1,522 research outputs found
The Red Queen visits Minkowski Space
When Alice went `Through the Looking Glass' [1], she found herself in a
situation where she had to run as fast as she could in order to stay still. In
accordance with the dictum that truth is stranger than fiction, we will see
that it is possible to find a situation in special relativity where running
towards one's target is actually counter-productive. Although the situation is
easily analysed algebraically, the qualitative properties of the analysis are
greatly illuminated by the use of space-time diagrams
Classification of Solutions to the Plane Extremal Distance Problem for Bodies With Smooth Boundaries
Folate-mediated tumor cell targeting of liposome-entrapped doxorubicin in vitro
AbstractReceptors for the vitamin folic acid are frequently overexpressed on epithelial cancer cells. To examine whether this overexpression might be exploited to specifically deliver liposome-encapsulated drug molecules in vitro, folate-targeted liposomes were prepared by incorporating 0.1 mol% of a folate-polyethyleneglycol-distearoylphosphatidylethanolamine (folate-PEG-DSPE) construct into the lipid bilayer, and were loaded with doxorubicin (DOX), an anti-cancer drug. Uptake of folate-PEG-liposomal DOX by KB cells was 45-fold higher than that of non-targeted liposomal DOX, and 1.6-times higher than that of free DOX, while the cytotoxicity was 86 and 2.7-times higher, respectively. Folate-targeting is fully compatible with PEG-coating of the liposomes, since incorporation of 4 mol% PEG2000-DSPE does not reduce the uptake or cytotoxicity of folate-PEG-liposomal DOX. Uptake of folate-PEG-liposomes was inhibited by 1 mM free folic acid but was unaffected by physiological concentrations of folate. In HeLa/W138 co-cultures, folate-PEG-liposomes encapsulating calcein, a fluorescent dye, were found to be almost exclusively internalized by the HeLa cells which overexpress the folate receptors. We suggest that folate targeting constitutes a possible mechanism for improving the specificity of PEG-coated liposomes for cancer cells
Speed Limits in General Relativity
Some standard results on the initial value problem of general relativity in
matter are reviewed. These results are applied first to show that in a well
defined sense, finite perturbations in the gravitational field travel no faster
than light, and second to show that it is impossible to construct a warp drive
as considered by Alcubierre (1994) in the absence of exotic matter.Comment: 7 pages; AMS-LaTeX; accepted for publication by Classical and Quantum
Gravit
Light Sheets and the Covariant Entropy Conjecture
We examine the holography bound suggested by Bousso in his covariant entropy
conjecture, and argue that it is violated because his notion of light sheet is
too generous. We suggest its replacement by a weaker bound.Comment: 5 pages, to appear in Classical and Quantum Gravit
Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium
We investigate models for the photoionization of the widespread diffuse
ionized gas in galaxies. In particular we address the long standing question of
the penetration of Lyman continuum photons from sources close to the galactic
midplane to large heights in the galactic halo. We find that recent
hydrodynamical simulations of a supernova-driven interstellar medium have low
density paths and voids that allow for ionizing photons from midplane OB stars
to reach and ionize gas many kiloparsecs above the midplane. We find ionizing
fluxes throughout our simulation grids are larger than predicted by one
dimensional slab models, thus allowing for photoionization by O stars of low
altitude neutral clouds in the Galaxy that are also detected in Halpha. In
previous studies of such clouds the photoionization scenario had been rejected
and the Halpha had been attributed to enhanced cosmic ray ionization or
scattered light from midplane H II regions. We do find that the emission
measure distributions in our simulations are wider than those derived from
Halpha observations in the Milky Way. In addition, the horizontally averaged
height dependence of the gas density in the hydrodynamical models is lower than
inferred in the Galaxy. These discrepancies are likely due to the absence of
magnetic fields in the hydrodynamic simulations and we discuss how
magnetohydrodynamic effects may reconcile models and observations.
Nevertheless, we anticipate that the inclusion of magnetic fields in the
dynamical simulations will not alter our primary finding that midplane OB stars
are capable of producing high altitude diffuse ionized gas in a realistic
three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure
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