1,272 research outputs found
Questioning the Equivalence Principle
The Equivalence Principle (EP) is not one of the ``universal'' principles of
physics (like the Action Principle). It is a heuristic hypothesis which was
introduced by Einstein in 1907, and used by him to construct his theory of
General Relativity. In modern language, the (Einsteinian) EP consists in
assuming that the only long-range field with gravitational-strength couplings
to matter is a massless spin-2 field. Modern unification theories, and notably
String Theory, suggest the existence of new fields (in particular, scalar
fields: ``dilaton'' and ``moduli'') with gravitational-strength couplings. In
most cases the couplings of these new fields ``violate'' the EP. If the field
is long-ranged, these EP violations lead to many observable consequences
(variation of ``constants'', non-universality of free fall, relative drift of
atomic clocks,...). The best experimental probe of a possible violation of the
EP is to compare the free-fall acceleration of different materials.Comment: 14 pages, contribution to the ONERA workshop on "Missions spatiales
en physique fondamentale" (Chatillon, 18-19 Jan 2001), to appear in a special
issue of the Comptes Rendus de l'Academie des Sciences (Paris), edited by C.
Borde and P. Toubou
Hyaluronan and CD44 antagonize mitogen-dependent cyclin D1 expression in mesenchymal cells
High molecular weight (HMW) hyaluronan (HA) is widely distributed in the extracellular matrix, but its biological activities remain incompletely understood. We previously reported that HMW-HA binding to CD44 antagonizes mitogen-induced S-phase entry in vascular smooth muscle cells (SMCs; Cuff, C.A., D. Kothapalli, I. Azonobi, S. Chun, Y. Zhang, R. Belkin, C. Yeh, A. Secreto, R.K. Assoian, D.J. Rader, and E. PurĂ©. 2001. J. Clin. Invest. 108:1031â1040); we now characterize the underlying molecular mechanism and document its relevance in vivo. HMW-HA inhibits the mitogen-dependent induction of cyclin D1 and down-regulation of p27kip1 in vascular SMCs. p27kip1 messenger RNA levels were unaffected by HMW-HA, but the expression of Skp2, the rate-limiting component of the SCF complex that degrades p27kip1, was reduced. Rescue experiments identified cyclin D1 as the primary target of HMW-HA. Similar results were observed in fibroblasts, and these antimitogenic effects were not detected in CD44-null cells. Analysis of arteries from wild-type and CD44-null mice showed that the effects of HMW-HA/CD44 on cyclin D1 and Skp2 gene expression are detected in vivo and are associated with altered SMC proliferation after vascular injury
The Kontsevich Connection on the Moduli Space of FZZT Liouville Branes
We point out that insertions of matrix fields in (connected amputated)
amplitudes of (generalized) Kontsevich models are given by covariant
derivatives with respect to the Kontsevich moduli. This implies that
correlators are sections of symmetric products of the (holomorphic) tangent
bundle on the (complexified) moduli space of FZZT Liouville branes. We discuss
the relation of Kontsevich parametrization of moduli space with that provided
by either the (p,1) or the (1,p) boundary conformal field theories. It turns
out that the Kontsevich connection captures the contribution of contact terms
to open string amplitudes of boundary cosmological constant operators in the
(1,p) minimal string models. The curvature of the connection is of type (1,1)
and has delta-function singularities at the points in moduli space where
Kontsevich kinetic term vanishes. We also outline the extention of our
formalism to the c=1 string at self-dual radius and discuss the problems that
have to be understood to reconciliate first and second quantized approaches in
this case.Comment: 34 pages, 2 eps figures, LaTex; typos corrected (including title);
more typos fixed, including those in Eqs.(72) and (132
Self-organization on surfaces: foreword
After decades of work, the growth of continuous thin films, i.e.,
two-dimensional structures, is progressively becoming a technological issue
more than a field of fundamental research. Incidentally self-organization of
nanostructures on surfaces is now an important field of research, i.e.,
structures of dimensionality one or zero, with a steep rise of attention in the
past five years. Whereas self-organization was initially motivated by potential
applications, it has up to now essentially contributed to the advancement of
fundamental science in low dimensions, as model systems could be produced that
could not have been fabricated by lithography. This Special Issue aims at
giving a cross-community timely overview of the field. The Issue gathers a
broad panel of articles covering various self-organization mechanisms, specific
structural characterization, physical properties, and current trends in
extending the versatility of growth. The materials mostly covered here are
semiconductors and magnetic materials.Comment: Foreword of the Editor to Special Issue on Self-organization on
surface
Finite and infinite speed of propagation for porous medium equations with fractional pressure
We study a porous medium equation with fractional potential pressure: for
, and . To be specific, the problem is posed for
, , and . The initial data is assumed
to be a bounded function with compact support or fast decay at infinity. We
establish existence of a class of weak solutions for which we determine
whether, depending on the parameter , the property of compact support is
conserved in time or not, starting from the result of finite propagation known
for . We find that when the problem has infinite speed of
propagation, while for it has finite speed of propagation.
Comparison is made with other nonlinear diffusion models where the results are
widely different.Comment: 6 pages, 1 figur
"Do screen captures in manuals make a difference?": a comparison between textual and visual manuals
Examines the use of screen captures in manuals. Three types of manuals were compared: one textual and two visual. The two visual manuals differed in the type of screen capture that was used. One had screen captures that showed only the relevant part of the screen, whereas the other consisted of captures of the full screen. All manuals contained exactly the same textual information. We examined immediate use on time (use as a job aid) and on learning (use as a teacher). For job-aid purposes, there was no difference between the manuals. The visual manual with full-screen captures and the textual manual were both better for learning than the visual manual with partial screen captures. We found no effect on user motivation. The tentative conclusion of this study is that screen captures seem not to be vital for learning or immediate use. If one opts for including screen captures, then the conclusion is that full-screen captures are better than partial one
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Distinctive growth requirements and gene expression patterns distinguish progenitor B cells from pre-B cells.
Long-term bone marrow cultures have been useful in determining gene expression patterns in pre-B cells and in the identification of cytokines such as interleukin 7 (IL-7). We have developed a culture system to selectively grow populations of B lineage restricted progenitors (pro-B cells) from murine bone marrow. Pro-B cells do not grow in response to IL-7, Steel locus factor (SLF), or a combination of the two. c-kit, the SLF receptor, and the IL-7 receptor are both expressed by pro-B cells, indicating that the lack of response is not simply due to the absence of receptors. Furthermore, SLF is not necessary for the growth of pro-B cells since they could be expanded on a stromal line derived from Steel mice that produces no SLF. IL-7 responsiveness in pre-B cells is associated with an increase in n-myc expression and is correlated with immunoglobulin (Ig) gene rearrangements. Although members of the ets family of transcription factors and the Pim-1 kinase are expressed by pro-B cells, n-myc is not expressed. Pro-B cells maintain Ig genes in the germline configuration, which is correlated with a low level of recombination activating genes 1 and 2 (Rag-1 and 2) mRNA expression, but high expression of sterile mu and terminal deoxynucleotidyl transferase. Pro-B cells are unable to grow separated from the stromal layer by a porous membrane, indicating that stromal contact is required for growth. These results suggest that pro-B cells are dependent on alternative growth signals derived from bone marrow stroma and can be distinguished from pre-B cells by specific patterns of gene expression
Will Fat Taxes Cause Americans to Become Fatter? Some Evidence from US Meats
Price and income elasticities of fat from meats are estimated by decomposing composite demand for meat into the produ ct of total calories, the fraction of calories eat as fat, and a residual measure of quality. This demand-characteristic system provides estimates of the impact of prices and income on the fraction of calories eaten as fat as well as their affect on the total consumption of fat. Empirical estimates of the comp ensated own-price elasticities of meats suggest that a fat tax designed to raise revenues to finance nutritional education efforts may increase the total consumption of fat.Agricultural and Food Policy, Demand and Price Analysis, Food Consumption/Nutrition/Food Safety,
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Heterodon nasicus
Number of Pages: 10Integrative BiologyGeological Science
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