EPJ Plus Highlight - Simulations’ Achille’s heel

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Example of periodic repetition of a small system. © D. Frenkel et al.

What can go wrong when computer simulations are applied outside their original context

In an article just published in EPJ Plus, Daan Frenkel from the University of Cambridge, UK, outlines the many pitfalls associated with simulation methods such as Monte Carlo algorithms or other commonly used molecular dynamics approaches.

The context of this paper is the exponential development of computing power in the past 60 years, estimated to have increased by a factor of 1015, in line with Moore’s law. Today, short simulations can reproduce a system the size of a bacterium.

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EPJ Plus Highlight - LARES: a nearly ideal satellite to test fundamental physics

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The LARES satellite (courtesy of ASI)

The discovery of the accelerating expansion of the Universe, thought to be driven by a mysterious form of ‘dark energy’ constituting most of the Universe, has further revived the interest in testing Einstein’s theory of General Relativity (GR). Frame-dragging in the gravitational field generated by a rotating body or by a current of mass-energy is one of the most fascinating phenomena predicted by GR. The recently launched LARES (Laser RElativity Satellite) space mission is aimed at improving of about an order of magnitude the accuracy of the previous frame-dragging measurements by the LAGEOS and LAGEOS 2 satellites, using GRACE-derived Earth gravity determinations. After some years of orbital analysis of LARES, LAGEOS and LAGEOS 2 satellite laser ranging data, we should reach a few percent uncertainty in testing frame-dragging.

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EPJ Plus Focus Point - Interdisciplinary Science with Cosmic Rays

Interdisciplinary Science with Cosmic Rays Guest Editors: Antonio Bueno and Lawrence Wiencke

The Pierre Auger Observatory, located near the base of the Argentinean Andes, is the largest cosmic ray facility in the world. Spanning 3,000 km2, its complementary detector systems use the troposphere as a giant calorimeter to measure the highest-energy subatomic particles known to mankind. Because this instrument observes both the earth and the cosmos in unique ways, its interdisciplinary significance extends to the atmospheric and earth sciences. The articles comprising this EPJ Plus Focus Point highlight examples of observations in these fields and discuss several beginning projects. As shown in the introductory remarks (A. Watson), these articles are intended to reach a broad audience, both in order to stimulate discussion and to encourage new collaborative efforts of an interdisciplinary nature.

The observatory is introduced by Wiencke et al. with examples such as a major earthquake that was observed by the observatory’s surface detector. A proposed seismic sensor array to be located at the observatory is also described by Ruigrok et al. The observatory’s ground-based atmospheric monitoring program is arguably the most extensive in the southern hemisphere. Aerosols and clouds play a complicated role in the earth’s climate and there are fewer detailed measurementsavailable from the southern hemisphere. The measurements of the atmospheric molecular component are described by Keilhauer et al., including a comparison between local radiosonde measurements and extrapolations from the Global Data Assimilation System. The subsequent article by Louedec et al. reviews methods used at the observatory to characterizeaerosols and atmospheric clarity, and outlines a proposed project to study the origin and transport of iron-rich aerosols that play a role in biological processes in the southern ocean. The article by Tonachini et al. discusses the lidar systems at the observatory. The demonstration how measurements of clouds are used to ground truth comparisons with GOES satellite datais the subject of the article by Chirinos et al. Next, Mussa et al. describe the serendipitous observation of transient luminescent events (ELVES) created above some thunderstorms. This article also demonstrates a detailed measurement of anELVE’s time evolution using the observatory’s air fluorescence detector. Finally, cosmic ray air showers have been proposed as a possible trigger mechanism for lightning. A closing article (Brown et al.) discusses a lightning detection system planned to test this hypothesis.

To view this focus point and others already published, please click here.

EPJ Plus Focus Point - Deep Underground Science Laboratories and Projects

Deep Underground Science Laboratories and Projects Guest Editor: Alessandro Bettini

Physicists have developed a theoretical description of the elementary building blocks of matter and of the basic forces of Nature, called the Standard Model. It is the most comprehensive theory ever developed and has been tested with high precision up to energies of a few hundred times the proton mass. A new collider, the LHC, has started to work at still higher energies, discovering the last missing element of the SM, the so-called Higgs boson. However we know already that this, and any accelerator of the future, will not be sufficient.

The reason is that three of the four basic forces of Nature, namely strong, electromagnetic and weak, seem to become equal at high energies. Unfortunately the energy scale of the unification is extremely high, so high that we will never be able to reach it with an accelerator. Even higher is the Planck scale, the Big Bang energy, at which, presumably, also the fourth force, gravitation, becomes unified. We can exploit an indirect way, because phenomena characterised by a high-energy scale do, in fact, happen naturally even at the lower, every day, energies. However the higher their intrinsic energy scale is, the more rarely they happen.

The deep underground laboratories are dedicated to the search for these natural, but extremely rare nuclear and subnuclear phenomena, requiring a very low radioactive background environment. The background is due to cosmic rays and to decays of radioactive nuclei present, in traces, in all materials. Underground laboratories, shielded from cosmic and radioactive radiations, have been built in Europe, Japan and North America. More are planned in China, India and South America. They differ in size, depth and organisation, but their scientific programmes are similar and complementary. Other disciplines, like geodynamics and biology, can profit from the unique environment of the underground facilities.

The author of the paper of each laboratory or project is the Director of the given Lab or the PI of the given project.

To view this focus point and others already published, please click here.

EPJ Plus Highlight – Statistical uncertainty in line shift and width interpretation

A general statistical analysis (classical statistics) is a common experimental procedure to determine the uncertainty of photon statistics in measuring a line shift and width. Given the importance of taking into account the background as well as the measured signal in any photon measurement, the paper describes both the perfect spectrometer measurements with a zero and nonzero background as well as the case of an imperfect spectrometer.

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EPJ Plus – Determination of a time shift in the OPERA setup using high-energy horizontal muons in the LVD and OPERA detectors

Distribution of the ?t = tLVD - t*OPERA for corrected events. All the events of each year are grouped into one single point with the exception of 2008, which is subdivided into three periods.

The halls of the INFN Gran Sasso Laboratory (LNGS) were built in the 1980s based on the design of A. Zichichi and oriented towards CERN for experiments on neutrino beams. In 2006, the CERN Neutrinos to Gran Sasso (CNGS) beam was ready and the search could start for tau-neutrino appearances in the muon-neutrino beam produced at CERN. The OPERA detector was designed and built for this purpose.

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EPJ Plus – The “inertia of heat” concept revisited

What is the general relativistic version of the Navier-Stokes-Fourier dissipative hydrodynamics? Surprisingly, no satisfactory answer to this question is known today. Eckart's early solution [Eckart, Phys. Rev. 58, 919 (1940)], is considered outdated on many grounds: the instability of its equilibrium states, ill-posed initial-value formulation, inconsistency with linear irreversible thermodynamics, etc. Although alternative theories have been proposed recently, none appears to have won the consensus.

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EPJ Plus - A note on Pöschl-Teller black holes

An interesting feature of black holes is the existence of quasi-normal modes, arising because the system has a peak in the wave potential (scalar, electromagnetic, or gravitational waves). The quasi-normal mode is excited when a disturbance is put in the field near but outside the black hole, (like a wave packet roughly in a circular orbit near the peak). The excitation then propagates outward and inward and decays.

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EPJ Plus - The twin paradox in a cosmological context

The twin paradox has been a source of debate since it was discovered by Einstein. It can be analytically verified assuming the existence of global nonrotating inertial frames.

The natural nonrotating frame and its identification with "fixed stars" is an aspect of Mach's Principle, which holds that the totality of matter in the universe determines the inertial frames.

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EPJ Plus - Discovering Technicolor

Cartoon of the Minimal Walking Technicolor Model extension of the SM.

At present there are no known elementary scalar fields. A possible candidate is the as yet undiscovered Higgs particle; however it could well be that this elusive particle is instead composite. This possibility is exhaustively examined in this article, which is both tutorial and extensive review, classifying the diverse technicolor models as extensions of the Standard Model of particle physics.

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