Relativistic analysis of the LISA long range optical links

The joint ESA/NASA LISA mission consists in three spacecraft on heliocentric orbits, flying in a triangular formation of 5 Mkm each side, linked by infrared optical beams. The aim of the mission is to detect gravitational waves in a low frequency band. For properly processing the science data, the propagation delays between spacecraft must be accurately known. We thus analyse the propagation of light between spacecraft in order to systematically derive the relativistic effects due to the static curvature of the Schwarzschild spacetime in which the spacecraft are orbiting with time-varying light-distances. In particular, our analysis allows to evaluate rigorously the Sagnac effect, and the gravitational (Einstein) redshift.Comment: 6 figures; accepted for publication in PR

Transcription factor LSF facilitiates lysine methylation of α-tubulin by microtubule-associated SET8

Microtubules are critical for mitosis, cell motility, and protein and organelle transport, and are a validated target for anticancer drugs. However, tubulin regulation and recruitment in these cellular processes is less understood. Post-translational modifications of tubulin are proposed to regulate microtubule functions and dynamics. Although many such modifications have been investigated, tubulin methylations and enzymes responsible for methylation have only recently begun to be described. Here we report that N-lysine methyl transferase KMT5A (SET8/PR-Set7), which methylates histone H4K20, also methylates α-tubulin. Furthermore, the transcription factor LSF binds both tubulin and SET8, and enhances α-tubulin methylation in vitro, countered by FQI1, a specific small molecule inhibitor of LSF. Thus, the three proteins SET8, LSF, and tubulin, all essential for mitotic progression, interact with each other. Overall, these results point to dual functions for both SET8 and LSF not only in chromatin regulation, but also for cytoskeletal modification.First author draf

What Constitutes an Explanation in Biology?

One of biology's fundamental aims is to generate understanding of the living world around—and within—us. In this chapter, I aim to provide a relatively nonpartisan discussion of the nature of explanation in biology, grounded in widely shared philosophical views about scientific explanation. But this discussion also reflects what I think is important for philosophers and biologists alike to appreciate about successful scientific explanations, so some points will be controversial, at least among philosophers. I make three main points: (1) causal relationships and broad patterns have often been granted importance to scientific explanations, and they are in fact both important; (2) some explanations in biology cite the components of or processes in systems that account for the systems’ features, whereas other explanations feature large-scale or structural causes that influence a system; and (3) there can be multiple different explanations of a given biological phenomenon, explanations that respond to different research aims and can thus be compatible with one another even when they may seem to disagree

Gravitational waves about curved backgrounds: a consistency analysis in de Sitter spacetime

Gravitational waves are considered as metric perturbations about a curved background metric, rather than the flat Minkowski metric since several situations of physical interest can be discussed by this generalization. In this case, when the de Donder gauge is imposed, its preservation under infinitesimal spacetime diffeomorphisms is guaranteed if and only if the associated covector is ruled by a second-order hyperbolic operator which is the classical counterpart of the ghost operator in quantum gravity. In such a wave equation, the Ricci term has opposite sign with respect to the wave equation for Maxwell theory in the Lorenz gauge. We are, nevertheless, able to relate the solutions of the two problems, and the algorithm is applied to the case when the curved background geometry is the de Sitter spacetime. Such vector wave equations are studied in two different ways: i) an integral representation, ii) through a solution by factorization of the hyperbolic equation. The latter method is extended to the wave equation of metric perturbations in the de Sitter spacetime. This approach is a step towards a general discussion of gravitational waves in the de Sitter spacetime and might assume relevance in cosmology in order to study the stochastic background emerging from inflation.Comment: 17 pages. Misprints amended in Eqs. 50, 54, 55, 75, 7

From Cambridge Keynesian to Institutional Economist: The Unnoticed Contributions of Robert Neild

This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.Robert Neild (born 1924) has made a major contribution to economics and to peace studies. This paper provides a brief sketch of Neild’s life and work. While noting his research in economic policy and peace studies, this essay devotes more attention to his largely-unnoticed contributions to institutional and evolutionary economics since 1984. These are important in their own right, but they are especially notable because Cambridge heterodox economists have been devoted mainly to other approaches, including Marxism and post-Keynesianism. Neild’s distinctive contribution is partly explained by his closeness to both Nicholas Kaldor and Gunnar Myrdal. Myrdal made explicit his adherence to the original American institutionalism: Neild extended that link to Cambridge.Peer reviewedFinal Published versio

Heat Kernel Asymptotics on Homogeneous Bundles

We consider Laplacians acting on sections of homogeneous vector bundles over symmetric spaces. By using an integral representation of the heat semi-group we find a formal solution for the heat kernel diagonal that gives a generating function for the whole sequence of heat invariants. We argue that the obtained formal solution correctly reproduces the exact heat kernel diagonal after a suitable regularization and analytical continuation.Comment: 29 pages, Proceedings of the 2007 Midwest Geometry Conference in Honor of Thomas P. Branso

Complex Kerr Geometry and Nonstationary Kerr Solutions

In the frame of the Kerr-Schild approach, we consider the complex structure of Kerr geometry which is determined by a complex world line of a complex source. The real Kerr geometry is represented as a real slice of this complex structure. The Kerr geometry is generalized to the nonstationary case when the current geometry is determined by a retarded time and is defined by a retarded-time construction via a given complex world line of source. A general exact solution corresponding to arbitrary motion of a spinning source is obtained. The acceleration of the source is accompanied by a lightlike radiation along the principal null congruence. It generalizes to the rotating case the known Kinnersley class of "photon rocket" solutions.Comment: v.3, revtex, 16 pages, one eps-figure, final version (to appear in PRD), added the relation to twistors and algorithm of numerical computations, English is correcte

Multiquadric interpolation: Surface fitting in three-dimensional space

Using mathematics to solve a problem does not always yield a perfect or absolute answer but may instead yield an approximate solution. We can try to approximate the solution as precisely as possible by using the mathematical tools and skills that are available to us or we could try to discover new methods which would enable us to find good approximations. It is important that we have precise approximating tools to begin with, so that we may preserve as much accuracy as possible. We can find such problems in the world around us. For instance, if we try to construct a topographical map of a mountainous region, first we gather data by measuring some elevations and locations. The data is then used to construct the map. We now realize that because measuring every dip and valley of the area would be an impossible task, the map must be constructed from a set of random points. The next step is either to guess about the elevations between the data points, if there are enough points close enough together, or to estimate these elevations mathematically. Since we would like to finish constructing the map by taking small regions around the known data points and finding approximating functions which, when graphed, will represent as precisely as possible the surface of the region, an entirely new problem arises. These surfaces around the known points cannot be easily calculated by use of simple functions. We now need to use these few, random data point to find an approximating surface by means of an interpolation method

¿Por qué el estegosaurio tiene placas, o es la biología una ciencia de segunda clase porque piensa en términos de fines?

There is something distinctively different about explanation in the biological sciences, as opposed to explanation in the physical sciences. In the former one has functional arguments, arguments making reference to what Aristotle called “final causes.” As in: “The function of the plates on the back of the Stegosaurus was to keep the body at a constant temperature.” Since the Scientific Revolution, such explanations have been forbidden in the physical sciences. Does this then mean that biology is second rate, as is suggested by many including Immanuel Kant? It is argued that the Darwinian mechanism of natural selection explains why there is need of functional explanation in biology and that once this point is grasped, there is no reason to judge biology second rate.Hay algo distintivamente diferente en la explicación propia de las ciencias biológicas, en tanto opuesta a la explicación en las ciencias físicas. En las primeras tenemos argumentos funcionales, argumentos que hacen referencia a lo que Aristóteles llamó “causas finales”; por ejemplo, “La función de las placas de la parte posterior del estegosaurio era mantener el cuerpo a una temperatura constante”. Desde la Revolución Científica, explicaciones de este tipo han sido prohibidas en la física. ¿Significa esto que la biología es de segunda clase, como sugieren algunos, entre ellos Immanuel Kant? Se defenderá en este artículo que el mecanismo darwiniano de selección natural explica por qué hay necesidad de una explicación funcional en biología y, que una vez que se comprende este punto, no hay razón para juzgar a la biología como ciencia de segunda clase

Evolutionary biology and beliefs : how ideology can draw different social stances from science

Agreeing that there are often strong connections between fields of science and the ideological convictions of those producing the science, this essay shows that the connections are often complex and rarely straightforward. Taking the example of evolutionary biology, by looking at three key figures ? Herbert Spencer, Charles Darwin and Alfred Russel Wallace ? it is shown how very different social beliefs can lead to very different social conclusions being drawn from one?s science. It is argued that this message should be kept firmly in mind by those who today would draw social conclusions from science, for instance suggesting that Darwinian evolutionary biology leads straight to the social philosophy of the Third Reich. The truth is always far more complex