‘The President is dead, long live the acting President!’

The uncertainty of Nigeria’s leadership is over: President Umaru Yar’Adua died yesterday. Elected in 2007 Yar’Adua was, in what observers claim to have been a flawed and rigged election, elevated to the top post in Nigeria. Already at the time when he assumed office his health was reported to be fragile as he was suffering from heart and kidney problems

Dust and gas emission from cometary nuclei: the case of comet 67P/Churyumov-Gerasimenko

Comets display with decreasing solar distance an increased emission of gas and dust particles, leading to the formation of the coma and tail. Spacecraft missions provide insight in the temporal and spatial variations of the dust and gas sources located on the cometary nucleus. For the case of comet 67P/Churyumov-Gerasimenko (67P/C-G), the long-term observations from the Rosetta mission point to a homogeneous dust emission across the entire illuminated surface. Despite the homogeneous initial distribution, a collimation in jet-like structures becomes visible. We propose that this observation is linked directly to the complex shape of the nucleus and projects concave topographical features into the dust coma. To test this hypothesis, we put forward a gas-dust description of 67P/C-G, where gravitational and gas forces are accurately determined from the surface mesh and the rotation of the nucleus is fully incorporated. The emerging jet-like structures persist for a wide range of gas-dust interactions and show a dust velocity dependent bending.Comment: 17 pages, with 7 figures. To appear in Advances in Physics X (2018

Efficiently Reconfiguring a Connected Swarm of Labeled Robots

When considering motion planning for a swarm of n labeled robots, we need to rearrange a given start configuration into a desired target configuration via a sequence of parallel, continuous, collision-free robot motions. The objective is to reach the new configuration in a minimum amount of time; an important constraint is to keep the swarm connected at all times. Problems of this type have been considered before, with recent notable results achieving constant stretch for not necessarily connected reconfiguration: If mapping the start configuration to the target configuration requires a maximum Manhattan distance of d, the total duration of an overall schedule can be bounded to ?(d), which is optimal up to constant factors. However, constant stretch could only be achieved if disconnected reconfiguration is allowed, or for scaled configurations (which arise by increasing all dimensions of a given object by the same multiplicative factor) of unlabeled robots. We resolve these major open problems by (1) establishing a lower bound of ?(?n) for connected, labeled reconfiguration and, most importantly, by (2) proving that for scaled arrangements, constant stretch for connected reconfiguration can be achieved. In addition, we show that (3) it is NP-hard to decide whether a makespan of 2 can be achieved, while it is possible to check in polynomial time whether a makespan of 1 can be achieved

The photoelectric effect in external fields

Atoms and negative ions interacting with laser photons yield a coherent source of photoelectrons. Applying external fields to photoelectrons gives rise to interesting and valuable interference phenomena. We analyze the spatial distribution of the photocurrent using elementary quantum methods. The photoelectric effect is shown to be an interesting example for the use of coherent particle sources in quantum mechanics.Comment: Contribution to the Einstein special issue, slightly updated reference

Integrating or Not-Integrating—That is the Question. Effects of Integrated Instruction on the Development of Pre-Service Biology Teachers’ Professional Knowledge

For successful classroom instruction, teachers require a well-founded knowledge base consisting of the three knowledge facets pedagogical-psychological knowledge (PK), content knowledge (CK), and pedagogical content knowledge (PCK). However, there is not yet clarity about the circumstances and instructional pathways through which teachers can best develop these knowledge facets. In an experimental study (N = 118 pre-service biology teachers), we investigated the effects of separated instruction (knowledge facets were treated successively without linking) or integrated instruction (knowledge facets were presented in an interrelated way) on PK, CK, PCK and the application of PCK in a video-based assessment tool in comparison to a control group (receiving no instruction). Both pathways of instruction were provided by a lecturer on the curricular topic of senses and sensory organs, exemplified for the topic skin. Results point to the effectiveness of both ways of instruction in terms of knowledge increases for CK and PCK. In addition, working with the video-based assessment tool may have had an additional effect on PCK. No effects for PK could be found, possibly due to a ceiling effect. Moreover, there was no effect of the intervention on the application of PCK. However, tendencies in descriptive results indicating a possible advantage concerning separated or integrated instruction with regard to CK and PCK are discussed. Overall, our results indicate that the use of video-based tools can complement instructional approaches to knowledge acquisition