Getting the Lorentz transformations without requiring an invariant speed

The structure of the Lorentz transformations follows purely from the absence of privileged inertial reference frames and the group structure (closure under composition) of the transformations---two assumptions that are simple and physically necessary. The existence of an invariant speed is \textit{not} a necessary assumption, and in fact is a consequence of the principle of relativity (though the finite value of this speed must, of course, be obtained from experiment). Von Ignatowsky derived this result in 1911, but it is still not widely known and is absent from most textbooks. Here we present a completely elementary proof of the result, suitable for use in an introductory course in special relativity.Comment: 4 pages, 1 figur

Switchable Adhesion of Soft Composites Induced by a Magnetic Field

Switchable adhesives have the potential to improve the manufacturing and recycling of parts and to enable new modes of motility for soft robots. Here, we demonstrate magnetically-switchable adhesion of a two-phase composite to non-magnetic objects. The composite's continuous phase is a silicone elastomer, and the dispersed phase is a magneto-rheological fluid. The composite is simple to prepare, and to mould to different shapes. When a magnetic field is applied, the magneto-rheological fluid develops a yield stress, which dramatically enhances the composite's adhesive properties. We demonstrate up to a nine-fold increase of the pull-off force of non-magnetic objects in the presence of a 250 mT field

Flux creep in Bi2Sr2CaCu2O8(sub +x) single crystals

The results of a magnetic study on a Bi2Sr2CaCu2O(8+x) single crystal are reported. Low field susceptibility (dc and ac), magnetization cycles and time dependent measurements were performed. With increasing the temperature the irreversible regime of the magnetization cycles is rapidly restricted to low fields, showing that the critical current J(sub c) becomes strongly field dependent well below T(sub c). At 2.4 K the critical current in zero field, determined from the remanent magnetization by using the Bean formula for the critical state, is J(sub c) = 2 10(exp 5) A/sq cm. The temperature dependence of J(sub c) is satisfactorily described by the phenomenological law J(sub c) = J(sub c) (0) (1 - T/T(sub c) (sup n), with n = 8. The time decay of the zero field cooled magnetization and of the remanent magnetization was studied at different temperatures for different magnetic fields. The time decay was found to be logarithmic in both cases, at least at low temperatures. At T = 4.2 K for a field of 10 kOe applied parallel to the c axis, the average pinning energy, determined by using the flux creep model, is U(sub o) = 0.010 eV

Magnetic dipoles and electric currents

We discuss several similarities and differences between the concepts of electric and magnetic dipoles. We then consider the relation between the magnetic dipole and a current loop and show that in the limit of a pointlike circuit, their magnetic fields coincide. The presentation is accessible to undergraduate students with a knowledge of the basic ideas of classical electromagnetism.Comment: 7 pages, 1 figur

The Supersymmetric Ward-Takahashi Identity in 1-Loop Lattice Perturbation Theory. I. General Procedure

The one-loop corrections to the lattice supersymmetric Ward-Takahashi identity (WTi) are investigated in the off-shell regime. In the Wilson formulation of the N=1 supersymmetric Yang-Mills (SYM) theory, supersymmetry (SUSY) is broken by the lattice, by the Wilson term and is softly broken by the presence of the gluino mass. However, the renormalization of the supercurrent can be realized in a scheme that restores the continuum supersymmetric WTi (once the on-shell condition is imposed). The general procedure used to calculate the renormalization constants and mixing coefficients for the local supercurrent is presented. The supercurrent not only mixes with the gauge invariant operator $T_\mu$. An extra mixing with other operators coming from the WTi appears. This extra mixing survives in the continuum limit in the off-shell regime and cancels out when the on-shell condition is imposed and the renormalized gluino mass is set to zero. Comparison with numerical results are also presented.Comment: 16 pages, 2 figures. Typos error correcte

FK Comae Berenices, King of Spin: The COCOA-PUFS Project

COCOA-PUFS is an energy-diverse, time-domain study of the ultra-fast spinning, heavily spotted, yellow giant FK Com (HD117555; G4 III). This single star is thought to be a recent binary merger, and is exceptionally active by measure of its intense ultraviolet and X-ray emissions, and proclivity to flare. COCOA-PUFS was carried out with Hubble Space Telescope in the UV (120-300 nm), using mainly its high-performance Cosmic Origins Spectrograph, but also high-precision Space Telescope Imaging Spectrograph; Chandra X-ray Observatory in the soft X-rays (0.5-10 keV), utilizing its High-Energy Transmission Grating Spectrometer; together with supporting photometry and spectropolarimetry in the visible from the ground. This is an introductory report on the project. FK Com displayed variability on a wide range of time scales, over all wavelengths, during the week-long main campaign, including a large X-ray flare; "super-rotational broadening" of the far-ultraviolet "hot-lines" (e.g., Si IV 139 nm (T~80,000 K) together with chromospheric Mg II 280 nm and C II 133 nm (10,000-30,000 K); large Doppler swings suggestive of bright regions alternately on advancing and retreating limbs of the star; and substantial redshifts of the epoch-average emission profiles. These behaviors paint a picture of a highly extended, dynamic, hot (10 MK) coronal magnetosphere around the star, threaded by cooler structures perhaps analogous to solar prominences, and replenished continually by surface activity and flares. Suppression of angular momentum loss by the confining magnetosphere could temporarily postpone the inevitable stellar spindown, thereby lengthening this highly volatile stage of coronal evolution.Comment: to be published in ApJ

Board of Directors' characteristics and environmental SDGs adoption: an international study

Drivers of environmentally conscious firm behaviour have gained increasing attention over past decades. The Board of Directors holds a central role in corporate decision-making, and previous empirical evidence suggests that its characteristics could influence corporate environmental performance. This paper contributes to the literature with the first evidence of the influence certain board characteristics have on whether a firm ulti-mately supports one or more environmental SDGs. Our focus is on board size, gender diversity, board independence and CEO duality. Logistic and fractional regressions on 4417 globally listed firms highlight that board size, the share of female directors, and the share of independent directors are significant drivers of support for environmental SDGs. The results and insights revealed in this study should be helpful to policymakers, investors and corporations in evaluating the effectiveness of corporate governance characteristics and fostering corporate contributions to the 2030 Agenda

Critical determinants influencing consumers’ decision-making process to buy green cosmetics. A systematic literature review

The current Systematic Literature Review (SLR) represents the first attempt to systematically classify the factors influencing consumers' decision-making process to purchase green cosmetics, based on a review of 60 studies from 2007 to 2022. The factors were classified using the Stimulus-Organism-Response (SOR) paradigm as the theoretical framework. The findings of the SLR indicate that consumers' decision-making process is primarily driven by socio-psychological stimuli, such as environmental concern, health consciousness, and social norms. These stimuli trigger internal states in consumers, which consist of cognitive and affective states. The internal states are represented by facilitators, such as attitude towards green cosmetics, perceived product quality, and product knowledge, as well as inhibitors, such as perceived high price, skepticism, and greenwashing. Among the final responses, purchase intention was found to be the most detected in the reviewed studies. Regarding socio-demographic characteristics, the segment of green cosmetic consumers is mainly characterized by women, employed individuals, with a high level of education and income. The study also highlights the core limitations of the existing literature and proposes a research agenda for future investigations

The intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud

We present Hubble Space Telescope ACS deep photometry of the intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud. By using this photometric dataset, we have determined the degree of ellipticity of the cluster ($\epsilon$=0.14$\pm$0.03) and the radial density profile. This profile is well reproduced by a standard King model with an extended core (r_c=24.5'') and a low concentration (c=1.16), indicating that the cluster has not experienced the collapse of the core. We also derived the cluster age, by using the Pisa Evolutionary Library (PEL) isochrones, with three different amount of overshooting (namely, $\Lambda_{os}$=0.0, 0.10 and 0.25). From the comparison of the observed Color-Magnitude Diagram (CMD) and Main Sequence (MS) Luminosity Function (LF) with the theoretical isochrones and LFs, we find that only models with the inclusion of some overshooting ($\Lambda_{os}$=0.10-0.25) are able to reproduce the observables. By using the magnitude difference $\delta V_{SGB}^{He-Cl}=0.90$ between the mean level of the He-clump and the flat region of the SGB, we derive an age $\tau$=1.4$\pm$0.2 Gyr.Comment: Accepted to publication by A