Uniqueness of Petrov type D spatially inhomogeneous irrotational silent models

The consistency of the constraint with the evolution equations for spatially inhomogeneous and irrotational silent (SIIS) models of Petrov type I, demands that the former are preserved along the timelike congruence represented by the velocity of the dust fluid, leading to \emph{new} non-trivial constraints. This fact has been used to conjecture that the resulting models correspond to the spatially homogeneous (SH) models of Bianchi type I, at least for the case where the cosmological constant vanish. By exploiting the full set of the constraint equations as expressed in the 1+3 covariant formalism and using elements from the theory of the spacelike congruences, we provide a direct and simple proof of this conjecture for vacuum and dust fluid models, which shows that the Szekeres family of solutions represents the most general class of SIIS models. The suggested procedure also shows that, the uniqueness of the SIIS of the Petrov type D is not, in general, affected by the presence of a non-zero pressure fluid. Therefore, in order to allow a broader class of Petrov type I solutions apart from the SH models of Bianchi type I, one should consider more general ``silent'' configurations by relaxing the vanishing of the vorticity and the magnetic part of the Weyl tensor but maintaining their ``silence'' properties i.e. the vanishing of the curls of $E_{ab},H_{ab}$ and the pressure $p$.Comment: Latex, 19 pages, no figures;(v2) some clarification remarks and an appendix are added; (v3) minor changes to match published versio

Fluence and polarisation dependence of GaAs based Lateral Photo-Dember terahertz emitters

We characterise THz output of lateral photo-Dember (LPD) emitters based on semi-insulating (SI), unannealed and annealed low temperature grown (LTG) GaAs. Saturation of THz pulse power with optical fluence is observed, with unannealed LTG GaAs showing highest saturation fluence at 1.1 ± 0.1 mJ cm-2. SI-GaAs LPD emitters show a flip in signal polarity with optical fluence that is attributed to THz emission from the metal-semiconductor contact. Variation in optical polarisation affects THz pulse power that is attributed to a local optical excitation near the metal contact

Self-similar Bianchi models: II. Class B models

In a companion article (referred hearafter as paper I) a detailed study of the simply transitive Spatially Homogeneous (SH) models of class A concerning the existence of a simply transitive similarity group has been given. The present work (paper II) continues and completes the above study by considering the remaining set of class B models. Following the procedure of paper I we find all SH models of class B subjected only to the minimal geometric assumption to admit a proper Homothetic Vector Field (HVF). The physical implications of the obtained geometric results are studied by specialising our considerations to the case of vacuum and $\gamma -$law perfect fluid models. As a result we regain all the known exact solutions regarding vacuum and non-tilted perfect fluid models. In the case of tilted fluids we find the \emph{general }self-similar solution for the exceptional type VI$_{-1/9}$ model and we identify it as equilibrium point in the corresponding dynamical state space. It is found that this \emph{new} exact solution belongs to the subclass of models $n_\alpha ^\alpha =0$, is defined for $\gamma \in (\frac 43,\frac 32)$ and although has a five dimensional stable manifold there exist always two unstable modes in the restricted state space. Furthermore the analysis of the remaining types, guarantees that tilted perfect fluid models of types III, IV, V and VII$_h$ cannot admit a proper HVF strongly suggesting that these models either may not be asymptotically self-similar (type V) or may be extreme tilted at late times. Finally for each Bianchi type, we give the extreme tilted equilibrium points of their state space.Comment: Latex, 15 pages, no figures; to appear in Classical Quantum Gravity (uses iopart style/class files); (v2) minor corrections to match published versio

Mergers of municipalities in Greek local government. An evaluation by means of a S.W.O.T. analysis

Σκοπός της συγκεκριμένης εργασίας είναι η κριτική προσέγγιση και η αξιολόγηση των μεταρρυθμίσεων για την τοπική αυτοδιοίκηση, οι οποίες συντελέστηκαν στην Ελλάδα κατά την τελευταία δεκαπενταετία. Στόχος των μεταρρυθμίσεων ήταν η δημιουργία ισχυρής τοπικής αυτοδιοίκησης, δηλαδή ισχυρών και αποτελεσματικών δήμων, ικανών να σχεδιάζουν και να υλοποιούν πολιτικές τοπικής ανάπτυξης, καθώς και να παρέχουν αποτελεσματικές και ποιοτικά αναβαθμισμένες υπηρεσίες στους πολίτες. Για την επίτευξη του σκοπού της εργασίας χρησιμοποιείται η μέθοδος της S.W.O.T. ανάλυσης, προκειμένου να εντοπιστούν τα πλεονεκτήματα και τα μειονεκτήματα, οι ευκαιρίες και οι απειλές του Προγράμματος «Καλλικράτης». Από τα αποτελέσματα της έρευνας προκύπτουν τα θετικά στοιχεία της μεταρρύθμισης, που τα κύρια είναι: η ενίσχυση της τοπικής αυτοδιοίκησης με νέες αρμοδιότητες και πόρους και επομένως η βελτίωση της αποτελεσματικότητάς της, η μείωση του λειτουργικού κόστους και η προώθηση της ενεργούς συμμετοχής των δημοτών στα κοινά, με την πραγματοποίηση των τοπικών δημοψηφισμάτων. Από την άλλη, προκύπτουν και απειλές για τη μεταρρύθμιση, που σχετίζονται με την ευρύτερη δυσμενή οικονομική συγκυρία, τις διαφωνίες των πολιτικών δυνάμεων για την ουσία της μεταρρύθμισης, τα όρια των νέων δήμων και τις ανάγκες σε προσωπικό.The aim of this paper is to evaluate the reforms in local government that took place in Greece during the last fi fteen years. The aim of the reforms was the creation of strong local government entities, i.e. powerful and effi cient local authorities, able to plan and implement local development policies, and to provide effective and qualitatively superior services to citizens. SWOT analysis is used in order to identify the advantages and disadvantages, opportunities and threats of the “Kallikrates” Program. Results indicate that the new reform offers to local government new powers and resources and therefore leads to improved effectiveness; reduced operating costs and promotion of the active participa tion of citizens in public affairs, by means of local referendums. However, there are also threats created for the reform, which are related to the broader fi nancial crisis, the disputes of political powers on the substance of the reform, the boundaries of new municipalities and the needs in personnel

ChatGPT: ascertaining the self-evident. The use of AI in generating human knowledge

The fundamental principles, potential applications, and ethical concerns of ChatGPT are analyzed and discussed in this study. Since ChatGPT emerged, it has gained a rapidly growing popularity, with more than 600 million users today. The development of ChatGPT was a significant mile-stone, as it demonstrated the potential of large-scale language models to generate natural language responses that are almost indistinguishable from those of a human. ChatGPT's operational principles, prospective applications, and ability to advance a range of human endeavours are discussed in the paper. However, much of the work discusses and poses moral and other problems that rely on the subject. To document the latter, we submitted 14 queries and captured the ChatGPT responses. ChatGPT appeared to be honest, self-knowledgeable, and careful with its answers. The authors come to the realization that since AI is already a part of society, the pervasiveness of the ChatGPT tool to the general public has once again brought to light concerns regarding AI in general. Still, they have moved from the domain of scientific community collective reflection at a conceptual level to everyday practice this time.Comment: 20 pages, 2 figure