27,067 research outputs found
Comment on "Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars"
In their Letter [Phys. Rev. Lett. 92, 121302 (2004)] (also [Astron.
Astrophys. 417, 853 (2004)]), Srianand et al. analysed optical spectra of
heavy-element species in 23 absorption systems along background quasar
sight-lines, reporting limits on relative variations in the fine-structure
constant: da/a=(-0.06+/-0.06) x 10^{-5}. Here we demonstrate basic flaws in
their analysis, using the same data and absorption profile fits, which led to
spurious values of da/a and significantly underestimated uncertainties. We
conclude that these data and fits offer no stringent test of previous evidence
for a varying alpha.
In their Reply (arXiv:0711.1742) to this Comment, Srianand et al. state or
argue several points regarding their original analysis and our new analysis. We
discuss these points here, dismissing all of them because they are demonstrably
incorrect or because they rely on a flawed application of simple statistical
arguments.Comment: 1+2 pages, 1 EPS figure. Page 1 accepted as PRL Comment on
arXiv:astro-ph/0402177 . Further details available in arXiv:astro-ph/0612407
. v2: Added critical discussion of Reply from Srianand et al.
(arXiv:0711.1742
Time evolution of the fine structure constant
We present a short review of the current quasar (QSO) absorption line
constraints on possible variation of the fine structure constant, alpha =
e^2/(hbar*c). Particular attention is paid to recent optical Keck/HIRES spectra
of 49 absorption systems which indicate a smaller alpha in the past (Murphy et
al. 2001, Webb et al. 2001). Here we present new preliminary results from 128
absorption systems: da/a = (-0.57 +/- 0.10) x 10^{-5} over the redshift range
0.2 < z < 3.7, in agreement with the previous results. Known potential
systematic errors cannot explain these results. We compare them with strong
`local' constraints and discuss other (radio and millimetre-wave) QSO
absorption line constraints on variations in alpha^2 * g_p and alpha^2 * g_p *
m_e/m_p (g_p is the proton g-factor and m_e/m_p is the electron/proton mass
ratio). Finally, we discuss future efforts to rule out or confirm the current
5.7 sigma optical detection.Comment: Invited review at the XXII Physics in Collision Conference (PIC02),
Stanford, CA, USA, June 2002, 11 pages, LaTeX, 7 eps figures. PSN FRA T0
Spin-dependent recombination in Czochralski silicon containing oxide precipitates
Electrically detected magnetic resonance is used to identify recombination
centers in a set of Czochralski grown silicon samples processed to contain
strained oxide precipitates with a wide range of densities (~ 1e9 cm-3 to ~
7e10 cm-3). Measurements reveal that photo-excited charge carriers recombine
through Pb0 and Pb1 dangling bonds and comparison to precipitate-free material
indicates that these are present at both the sample surface and the oxide
precipitates. The electronic recombination rates vary approximately linearly
with precipitate density. Additional resonance lines arising from iron-boron
and interstitial iron are observed and discussed. Our observations are
inconsistent with bolometric heating and interpreted in terms of spin-dependent
recombination. Electrically detected magnetic resonance is thus a very powerful
and sensitive spectroscopic technique to selectively probe recombination
centers in modern photovoltaic device materials.Comment: 8 pages, 8 figure
Determination of airplane model structure from flight data by using modified stepwise regression
The linear and stepwise regressions are briefly introduced, then the problem of determining airplane model structure is addressed. The MSR was constructed to force a linear model for the aerodynamic coefficient first, then add significant nonlinear terms and delete nonsignificant terms from the model. In addition to the statistical criteria in the stepwise regression, the prediction sum of squares (PRESS) criterion and the analysis of residuals were examined for the selection of an adequate model. The procedure is used in examples with simulated and real flight data. It is shown that the MSR performs better than the ordinary stepwise regression and that the technique can also be applied to the large amplitude maneuvers
Why will rat's go where rats will not
Experimental evidence indicates that regular plurals are nearly always omitted from English compounds (e.g., rats-eater) while irregular plurals may be included within these structures (e.g., mice-chaser). This phenomenon is considered to be good evidence to support the dual mechanism model of morphological processing (Pinker & Prince, 1992). However, evidence from neural net modelling has shown that a single route associative memory based account might provide an equally, if not more, valid explanation of the compounding phenomenon
Plural morphology in compounding is not good evidence to support the dual mechanism model
The compounding phenomena is considered to be good evidence to support the dual mechanism model of morphological processing (Pinker & Prince, 1992). However evidence from initial neural net modeling has shown that a single route associative memory based account might provide an equally, if not more valid explanation of the treatment of plurals in compounds. Further neural net modeling and empirical work is proposed to test this single route accoun
Is there further evidence for spatial variation of fundamental constants?
The detection of a spatial variation of the fine-structure constant, alpha,
based on study of quasar absorption systems has recently been reported. The
physics that causes this alpha-variation should have other observable
manifestations, and this motivates us to look for complementary astrophysical
effects. In this paper we propose a method to test whether spatial variation of
fundamental constants existed during the epoch of big bang nucleosynthesis.
Using existing measurements of primordial deuterium abundance we find very weak
indications that such a signature might exist, but the paucity of measurements
precludes any firm conclusion. We also examine existing quasar absorption
spectra data that are sensitive to variation of the electron-to-proton mass
ratio, mu, and x = (alpha^2 mu g_p) for spatial variation.Comment: 6 pages, 1 figur
Hydrocarbons in the banana leaf, Musa sapientum
Mass spectrometry, spectroscopy, and thin layer and gas chromatography studies of hydrocarbons in banana leave
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