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Fundamental Limit (fundamental + limit)

Distribution by Scientific Domains

Chemistry	33%

Selected Abstracts

Soft X-ray radiation-damage studies in PMMA using a cryo-STXM

JOURNAL OF SYNCHROTRON RADIATION, Issue 3 2003
Tobias Beetz
Radiation damage sets a fundamental limit for studies with ionizing radiation; cryo-methods are known to ease these limits. Here, measurements on mass loss and the decrease in the C=O bond density as measured by oxygen-edge XANES (NEXAFS) spectroscopy in thin films of poly(methylmethacrylate) (PMMA), studied in a vacuum, are reported. While cryo-methods allow more than 95% of the mass to remain at doses up to ,Gy, there is little difference in C=O bond density versus dose between 298,K and 113,K sample temperatures. At both temperatures the critical dose for bond breaking is ,15 × 106,Gy. [source]

Limits on the detectability of the CMB B-mode polarization imposed by foregrounds

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005
M. Tucci
ABSTRACT We investigate which practical constraints are imposed by foregrounds on the detection of the B-mode polarization generated by gravitational waves, in the case of experiments of the type currently being planned. As the B-mode signal is probably dominated by foregrounds at all frequencies, the detection of the cosmological component depends drastically on our ability to remove foregrounds. We provide an analytical expression with which to estimate the level of the residual polarization for Galactic foregrounds, according to the method employed for their subtraction. We interpret this result in terms of the lower limit of the tensor-to-scalar ratio r that allows us to disentangle the cosmological B-mode polarization from the foreground contribution. Polarized emission from extragalactic radio sources and gravitational lensing is also taken into account. As a first approach, we consider the ideal limit of an instrumental noise-free experiment: for full-sky coverage and a resolution of 1°, we obtain a limit of r, 10,4. This value can be improved by high-resolution experiments and, in principle, there is no clear fundamental limit on the detectability of the polarization of gravitational waves. Our analysis is also applied to planned or hypothetical future polarization experiments, taking into account expected noise levels. [source]

Detection and characterization of ultra-thin films with neutron reflectometry

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2009
Z. Tun
Specular reflectometry, being a technique based on interference between coherent X-ray or neutron beams, is considered to have a fundamental limit in sensing the presence of films that are too thin for the maximum momentum transfer, Qmax, to which reflectivity has been measured. However, it is known both experimentally and from simulations that an ultra-thin film, with thickness t << 2,/Qmax, can be detected if it exists sandwiched between two contrast-matched media. This possibility is qualitatively explained using phase-vector diagrams. The diagrams also show that the detection is through unmistakable shifts of the interference maxima and minima, and that the scattering-length density of the ultra-thin film determined by least-squares analysis is unique. [source]

Signal representation and approximation,fundamental limits

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 5 2007
Holger Boche
The expansion of functions in orthonormal bases is an important analytical and practical instrument in many different areas such as in signal processing, in system and information theory and in communications. However, the selection of an optimal basis is a non-trivial task in general and depends strongly on the performance measure of the concrete problem. This paper considers the basis selection problem for three different applications, starting with a problem from system theory, looking on entropy based methods from information theory, and finally it investigates the peak-to-average power ratio problem in communication systems. In particular, it is investigated under which conditions the problems are solvable, that is under which conditions there exists an appropriate basis. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Predicting the impacts of climate change on the distribution of species: are bioclimate envelope models useful?

GLOBAL ECOLOGY, Issue 5 2003
Richard G. Pearson
ABSTRACT Modelling strategies for predicting the potential impacts of climate change on the natural distribution of species have often focused on the characterization of a species' bioclimate envelope. A number of recent critiques have questioned the validity of this approach by pointing to the many factors other than climate that play an important part in determining species distributions and the dynamics of distribution changes. Such factors include biotic interactions, evolutionary change and dispersal ability. This paper reviews and evaluates criticisms of bioclimate envelope models and discusses the implications of these criticisms for the different modelling strategies employed. It is proposed that, although the complexity of the natural system presents fundamental limits to predictive modelling, the bioclimate envelope approach can provide a useful first approximation as to the potentially dramatic impact of climate change on biodiversity. However, it is stressed that the spatial scale at which these models are applied is of fundamental importance, and that model results should not be interpreted without due consideration of the limitations involved. A hierarchical modelling framework is proposed through which some of these limitations can be addressed within a broader, scale-dependent context. [source]

Decision Structure and the Problem of Scale in Deliberation

COMMUNICATION THEORY, Issue 2 2006
Raymond J. Pingree
Deliberation has been limited to small groups because coherence seems to require full reception, meaning that all participants receive all messages sent. Assuming that full reception actually leads to coherence ignores fundamental limits of human memory and group processes. Full reception is also not the only route to coherence because the forms of coherence desired in deliberation are decision specific and because all deliberations at least implicitly contain a structure of subdecisions. Coherent deliberation is plausible at large scales, without full reception, via a theoretical model called decision-structured deliberation. This model allows coherent contributions by participants who are unaware of large parts of the discussion and may reduce negative effects of limits of memory and group processes on the quality of deliberation. [source]