4,357 research outputs found
Real Time Relativity: exploration learning of special relativity
Real Time Relativity is a computer program that lets students fly at
relativistic speeds though a simulated world populated with planets, clocks,
and buildings. The counterintuitive and spectacular optical effects of
relativity are prominent, while systematic exploration of the simulation allows
the user to discover relativistic effects such as length contraction and the
relativity of simultaneity. We report on the physics and technology
underpinning the simulation, and our experience using it for teaching special
relativity to first year university students
Targeted search for continuous gravitational waves: Bayesian versus maximum-likelihood statistics
We investigate the Bayesian framework for detection of continuous
gravitational waves (GWs) in the context of targeted searches, where the phase
evolution of the GW signal is assumed to be known, while the four amplitude
parameters are unknown. We show that the orthodox maximum-likelihood statistic
(known as F-statistic) can be rediscovered as a Bayes factor with an unphysical
prior in amplitude parameter space. We introduce an alternative detection
statistic ("B-statistic") using the Bayes factor with a more natural amplitude
prior, namely an isotropic probability distribution for the orientation of GW
sources. Monte-Carlo simulations of targeted searches show that the resulting
Bayesian B-statistic is more powerful in the Neyman-Pearson sense (i.e. has a
higher expected detection probability at equal false-alarm probability) than
the frequentist F-statistic.Comment: 12 pages, presented at GWDAW13, to appear in CQ
The age of data-driven proteomics : how machine learning enables novel workflows
A lot of energy in the field of proteomics is dedicated to the application of challenging experimental workflows, which include metaproteomics, proteogenomics, data independent acquisition (DIA), non-specific proteolysis, immunopeptidomics, and open modification searches. These workflows are all challenging because of ambiguity in the identification stage; they either expand the search space and thus increase the ambiguity of identifications, or, in the case of DIA, they generate data that is inherently more ambiguous. In this context, machine learning-based predictive models are now generating considerable excitement in the field of proteomics because these predictive models hold great potential to drastically reduce the ambiguity in the identification process of the above-mentioned workflows. Indeed, the field has already produced classical machine learning and deep learning models to predict almost every aspect of a liquid chromatography-mass spectrometry (LC-MS) experiment. Yet despite all the excitement, thorough integration of predictive models in these challenging LC-MS workflows is still limited, and further improvements to the modeling and validation procedures can still be made. In this viewpoint we therefore point out highly promising recent machine learning developments in proteomics, alongside some of the remaining challenges
Dynamics of Plume-Triple Junction Interaction: Results from a Series of Three-Dimensional Numerical Models and Implications for the Formation of Oceanic Plateaus
Mantle plumes rising in the vicinity of mid-ocean ridges often generate anomalies in melt production and seafloor depth. This study investigates the dynamical interactions between a mantle plume and a ridge-ridge-ridge triple junction, using a parameter space approach and a suite of steady state, three-dimensional finite element numerical models. The top domain boundary is composed of three diverging plates, with each assigned half-spreading rates with respect to a fixed triple junction point. The bottom boundary is kept at a constant temperature of 1350Ā°C except where a two-dimensional, Gaussian-shaped thermal anomaly simulating a plume is imposed. Models vary plume diameter, plume location, the viscosity contrast between plume and ambient mantle material, and the use of dehydration rheology in calculating viscosity. Importantly, the model results quantify how plume-related anomalies in mantle temperature pattern, seafloor depth, and crustal thickness depend on the specific set of parameters. To provide an example, one way of assessing the effect of conduit position is to calculate normalized area, defined to be the spatial dispersion of a given plume at specific depth (here selected to be 50km) divided by the area occupied by the same plume when it is located under the triple junction. For one particular case modeled where the plume is centered in an intraplate position 100km from the triple junction, normalized area is just 55%. Overall, these models provide a framework for better understanding plateau formation at triple junctions in the natural setting and a tool for constraining subsurface geodynamical processes and plume properties
Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event
The discovery of the Sagittarius dwarf galaxy, which is being tidally
disrupted by and merging with the Milky Way, supports the view that the halo of
the Galaxy has been built up at least partially by the accretion of similar
dwarf systems. The Sagittarius dwarf contains several distinct populations of
stars, and includes M54 as its nucleus, which is the second most massive
globular cluster associated with the Milky Way. The most massive globular
cluster is omega Centauri, and here we report that omega Centauri also has
several distinct stellar populations, as traced by red-giant-branch stars. The
most metal-rich red-giant-branch stars are about 2 Gyr younger than the
dominant metal-poor component, indicating that omega Centauri was enriched over
this timescale. The presence of more than one epoch of star formation in a
globular cluster is quite surprising, and suggests that omega Centauri was once
part of a more massive system that merged with the Milky Way, as the
Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were
much more frequent in the early history of the Galaxy and omega Centauri
appears to be a relict of this era.Comment: 7 pages, 3 figures, Latex+nature.sty (included), To appear in
November 4th issue of Natur
Double Exchange Alone Does Not Explain the Resistivity of
The system with has
traditionally been modelled with a ``double exchange'' Hamiltonian, in which it
is assumed that the only relevant physics is the tendency of carrier hopping to
line up neighboring spins. We present a solution of the double exchange model,
show it is incompatible with many aspects of the resistivity data, and propose
that a strong electron-phonon interaction arising from a Jahn-Teller splitting
of the outer Mn d-level plays a crucial role.Comment: Figure available via concentional mail. Contact
[email protected]
Structure, magnetic and transport properties of Ti-substituted La0.7Sr0.3MnO3
Ti-substituted perovskites, La0.7Sr0.3Mn1-xTixO3, with x between 0 to 0.20,
were investigated by neutron diffraction, magnetization, electric resistivity,
and magnetoresistance (MR) measurements. All samples show a rhombohedral
structure (space group R3c) from 10 K to room temperature. At room temperature,
the cell parameters a, c and the unit cell volume increase with increasing Ti
content. However, at 10 K, the cell parameter a has a maximum value for x =
0.10, and decreases for x greater than 0.10, while the unit cell volume remains
nearly constant for x greater than 0.10. The average (Mn,Ti)-O bond length
increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with
increasing Ti content to its minimum value at x=0.15 at room temperature. Below
the Curie temperature T_C, the resistance exhibits metallic behavior for the x
_ 0.05 samples. A metal (semiconductor) to insulator transition is observed for
the x_ 0.10 samples. A peak in resistivity appears below T_C for all samples,
and shifts to a lower temperature as x increases. The substitution of Mn by Ti
decreases the 2p-3d hybridization between O and Mn ions, reduces the bandwidth
W, and increases the electron-phonon coupling. Therefore, the TC shifts to a
lower temperature and the resistivity increases with increasing Ti content. A
field-induced shift of the resistivity maximum occurs at x less than or equal
to 0.10. The maximum MR effect is about 70% for La0.7Sr0.3Mn0.8Ti0.2O3. The
separation of TC and the resistivity maximum temperature Tmax enhances the MR
effect in these compounds due to the weak coupling between the magnetic
ordering and the resistivity as compared with La0.7Sr0.3MnO3.Comment: zip fil
The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Profiles of Barred Galaxies
We present a study of 66 barred, early-type (S0-Sb) disk galaxies, focused on
the disk surface brightness profile outside the bar region and the nature of
Freeman Type I and II profiles, their origins, and their possible relation to
disk truncations. This paper discusses the data and their reduction, outlines
our classification system, and presents -band profiles and classifications
for all galaxies in the sample.
The profiles are derived from a variety of different sources, including the
Sloan Digital Sky Survey (Data Release 5). For about half of the galaxies, we
have profiles derived from more than one telescope; this allows us to check the
stability and repeatability of our profile extraction and classification. The
vast majority of the profiles are reliable down to levels of mu_R ~ 27 mag
arcsec^-2; in exceptional cases, we can trace profiles down to mu_R > 28. We
can typically follow disk profiles out to at least 1.5 times the traditional
optical radius R_25; for some galaxies, we find light extending to ~ 3 R_25.
We classify the profiles into three main groups: Type I (single-exponential),
Type II (down-bending), and Type III (up-bending). The frequencies of these
types are approximately 27%, 42%, and 24%, respectively, plus another 6% which
are combinations of Types II and III. We further classify Type II profiles by
where the break falls in relation to the bar length, and in terms of the
postulated mechanisms for breaks at large radii ("classical trunction" of star
formation versus the influence of the Outer Lindblad Resonance of the bar). We
also classify the Type III profiles by the probable morphology of the outer
light (disk or spheroid). Illustrations are given for all cases. (Abridged)Comment: 41 pages, 26 PDF figures. To appear in the Astronomical Journal.
Version with full-resolution figures available at
http://www.mpe.mpg.de/~erwin/research
Robot rights? Towards a social-relational justification of moral consideration \ud
Should we grant rights to artificially intelligent robots? Most current and near-future robots do not meet the hard criteria set by deontological and utilitarian theory. Virtue ethics can avoid this problem with its indirect approach. However, both direct and indirect arguments for moral consideration rest on ontological features of entities, an approach which incurs several problems. In response to these difficulties, this paper taps into a different conceptual resource in order to be able to grant some degree of moral consideration to some intelligent social robots: it sketches a novel argument for moral consideration based on social relations. It is shown that to further develop this argument we need to revise our existing ontological and social-political frameworks. It is suggested that we need a social ecology, which may be developed by engaging with Western ecology and Eastern worldviews. Although this relational turn raises many difficult issues and requires more work, this paper provides a rough outline of an alternative approach to moral consideration that can assist us in shaping our relations to intelligent robots and, by extension, to all artificial and biological entities that appear to us as more than instruments for our human purpose
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