Estimating Lyapunov exponents in billiards

Dynamical billiards are paradigmatic examples of chaotic Hamiltonian dynamical systems with widespread applications in physics. We study how well their Lyapunov exponent, characterizing the chaotic dynamics, and its dependence on external parameters can be estimated from phase space volume arguments, with emphasis on billiards with mixed regular and chaotic phase spaces. We show that in the very diverse billiards considered here the leading contribution to the Lyapunov exponent is inversely proportional to the chaotic phase space volume, and subsequently discuss the generality of this relationship. We also extend the well established formalism by Dellago, Posch, and Hoover to calculate the Lyapunov exponents of billiards to include external magnetic fields and provide a software implementation of it

Global infrasound observations and their relation to atmospheric tides and mountain waves

Niederfrequenter Schall (Infraschall) kann aufgrund geringer Dämpfung — je nach Zustand der Atmosphäre — Distanzen von wenigen hundert bis einigen tausend Kilometern zurücklegen. Diese Eigenschaft wird zur Registrierung atmosphärischer Explosionen genutzt: Nachdem das Kernwaffenteststoppabkommen (CTBT) von den Vereinten Nationen im Jahr 1996 zur Unterzeichnung aufgelegt worden war, wurde ein globales Messnetz (IMS) konzipiert, das in der Lage sein soll, Explosionen mit einer Ladungsstärke von mindestens einer Kilotonne TNT-Äquivalent weltweit zu detektieren. Sechzig Infraschallstationen des IMS registrieren entsprechende Druckschwankungen in Größenordnungen von 10^(-3) Pa bis 10 Pa. Diese Arbeit befasst sich mit der Bestimmung von natürlichen atmosphärischen Wellen verschiedener Skalen aus den barometrischen Datensätzen des IMS-Infraschallmessnetzes. Der Schwerpunkt liegt auf dem niederfrequenten Teil des registrierten Spektrums von Infraschallwellen, d. h. Perioden von 2 s bis 100 s. Zunächst wird gezeigt, dass sich die hohe Präzision der barometrischen Daten und die weltweite Verteilung der IMS-Stationen eignen, ein breites Spektrum atmosphärisch-dynamischer Phänomene zu quantifizieren. Auffällig sind dabei die atmosphärischen Gezeiten, die deutlich bei Perioden von 24 h, 12 h und 8 h nachweisbar sind. Die geografische und saisonale Variabilität dieser speziellen, großskaligen Form von atmosphärischen Schwerewellen werden mit dem IMS-Infraschallmessnetz akkurat erfasst. Die spektrale Analyse der differentiellen Druckdaten hebt zusätzlich zu Gezeiten auch einen kurzperiodischen Bereich hervor. Es handelt sich um kohärente Strukturen, genannt Mikrobarome, die nahezu permanent durch Interaktion gegenläufiger ozeanischer Wellen entstehen und weltweit als Infraschall mit Perioden von 2 s bis 10 s registriert werden. Die Wind- und Temperaturverteilung der mittleren Atmosphäre bestimmt die Detektierbarkeit der Signale, ist in Wettermodellen oft jedoch nicht präzise wiedergegeben. Zur Quantifizierung von Modellunsicherheiten wurden an der Infraschallstation IS26 im bayerischen Wald mithilfe eines Lidars Temperaturprofile im Höhenbereich von 20 km bis 90 km gemessen. Unter Verwendung eines Quellenmodells sowie einer atmosphärischen Dämpfungsrelation ließen sich die Unsicherheiten erstmalig in Mikrobaromamplituden übertragen. Dies ermöglichte bis zu 97 % der Detektionen hinsichtlich ihrer Variabilität in Ursprungsrichtung und Amplitude zu erklären. Mikrobarome und Gezeitenwellen unterscheiden sich aufgrund der verschiedenen Skalen in ihrer rücktreibenden Druckkraft bzw. Schwerkraft. Atmosphärische Schwerewellen kennzeichnen einen breiten Spektralbereich zwischen Infraschall und den Gezeiten. Orografische Infraschallwellen (engl. Mountain-associated Waves, MAWs), deren Entstehungsmechanismus bislang nicht abschließend erforscht worden ist, wurden hier auf einen Zusammenhang mit orografischen Schwerewellen untersucht. MAWs sind ähnlich wie Mikrobarome kohärente Strukturen, die mit Perioden von 10 s bis 100 s über Distanzen von tausenden Kilometern detektiert werden können. Die IMS-Daten ermöglichten im Rahmen dieser Arbeit erstmals eine globale Analyse der MAWs. Mittels Kreuzpeilung konnten globale Quellregionen dieses Phänomens monatsweise bestimmt werden. Es wird gezeigt, dass MAWs mit troposphärischen Winden korrelieren. Diese allein erklären jedoch nicht die saisonale Variabilität in MAW-Detektionen. Mögliche weitere in der Entstehung von MAWs relevante Prozesse werden diskutiert, zum Beispiel das Brechen orografischer Schwerewellen. Ein Vergleich mit aus Satellitendaten bestimmten orografischen Quellregionen von Schwerewellen deutet darauf hin, dass diese mit denen von MAWs übereinstimmen — und zwar auch in Regionen, in denen die vertikale Ausbreitung von Schwerewellen durch ein Windminimum in der Stratosphäre unterdrückt wird. Wenn sich das Wellenbrechen in weiteren Studien als primäre Anregung orografischer Infraschallwellen bestätigt, kann das Auftreten von orografischen Schwerewellen global mit dem IMS-Infraschallnetzwerk abgeleitet werden.Infrasound can propagate through the atmosphere over distances of hundreds to thousands of kilometers as a result of low absorption, depending on the state of the atmosphere. This property is utilized to record atmospheric explosions. Following the opening by the United Nations of the Comprehensive Nuclear-Test-Ban Treaty for signature in 1996, the International Monitoring System (IMS) was designed in order to detect explosions with a minimum yield of one kiloton of TNT equivalent worldwide. Sixty IMS infrasound stations have since been recording corresponding pressure fluctuations of the order of 10^(−3) Pa to 10 Pa. This thesis reports on the determination of atmospheric waves, on different scales, from the barometric datasets of the IMS infrasound network. The focus was on the low-frequency part of the recorded spectrum of infrasonic waves, i.e., periods of 2 s to 100 s. The high precision of the barometric data and the worldwide distribution of the IMS stations were utilized to characterize a broad spectrum of atmospheric-dynamic phenomena. Dominant features include the thermal atmospheric tides, which are clearly distinguished at periods of 24 h, 12 h and 8 h in spectral analyses. The IMS infrasound network allowed the accurate characterization of the geographic and seasonal variability of this specific large-scale type of atmospheric gravity wave. In addition to these tides, spectral analysis of the differential pressure data also highlights a short-period range (2–10 s). This reflects coherent structures — microbaroms — that almost permanently produce infrasound detections worldwide. Microbaroms originate from the interaction of opposing ocean surface waves. The distribution of winds and temperature in the middle atmosphere determines the detectability of such signals. In weather models, however, the middle atmosphere is only represented to a limited extent. To quantify model uncertainties, temperature profiles in the altitude range of 20 km to 90 km were measured at infrasound station IS26 in Germany using a mobile lidar system. Combining a source model with an atmospheric attenuation relation allowed the transfer of these uncertainties into microbarom amplitudes for the first time. These explained up to 97 % of the detections, in terms of their variability in origin and amplitude. Microbaroms and tidal waves differ in their restoring force — namely, pressure and buoyancy — due to their different scales. Atmospheric gravity waves reflect a broad spectral range between infrasound and the tides. A relation between orographic gravity waves and infrasonic mountain-associated waves (MAWs), of which the source generation mechanism has not yet been fully explored, was investigated here. Similarly to microbaroms, MAWs are coherent structures at periods of 10 s to 100 s that can propagate over distances of thousands of kilometers. The IMS data, for the first time, enabled a global analysis of MAWs. A cross-bearing method determined global source regions of this phenomenon on a monthly basis. It is shown that the MAWs correlate with tropospheric winds; however, the latter are not sufficient to explain the seasonal variability in MAW detections. Further possible processes being involved in the excitation of MAWs are discussed, including breaking orographic gravity waves. The comparison with gravity wave source regions, derived from satellite data, suggests that MAW source regions match those of orographic gravity waves, even when vertically propagating gravity waves are filtered due to a stratospheric wind minimum causing the waves to break. If this process, in future studies, turns out to induce the MAWs, their occurrence detected by the IMS infrasound network can allow monitoring orographic gravity wave activity globally

Making a Case for the State: Review essay on Paul du Gay and Thomas Lopdrup-Hjorth (2023), For Public Service: State, Office and Ethics

'For Public Service' is an important book. With their focus on the rationale of the State, the authors address what is needed to be a good public servant. Since New Public Management became omnipresent, they see reason to do so. Its 'market-mimicking' has turned public servants from role-bound personae into individuals, at the expense of attention to due process. More generally, a widespread anti-étatism has neglected the requirements inherent to acting on behalf of the state in an official capacity. Instead, the authors want to offer a positive account of the state. In their venture in political realism, they go far, adopting a relativist stance towards the impact of a humanist universalism. The authors emphasize structural elements but seem to ignore the politics of the state in action. Dilemmas of conduct in the practice of public office-holding need attention too. For instance, how are public servants to deal with discriminatory political directives?'For Public Service' ist ein bedeutendes Buch. Mit ihrem Fokus auf Grundüberlegungen bezüglich des Staates thematisieren die Autoren, was es bedeutet, öffentliche*r Bedienstete*r zu sein. Den Grund dafür liefert die aufkommende Allgegenwärtigkeit des New Public Management Ansatzes. Der 'marktähnliche' Ansatz des New Public Management hat das Verständnis von öffentlichen Bediensteten als an eine Rolle gebundenen Personen in das eines Individuums verändert, und das auf Kosten der Bedeutung ordnungsgemäßer Verfahren. Allgemeiner hat ein weitverbreiteter anti-étatism die Anforderungen, die mit dem Handeln im Auftrag des Staates in offizieller Funktion einhergehen, vernachlässigt. Dementgegen wollen die Autoren eine positive Darstellung des Staates bieten. In ihrem Vorstoß in politischem Realismus, nehmen sie eine relativistische Haltung gegenüber dem Einfluss eines humanistischen Universalismus ein. Die Autoren betonen strukturelle Elemente des Staates, scheinen jedoch die Politik des Staates in Aktion außer Acht zu lassen. Dilemmata in der konkreten Ausübung eines öffentlichen Amtes bedürfen es auch beachtet zu werden. Zum Beispiel: wie sollten öffentliche Bedienstete mit diskriminierenden politischen Anweisungen umgehen

Powers behind control: An essay on democracy

In contemporary Western democracies the role of government is not what it used to be. In the public discourse some authors claim to observe a ‘relocation of politics’, while others speak of a ‘democratic deficit’ in general. In this essay the relationship between democracy and governance is explored on a macro-level. The argument is that performance in the public domain and the decisions underlying it no longer in a direct way can be traced down to expressions of societal demands. This is because both the relationships between society and democracy and between politics and performance have become looser. In particular, the hierarchical relation between democracy and government has been replaced by a more horizontal pairing of democracy and governance. As the latter has multiple dimensions, entailing a range of activities performed at various spots by a variety of actors, it appears that democracy, as well, cannot appropriately get substance and form in a singular way anymore. Enhancing the visibility of who is involved in the processes leading to public decisions, combined with enlarging possibilities for accountability, ‘multi-localisation’ provides a conceptual perspective for rethinking contemporary democracy.Session 3: Institutional rearrangement of the public domai

Dimensions of Discretion: Specifying the Object of Street-Level Bureaucracy Research

__Abstract__ At the street level of the state public policies get their final form and substance. This being so, discretion is a key concept. The goal of this article is to specify discretion as a research object in the study of street-level bureaucracy. Therefore the theoretical views on discretion prevalent in juridical and other disciplines are explored. Discretion appears to be a multi-faceted concept. This finding has consequences for the analysis of discretion in the explanation of what happens in street-level bureaucracies

SMARTER CROWDWORK BY APPLYING SMART CONTRACTS?

Crowdworking is characterized by flexibly engaging a workforce that is recruited for only one task. After the task is completed, the crowdworker and the company do not collaborate anymore. To identify the best-suited crowdwoker for a certain task, crowd working platform providers offer different tools to match both parties. Matching a crowdworker with the required skills for a task is an important factor to complete a crowdworking campaign successfully. As of today, the principal agent theorem and high transaction costs decrease the effectiveness of such campaigns due to information asymmetries between crodworkers and crowdsourcers. Based on a qualitative approach, we structurally develop a currently existing crowdworking campaign process. We then theoretically analyze how smart contracts can support and simplify the identified process while reducing the challenges of transaction cost economics and the principal agent theorem

Revolutionizing Crowdworking Campaigns: Conquering Adverse Selection and Moral Hazard with the Help of Smart Contracts

Crowdworking is increasingly being applied by companies to outsource tasks beyond their core competencies flexibly and cost-effectively to an unknown group. However, the anonymous and financially incentivized nature of crowdworkers creates information asymmetries and conflicts of interest, leading to inefficiencies and intensifying the principal-agent problem. Our paper offers a solution to the widespread problem of inefficient Crowdworking campaigns. We first derive the currently applied Crowdworking campaign process based on a qualitative study. Subsequently, we identify the broadest adverse selection and moral hazard problems in the process. We then analyze how the blockchain application of smart contracts can counteract those challenges and develop a process model that maps a Crowdworking campaign using smart contracts. We explain how our developed process significantly reduces adverse selection and moral hazard at each stage. Thus, our research provides approaches to make online labor more attractive and transparent for companies and online workers