Excited granular matter driven by external forces is a rich pattern-forming system. It exists in numerous form from microscopic particles to macroscopic gravels. Granular systems with high density exhibit interesting properties such as clustering instability. In dissipative granular gases where collisions are inelastic, particles tend to form dense clusters. A slight variation of density, pressure or temperature leads to this peculiar physical process. Particle interactions, i.e. collisions, combined with gravity may lead to cluster formation.
The talk discusses how the temperature has an impact on the density fluctuation in the system and how the nature of the particle collisions is changing as the temperature decreases.
Literature: M.X. Lim, A. Souslov, V. Vitelli, and H. M. Jaeger, "Cluster formation by acoustic forces", Nature Phys. 15, 460–64 (2019); I. Goldhirsch, G. Zanetti, "Clustering Instability in Dissipative Gases", Phys. Rev. Lett. 70, 1619–22 (1993).

The Planck length is a fundamental minimum to the classical concept of spacetime. Beyond this scale the nature of spacetime must be reconsidered since quantum effects cannot be neglected anymore. These properties are responsible for why the Planck scale has an important role in present and future theories of quantum spacetime and quantum gravity. This talk uses elementary, heuristic and plausible arguments to find the Planck scale in 6 different thought experiments.
Literature: Ronald J. Adler, "Six easy roads to the Planck scale", Am. J. Phys. 78 (9), 925–32 (2010).

The topic of climate change, especially global warming, has gained more awareness in popular and scientific discussions during the last decades. Derrek J. Wilson and Julio Gea-Banacloche worked out a model for a simple estimation of the contribution of CO2 to the greenhouse effect. The main indicator for the impact is the so called "climate sensitivity", which gives the increase in temperature caused by a doubling of the concentration of CO2, under the main assumption of no feedbacks. The evaluation of this simple model is done in two steps. The first step is to give an upper bound for the climate sensitivity based on photons performing a random walk in the atmosphere. In the second step, the calculation is done using the radiative transfer equation and therefore the molecular absorption spectrum has to be taken into account. The second model is to be found in fairly good agreement with currently accepted values.
Literature: Derrek J. Wilson and Julio Gea-Banacloche, "Simple model to estimate the contribution of atmospheric CO2 to the Earth's greenhouse effect", Am. J. Phys. 80 (4), 306–15 (2012).

Small and jerky movements of the eye (so called Microsaccades), while we are fixing an object, are essential for high acuity vision. These movements can be considered as a stochastic process and are described by models of anomalous diffusion. In previous analyses of experimental data, it was found that the mean-squared-displacement on long time-scales shows oscillations. To explain these, Hermann, Metzler and Engbert propose a new model that combines a self-avoiding random walk with neuronal time-delay.
Literature: C.J.J. Herrmann, R. Metzler, and R. Engbert, "A self-avoiding walk with neural delays as a model of fixational eye movements", Sci. Rep. 7, 12958 (2017).

The recent observations of gravitational waves have excited the physic community and the general public. However, to many physicists, the tensor formalism used in general relativity to formalize gravitational waves seem rather challenging. Using analogies with the vector based theory of electromagnetism, Robert C. Hilborn presents a relatively simple model of gravitational wave radiation emitted by orbiting binaries. This model is able to describe the detected gravitational wave form observed by the LIGO-VIRGO observatories and predicted from numerical GR calculations except for wave forms that result from the final ring-down moments of a binary merger.
Literature: Robert C. Hilborn, "Gravitational waves from orbiting binaries without general relativity", Am. J. Phys. 86 (3), 186–97 (2018).

Light can be described as a transverse wave. The electric field of a light beam can oscillate in a direction perpendicular to the beam axis, but can also rotate around it. There are two forms of rotation. One results from the photon spin which is related to the (circular) polarization of the beam, it determines the spin angular momentum of light (SAM). The other is obtained from the wavefront of the light beam and determines its orbital angular momentum (OAM). In special shaped or inhomogenous light fields the electric field can also have longitudinal components. This is for example the case in twisted light where the wavefronts are helical due to a nonzero OAM and where spin and orbital angular momentum are antiparallel to each other. Longitudinal field components are usually neglected for light-matter interaction in the paraxial approximation. Quinteiro et al. compare a twisted light model with experimental data of quadrupole transitions excited in trapped ions to prove the relevance of longitudinal field components.
Literature: G. F. Quinteiro, Ferdinand Schmidt-Kaler and Christian T. Schmiegelow, "Twisted-Light-Ion Interaction: The Role of Longitudinal Fields", Phys. Rev. Lett. 119 (2017) 253203.