**Tim Dunker**

In the early 1930s, the positron, pair production, and, at last, positron annihilation were discovered. Over the years, several scientists have been credited with the discovery of the annihilation radiation. Commonly, Thibaud and Joliot have received credit for the discovery of positron annihilation. A conversation between Werner Heisenberg and Theodor Heiting prompted me to examine relevant publications, when these were submitted and published, and how experimental results were interpreted in the relevant articles. I argue that it was Theodor Heiting – usually not mentioned at all in relevant publications – who discovered positron annihilation, and that he should receive proper credit.

Read more at https://arxiv.org/pdf/1809.04815.pdf

# Is our universe one of many?

BY KER THAN

As physicists have delved deeper and deeper into nature’s mysteries, they have been forced to accept the unsettling fact that our universe is suspiciously fine-tuned to support life. The amount of matter in the universe, the mass of the electron, the strength of gravity – if the value of any of these deviated only a tiny bit from what they actually are, then galaxies and stars could not form and biological life could not exist. The best theory that physicists have come up with to explain this cosmic coincidence is called the String Theory Landscape.

The String Theory Landscape combines elements from two of the strangest and most enduring ideas in modern physics – string theory and cosmic inflation – to argue that we live in a multiverse made up of infinitely many “pocket universes,” of which our perfectly calibrated universe is just one. This five-part series tells the story of how theoretical physicists at Stanford helped develop the String Theory Landscape – and in the process sparked a fierce and still ongoing debate about what science is and what it should be…

Read more at https://news.stanford.edu/2018/09/10/landscape-theory/

# Snellius meets Schwarzschild

**Refraction of brachistochrones and time-like geodesics**

**Heinz-Jürgen Schmidt**

The brachistochrone problem can be solved either by variational calculus or by a skillful application of the Snellius’ law of refraction. This suggests the question whether also other variational problems can be solved by an analogue of the refraction law. In this paper we investigate the physically interesting case of free fall in General Relativity that can be formulated as a variational problem w. r. t. proper time. We state and discuss the corresponding refraction law for a special class of spacetime metrics including the Schwarzschild metric…

Read more at https://arxiv.org/pdf/1809.00355.pdf

# Analogy between thermal emission of nano objects and Hawking’s radiation

**Karl Joulain **

We analyze in this work some analogies between thermal emission of nano objects and Hawking’s radiation. We first focus on the famous expression of the black hole radiating temperature derived by Hawking in 1974 and consider the case of thermal emission of a small aperture made into a cavity (Ideal Blackbody). We show that an expression very similar to Hawking’s temperature determines a temperature below which an aperture in a cavity cannot be considered as standard blackbody radiating like T^4. Hawking’s radiation therefore appear as a radiation at a typical wavelength which is of the size of the horizon radius. In a second part, we make the analogy between the emission of particle-anti particle pairs near the black hole horizon and the scattering and coupling of thermally populated evanescent waves by a nano objects. We show here again that a temperature similar to the Hawking temperature determines the regimes where the scattering occur or where it is negligible.

Read more at https://arxiv.org/pdf/1808.08037.pdf

# Effects of exoplanetary gravity on human locomotor ability

**Nikola Poljak, Dora Klindzic, Mateo Kruljac**

At some point in the future, if mankind hopes to settle planets outside the Solar System, it will be crucial to determine the range of planetary conditions under which human beings could survive and function. In this article, we apply physical considerations to future exoplanetary biology to determine the limitations which gravity imposes on several systems governing the human body. Initially, we examine the ultimate limits at which the human skeleton breaks and muscles become unable to lift the body from the ground. We also produce a new model for the energetic expenditure of walking, by modelling the leg as an inverted pendulum. Both approaches conclude that, with rigorous training, humans could perform normal locomotion at gravity no higher than 4 g_{Earth}.

Read more at https://arxiv.org/pdf/1808.07417.pdf

# What Is a Black Hole?

**Erik Curiel**

Although black holes are objects of central importance across many fields of physics, there is no agreed upon definition for them, a fact that does not seem to be widely recognized. Physicists in different fields conceive of and reason about them in radically different, and often conflicting, ways. All those ways, however, seem sound in the relevant contexts. After examining and comparing many of the definitions used in practice, I consider the problems that the lack of a universally accepted definition leads to, and discuss whether one is in fact needed for progress in the physics of black holes. I conclude that, within reasonable bounds, the profusion of different definitions is in fact a virtue, making the investigation of black holes possible and fruitful in all the many different kinds of problems about them that physicists consider, although one must take care in trying to translate results between fields.

Read more at https://arxiv.org/pdf/1808.01507.pdf

# Quantum treatment of Verlinde’s entropic force conjecture

**A. Plastino, M. C. Rocca, G. L. Ferri**

Verlinde conjectured that gravitation is an emergent entropic force. This surprising conjecture was proved in [Physica A **505** (2018) 190] within a purely classical context. Here, we appeal to a quantum environment to deal with the conjecture in the case of bosons and consider also the classical limit of quantum mechanics (QM)….

Read more at https://arxiv.org/pdf/1808.01330.pdf