Ricci identities in higher dimensions

We explore connections between geometrical properties of null congruences and the algebraic structure of the Weyl tensor in n>4 spacetime dimensions. First, we present the full set of Ricci identities on a suitable "null" frame, thus completing the extension of the Newman-Penrose formalism to higher dimensions. Then we specialize to geodetic null congruences and study specific consequences of the Sachs equations. These imply, for example, that Kundt spacetimes are of type II or more special (like for n=4) and that for odd n a twisting geodetic WAND must also be shearing (in contrast to the case n=4).Comment: 8 pages. v2: typo corrected between Propositions 2 and 3. v3: typo in the last term in the first line of (11f) corrected, missing term on the r.h.s. of (11p) added, first sentence between Propositions 2 and 3 slightly change

Higher dimensional Kerr-Schild spacetimes

We investigate general properties of Kerr-Schild (KS) metrics in n>4 spacetime dimensions. First, we show that the Weyl tensor is of type II or more special if the null KS vector k is geodetic (or, equivalently, if T_{ab}k^ak^b=0). We subsequently specialize to vacuum KS solutions, which naturally split into two families of non-expanding and expanding metrics. After demonstrating that non-expanding solutions are equivalent to the known class of vacuum Kundt solutions of type N, we analyze expanding solutions in detail. We show that they can only be of the type II or D, and we characterize optical properties of the multiple Weyl aligned null direction (WAND) k. In general, k has caustics corresponding to curvature singularities. In addition, it is generically shearing. Nevertheless, we arrive at a possible "weak" n>4 extension of the Goldberg-Sachs theorem, limited to the KS class, which matches previous conclusions for general type III/N solutions. In passing, properties of Myers-Perry black holes and black rings related to our results are also briefly discussed.Comment: 33 pages. v2: minor changes, new reference

Vanishing Scalar Invariant Spacetimes in Higher Dimensions

We study manifolds with Lorentzian signature and prove that all scalar curvature invariants of all orders vanish in a higher-dimensional Lorentzian spacetime if and only if there exists an aligned non-expanding, non-twisting, geodesic null direction along which the Riemann tensor has negative boost order.Comment: final versio

Generalizations of pp-wave spacetimes in higher dimensions

We shall investigate $D$-dimensional Lorentzian spacetimes in which all of the scalar invariants constructed from the Riemann tensor and its covariant derivatives are zero. These spacetimes are higher-dimensional generalizations of $D$-dimensional pp-wave spacetimes, which have been of interest recently in the context of string theory in curved backgrounds in higher dimensions.Comment: 5 pages, RevTex, to appear in Physical Review

Comb mode filtering silver mirror cavity for spectroscopic distance measurement

In this work we present a design of an external optical cavity based on Fabry-Perot etalons applied to a 100 MHz Er-doped fiber optical frequency comb working at 1560 nm to increase its repetition frequency. A Fabry-Perot cavity is constructed based on a transportable cage system with two silver mirrors in plano-concave geometry including the mode-matching lenses, fiber coupled collimation package and detection unit. The system enables full 3D angle mirror tilting and x-y off axis movement as well as distance between the mirrors. We demonstrate the increase of repetition frequency by direct measurement of the beat frequency and spectrally by using the virtually imaged phased array images.ImPhys/Optic

SLOVAKIA. Critical junctures in the media transformation process.

In Slovakia, we record several critical junctures in the years 2000 – 2020, which had a significant impact on the media development, as well as on the opportunities and risks for deliberative communication and democracy. These included the advent of online media after 2000, the rise of social media since 2004, the worldwide economic crisis in 2008, the corruption case ‘Gorilla’ in 2011, the murder of the investigative journalist Ján Kuciak in 2018 and the COVID-19 pandemic since 2020. These events also affected the individual domains studied in Slovakia. Within the legal and ethical regulation, there are laws and standards that guarantee freedom of the press and the conduct of journalists is governed by multiple ethical codes. However, the most significant risks for the media in the journalistic domain are political influences and the pressures from media owners. These issues were also reflected in the decline in media credibility, as confirmed by research in the domain of media usage patterns. A rather positive phenomenon is, despite the persistent absence of empirical data that would support the claim, a sufficient number of academics who deal with the media competencies of media users. In summary, solid conditions (legislative, ethical, educational) for the media have been created in Slovakia, but the influences of politicians and media owners, job instability and poor financial evaluation of journalists remain a risk

SLOVAKIA. Risks and Opportunities Related to Media and Journalism Studies (2000–2020). Case Study on the National Research and Monitoring Capabilities.

Slovakia with a population of 5.3 million is one of the smaller countries in the Mediadelcom project, so the authors were able to identify and describe almost all relevant publications that contributed to the scholarly reflection on the four media domains in 2000 – 2020. The researchers mapped 865 publications, which included 707 academic and 159 non-academic outputs. Within the academic outputs, the project participants found that the most represented are academic articles, then academic books and finally academic book chapters. Most publications are written in Slovak and then in English. A smaller part of the body of literature is indexed in WoS or Scopus databases. Within the non-academic outputs, the project participants found that nonacademic articles, books and research reports have the largest representation. Most outputs are published in Slovak and without indexing in WoS or Scopus databases. Based on these findings, the authors claim that the media studies field has a well-established tradition in Slovakia, which is constantly evolving