbert space. The manifold M is isomorphic to an M-dimensional torus, TM. Finally, M additional expectation values of appropriately chosen nonlocal operators fix the quantum state uniquely. The method used here is the analog of an approach that has been applied successfully to the corresponding problem for a spin system
Long-Time Behavior of Macroscopic Quantum Systems: Commentary Accompanying the English Translation of John von Neumann's 1929 Article on the Quantum Ergodic Theorem
The renewed interest in the foundations of quantum statistical mechanics in
recent years has led us to study John von Neumann's 1929 article on the quantum
ergodic theorem. We have found this almost forgotten article, which until now
has been available only in German, to be a treasure chest, and to be much
misunderstood. In it, von Neumann studied the long-time behavior of macroscopic
quantum systems. While one of the two theorems announced in his title, the one
he calls the "quantum H-theorem", is actually a much weaker statement than
Boltzmann's classical H-theorem, the other theorem, which he calls the "quantum
ergodic theorem", is a beautiful and very non-trivial result. It expresses a
fact we call "normal typicality" and can be summarized as follows: For a
"typical" finite family of commuting macroscopic observables, every initial
wave function from a micro-canonical energy shell so evolves that for
most times in the long run, the joint probability distribution of these
observables obtained from is close to their micro-canonical
distribution.Comment: 34 pages LaTeX, no figures; v2: minor improvements and additions. The
English translation of von Neumann's article is available as arXiv:1003.213
On the Global Existence of Bohmian Mechanics
We show that the particle motion in Bohmian mechanics, given by the solution
of an ordinary differential equation, exists globally: For a large class of
potentials the singularities of the velocity field and infinity will not be
reached in finite time for typical initial values. A substantial part of the
analysis is based on the probabilistic significance of the quantum flux. We
elucidate the connection between the conditions necessary for global existence
and the self-adjointness of the Schr\"odinger Hamiltonian.Comment: 35 pages, LaTe
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