153,451 research outputs found

    Quantum buoyancy, generalized second law, and higher-dimensional entropy bounds

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    Bekenstein has presented evidence for the existence of a universal upper bound of magnitude 2πR/c2\pi R/\hbar c to the entropy-to-energy ratio S/ES/E of an arbitrary {\it three} dimensional system of proper radius RR and negligible self-gravity. In this paper we derive a generalized upper bound on the entropy-to-energy ratio of a (D+1)(D+1)-dimensional system. We consider a box full of entropy lowered towards and then dropped into a (D+1)(D+1)-dimensional black hole in equilibrium with thermal radiation. In the canonical case of three spatial dimensions, it was previously established that due to quantum buoyancy effects the box floats at some neutral point very close to the horizon. We find here that the significance of quantum buoyancy increases dramatically with the number DD of spatial dimensions. In particular, we find that the neutral (floating) point of the box lies near the horizon only if its length bb is large enough such that b/bC>F(D)b/b_C>F(D), where bCb_C is the Compton length of the body and F(D)DD/21F(D)\sim D^{D/2}\gg1 for D1D\gg1. A consequence is that quantum buoyancy severely restricts our ability to deduce the universal entropy bound from the generalized second law of thermodynamics in higher-dimensional spacetimes with D1D\gg1. Nevertheless, we find that the universal entropy bound is always a sufficient condition for operation of the generalized second law in this type of gedanken experiments.Comment: 6 page

    Sustainable deathstyles? The geography of green burials in Britain

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    In the context of a wider literature on ‘deathscapes’, we map the emergence of a new mode of burial and remembrance in Britain. Since a ‘green’ burial ground was established in Carlisle in 1993, sites for so-called ‘green, ‘natural’ or ‘woodland’ funerals have proliferated. There are now over 270 such sites in Britain. Drawing on a postal and email survey sent to all managers/owners and visits to 15 green burial grounds (enabling observations and semi-structured interviews with their managers), we chart their growth, establishment and regulation and describe the landscapes associated with them. This requires, and leads to, wider reflections on nature, capital, consumption, culture and the body

    The beta subunit of the signal recognition particle receptor is a transmembrane GTPase that anchors the alpha subunit, a peripheral membrane GTPase, to the endoplasmic reticulum membrane.

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    The signal recognition particle receptor (SR) is required for the cotranslational targeting of both secretory and membrane proteins to the endoplasmic reticulum (ER) membrane. During targeting, the SR interacts with the signal recognition particle (SRP) which is bound to the signal sequence of the nascent protein chain. This interaction catalyzes the GTP-dependent transfer of the nascent chain from SRP to the protein translocation apparatus in the ER membrane. The SR is a heterodimeric protein comprised of a 69-kD subunit (SR alpha) and a 30-kD subunit (SR beta) which are associated with the ER membrane in an unknown manner. SR alpha and the 54-kD subunits of SRP (SRP54) each contain related GTPase domains which are required for SR and SRP function. Molecular cloning and sequencing of a cDNA encoding SR beta revealed that SR beta is a transmembrane protein and, like SR alpha and SRP54, is a member of the GTPase superfamily. Although SR beta defines its own GTPase subfamily, it is distantly related to ARF and Sar1. Using UV cross-linking, we confirm that SR beta binds GTP specifically. Proteolytic digestion experiments show that SR alpha is required for the interaction of SRP with SR. SR alpha appears to be peripherally associated with the ER membrane, and we suggest that SR beta, as an integral membrane protein, mediates the membrane association of SR alpha. The discovery of its guanine nucleotide-binding domain, however, makes it likely that its role is more complex than that of a passive anchor for SR alpha. These findings suggest that a cascade of three directly interacting GTPases functions during protein targeting to the ER membrane

    Improved Algorithms for Decremental Single-Source Reachability on Directed Graphs

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    Recently we presented the first algorithm for maintaining the set of nodes reachable from a source node in a directed graph that is modified by edge deletions with o(mn)o(mn) total update time, where mm is the number of edges and nn is the number of nodes in the graph [Henzinger et al. STOC 2014]. The algorithm is a combination of several different algorithms, each for a different mm vs. nn trade-off. For the case of m=Θ(n1.5)m = \Theta(n^{1.5}) the running time is O(n2.47)O(n^{2.47}), just barely below mn=Θ(n2.5)mn = \Theta(n^{2.5}). In this paper we simplify the previous algorithm using new algorithmic ideas and achieve an improved running time of O~(min(m7/6n2/3,m3/4n5/4+o(1),m2/3n4/3+o(1)+m3/7n12/7+o(1)))\tilde O(\min(m^{7/6} n^{2/3}, m^{3/4} n^{5/4 + o(1)}, m^{2/3} n^{4/3+o(1)} + m^{3/7} n^{12/7+o(1)})). This gives, e.g., O(n2.36)O(n^{2.36}) for the notorious case m=Θ(n1.5)m = \Theta(n^{1.5}). We obtain the same upper bounds for the problem of maintaining the strongly connected components of a directed graph undergoing edge deletions. Our algorithms are correct with high probabililty against an oblivious adversary.Comment: This paper was presented at the International Colloquium on Automata, Languages and Programming (ICALP) 2015. A full version combining the findings of this paper and its predecessor [Henzinger et al. STOC 2014] is available at arXiv:1504.0795

    A simultaneous generalization of independence and disjointness in boolean algebras

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    We give a definition of some classes of boolean algebras generalizing free boolean algebras; they satisfy a universal property that certain functions extend to homomorphisms. We give a combinatorial property of generating sets of these algebras, which we call n-independent. The properties of these classes (n-free and omega-free boolean algebras) are investigated. These include connections to hypergraph theory and cardinal invariants on these algebras. Related cardinal functions, nnInd, which is the supremum of the cardinalities of n-independent subsets; i_n, the minimum size of a maximal n-independent subset; and i_omega, the minimum size of an omega-independent subset, are introduced and investigated. The values of i_n and i_omega on P(omega)/fin are shown to be independent of ZFC.Comment: Sumbitted to Orde

    Effective temperature for black holes

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    The physical interpretation of black hole's quasinormal modes is fundamental for realizing unitary quantum gravity theory as black holes are considered theoretical laboratories for testing models of such an ultimate theory and their quasinormal modes are natural candidates for an interpretation in terms of quantum levels. The spectrum of black hole's quasinormal modes can be re-analysed by introducing a black hole's effective temperature which takes into account the fact that, as shown by Parikh and Wilczek, the radiation spectrum cannot be strictly thermal. This issue changes in a fundamental way the physical understanding of such a spectrum and enables a re-examination of various results in the literature which realizes important modifies on quantum physics of black holes. In particular, the formula of the horizon's area quantization and the number of quanta of area result modified becoming functions of the quantum "overtone" number n. Consequently, the famous formula of Bekenstein-Hawking entropy, its sub-leading corrections and the number of microstates are also modified. Black hole's entropy results a function of the quantum overtone number too. We emphasize that this is the first time that black hole's entropy is directly connected with a quantum number. Previous results in the literature are re-obtained in the limit n \to \infty.Comment: 10 pages,accepted for publication in Journal of High Energy Physics. Comments are welcom

    DBpedia's triple pattern fragments: usage patterns and insights

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    Queryable Linked Data is published through several interfaces, including SPARQL endpoints and Linked Data documents. In October 2014, the DBpedia Association announced an official Triple Pattern Fragments interface to its popular DBpedia dataset. This interface proposes to improve the availability of live queryable data by dividing query execution between clients and servers. In this paper, we present a usage analysis between November 2014 and July 2015. In 9 months time, the interface had an average availability of 99.99 %, handling 16,776,170 requests, 43.0% of which were served from cache. These numbers provide promising evidence that low-cost Triple Pattern Fragments interfaces provide a viable strategy for live applications on top of public, queryable datasets
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