Monsters, black holes and the statistical mechanics of gravity

We review the construction of monsters in classical general relativity. Monsters have finite ADM mass and surface area, but potentially unbounded entropy. From the curved space perspective they are objects with large proper volume that can be glued on to an asymptotically flat space. At no point is the curvature or energy density required to be large in Planck units, and quantum gravitational effects are, in the conventional effective field theory framework, small everywhere. Since they can have more entropy than a black hole of equal mass, monsters are problematic for certain interpretations of black hole entropy and the AdS/CFT duality. In the second part of the paper we review recent developments in the foundations of statistical mechanics which make use of properties of high-dimensional (Hilbert) spaces. These results primarily depend on kinematics -- essentially, the geometry of Hilbert space -- and are relatively insensitive to dynamics. We discuss how this approach might be adopted as a basis for the statistical mechanics of gravity. Interestingly, monsters and other highly entropic configurations play an important role.Comment: 9 pages, 4 figures, revtex; invited Brief Review to be published in Modern Physics Letters

Plasma synthesis of single crystal silicon nanoparticles for novel electronic device applications

Single-crystal nanoparticles of silicon, several tens of nm in diameter, may be suitable as building blocks for single-nanoparticle electronic devices. Previous studies of nanoparticles produced in low-pressure plasmas have demonstrated the synthesis nanocrystals of 2-10 nm diameter but larger particles were amorphous or polycrystalline. This work reports the use of a constricted, filamentary capacitively coupled low-pressure plasma to produce single-crystal silicon nanoparticles with diameters between 20-80 nm. Particles are highly oriented with predominant cubic shape. The particle size distribution is rather monodisperse. Electron microscopy studies confirm that the nanoparticles are highly oriented diamond-cubic silicon.Comment: accepted for publication in Plasma Physics and Controlled Fusion, scheduled for Dec. 2004 F

Detection of Mitochondrial COII DNA Sequences in Ant Guts as a Method for Assessing Termite Predation by Ants

Termites and ants contribute more to animal biomass in tropical rain forests than any other single group and perform vital ecosystem functions. Although ants prey on termites, at the community level the linkage between these groups is poorly understood. Thus, assessing the distribution and specificity of ant termitophagy is of considerable interest.We describe an approach for quantifying ant-termite food webs by sequencing termite DNA (cytochrome c oxidase subunit II, COII) from ant guts and apply this to a soil-dwelling ant community from tropical rain forest in Gabon. We extracted DNA from 215 ants from 15 species. Of these, 17.2% of individuals had termite DNA in their guts, with BLAST analysis confirming the identity of 34.1% of these termites to family level or better. Although ant species varied in detection of termite DNA, ranging from 63% (5/7; Camponotus sp. 1) to 0% (0/7; Ponera sp. 1), there was no evidence (with small sample sizes) for heterogeneity in termite consumption across ant taxa, and no evidence for species-specific ant-termite predation. In all three ant species with identifiable termite DNA in multiple individuals, multiple termite species were represented. Furthermore, the two termite species that were detected on multiple occasions in ant guts were in both cases found in multiple ant species, suggesting that anttermite food webs are not strongly compartmentalised. However, two ant species were found to consume only Anoplotermes-group termites, indicating possible predatory specialisation at a higher taxonomic level. Using a laboratory feeding test, we were able to detect termite COII sequences in ant guts up to 2 h after feeding, indicating that our method only detects recent feeding events. Our data provide tentative support for the hypothesis that unspecialised termite predation by ants is widespread and highlight the use of molecular approaches for future studies of ant-termite food webs

Adenosine and lymphocyte regulation

Adenosine is a potent extracellular messenger that is produced in high concentrations under metabolically unfavourable conditions. Tissue hypoxia, consequent to a compromised cellular energy status, is followed by the enhanced breakdown of ATP leading to the release of adenosine. Through the interaction with A2 and A3 membrane receptors, adenosine is devoted to the restoration of tissue homeostasis, acting as a retaliatory metabolite. Several aspects of the immune response have to be taken into consideration and even though in general it is very important to dampen inflammation, in some circumstances, such as the case of cancer, it is also necessary to increase the activity of immune cells against pathogens. Therefore, adenosine receptors that are defined as ‘sensors–of metabolic changes in the local tissue environment may be very important targets for modulation of immune responses and drugs devoted to regulating the adenosinergic system are promising in different clinical situations

Assessment of MRI scanner performance for preclinical functional studies

Functional Magnetic Resonance Imaging (fMRI) based studies are rapidly expanding in the field of preclinical research. The majority of these studies use Echo Planar Imaging (EPI) to measure Blood Oxygenation Level Dependent (BOLD) signal contrasts in the brain. In such studies the magnitude and statistical significances of these contrasts are then related to brain function and cognition. It is assumed that any observed signal contrast is ultimately due to differences in biological state and that scanner performance is stable and repeatable between subjects and studies. However, due to confounding issues introduced by in vivo subjects, little work has been undertaken to test this basic assumption. As the BOLD signal contrasts generated in such experiments are often very low, even small changes in scanner performance may dominate the BOLD contrast, distorting any biological conclusions drawn. A series of fMRI phantoms were produced to measure scanner performance independent of biological subjects. These phantoms produce specified signal contrast levels on demand during an fMRI scan by means of current-induced magnetic field gradients. These were used to generate data sets that emulated the BOLD signal contrast of in vivo imaging. Two studies examining scanner performance were then conducted on high-field preclinical MRI scanners. Firstly, in a longitudinal study on a single scanner, measurements were taken over a number of days across a week long period and then every two months over a year long period. Secondly, the behaviour of four preclinical scanners (three at 7T, one at 9.4T) was comparatively assessed. Measurements of several imaging parameters including contrast generated and functional contrast to noise ratio (fCNR) were obtained in both studies. If the scanners involved are truly comparable then they should generate similar measurement values. Across both studies parameter measurements showed significant differences for identical contrast settings on the phantom. Although signal contrast itself proved very comparable across the studies fCNR proved to be highly variable. As well as these measurements of longer tem behaviour proving variable, short and mid-term signal stability displayed a wide range of variability. Variations in the level and quality of both signal and noise were observed. Modelling of signal changes based on fundamental physical principles was also performed for comparison. The impact of these behaviours and variations on in vivo studies could result in skewed biological conclusions at any single site, with some sites exhibiting greater problems than others. The multisite results suggest potential difficulties when comparing biological conclusions between sites, even when using identical imaging parameters. In summary, these results suggest that a cautious approach should be taken with the conclusions of both fMRI and associated resting state connectivity studies that use EPI as their acquisition sequence. Improvements to both the experimental design of studies and regular quality monitoring of scanners should be undertaken to minimise these effects. Clinical MRI scanners should also be assessed for similar aberrations in behaviour

Intracavity frequency doubling of a Nd:YAG laser with an organic nonlinear optical crystal

Your access to Appl. Phys. Lett. is provided through the subscription of Univ Nebraska-Lincoln Lib. What is this? Scitation Citing Articles | CrossRef Citing Articles | All Citing Articles [ Previous / Next Abstract | Issue Table of Contents | Bottom of Page ] Applied Physics Letters -- August 6, 1990 -- Volume 57, Issue 6, pp. 537-539 Full Text: [ PDF (639 kB) ] Order Rightslink Permissions for Reuse About Rightslink view MyArticles What is this? Select up to 20 articles at a time. Intracavity frequency doubling of a Nd:YAG laser with an organic nonlinear optical crystal Stephen Ducharme, W. P. Risk, W. E. Moerner, Victor Y. Lee, R. J. Twieg, and G. C. Bjorklund IBM Almaden Research Center, San Jose, California 95120-6099 (Received 15 March 1990; accepted 29 May 1990) We have demonstrated intracavity second-harmonic generation of green 532 nm light in a quasi-cw 1064 nm Nd:YAG laser using organic nonlinear optical crystals of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN) immersed in index matching fluid contained in an antireflection-coated cuvette. This technique permits crystals to be used directly from solution growth without polishing or antireflection coating them. Up to 0.56 mW peak power of 532 nm light was generated from 2.3 W of intracavity 1064 nm peak power in 100 µs pulses. We also report preliminary results on true cw intracavity harmonic generation with antireflection-coated DAN crystals