## The Quantum Eraser Paradox

Colm Bracken, Jonte R. Hance, Sabine Hossenfelder

The Delayed-Choice Quantum Eraser experiment is commonly interpreted as implying that in quantum mechanics a choice made at one time can influence an earlier event. We here suggest an extension of the experiment that results in a paradox when interpreted using a local realist interpretation combined with backward causation (“retrocausality”). We argue that resolving the paradox requires giving up the idea that, in quantum mechanics, a choice can influence the past, and that it instead requires a violation of Statistical Independence without retrocausality. We speculate what the outcome of the experiment would be.

read more at https://arxiv.org/abs/2111.09347

read also: …a comment… and the Reply…

## The First Interstellar Astronauts Will Not Be Human

Our ability to explore the cosmos by direct contact has been limited to a small number of lunar and interplanetary missions. However, the NASA Starlight program points a path forward to send small, relativistic spacecraft far outside our solar system via standoff directed-energy propulsion. These miniaturized spacecraft are capable of robotic exploration but can also transport seeds and organisms, marking a profound change in our ability to both characterize and expand the reach of known life. Here we explore the biological and technological challenges of interstellar space biology, focusing on radiation-tolerant microorganisms capable of cryptobiosis. Additionally, we discuss planetary protection concerns and other ethical considerations of sending life to the stars….

Read more at *Lantin et al https://arxiv.org/abs/2110.13080*

## Three Impossible Theories

**Leonard Susskind**

I will begin by conjecturing a cosmological generalization of black hole complementarity (also known as the central dogma). I will then discuss three theories and argue that they are inconsistent with second law of thermodynamics if the cosmological version of the dogma is correct. The three theories are: the big rip; cyclic cosmology; and the Farhi-Guth-Guven mechanism for creating inflating universes behind black hole horizons. … Read more at https://arxiv.org/abs/2107.11688

## From Einstein to spacetime

Gravitational wave astronomy involves people from around the world, all with their our own stories. Dr. Corey Gray, Caltech, is a member of the Siksika Nation (Northern Blackfoot tribe of Alberta) and Scottish. He is the lead operator at the Hanford Observatory of the Laser Interferometer Gravitational wave Observatory. In this hour-long public lecture, Dr. Gray presents a behind-the-scenes look at what it’s been like working at a land-based gravitational wave detector since 1998. He will share a “Top 3” list of his favorite detections as well as the experience of a son having the opportunity to recruit his mother to work with him because of language—the language of spacetime and the Blackfoot people.

The LIGO Scientific Collaboration made big news in 2016 by announcing what has been hailed as “the scientific breakthrough of the century:” the first direct detection of gravitational waves. This was a monumental discovery because it proved a prediction made 100 years earlier by Albert Einstein. LIGO has made many more detections over the years. These detections mark the beginning of a completely new field of science: gravitational wave astronomy.

Dr. Corey received his Bachelor of Science degrees in physics and applied mathematics from Humboldt State University in northern California. After graduation, he was hired as a detector operator by the California Institute of Technology to work for the LIGO Hanford Observatory in Washington state. As a member of the LIGO team, Corey’s work has included working with groups to help build the gravitational wave detector and also operating the detector as a member of the operator team.

He also enjoys outreach & science communication. Over the years he has given keynotes, plenary talks, public colloquia, conference panel sessions, and also a TEDx talk. His speaking engagements have taken him from Banff to Orlando, Montreal to Honolulu and many points in between. He especially loves to share the science of Einstein with Indigenous youth and other underrepresented groups.

## Low-cost experiment to measure the speed of light

Faraz Mehdi, Kiran M. Kolwankar

In this paper, we demonstrate a low-cost method to measure the speed of light. It uses instruments which are readily available in any undergraduate laboratory in a developing country and some components which are inexpensive. The method is direct as it measures the time of flight of the LASER beam and easy to implement. It will allow students to verify the finite value of the speed of light first hand. It can be part of the undergraduate syllabus as a regular experiment or a demonstration experiment.

read more at https://arxiv.org/abs/2108.06773

## Solid angles in perspective

**Paul Quincey**

The specialised uses of solid angles mean that they are quite unfamiliar quantities. This article, apart from making solid angles a little more familiar, brings out several topics of general interest, such as how units are interrelated and how equations depend on the choice of units. Although the steradian is commonly used as the unit for solid angle, another unit, the square degree, is used in astronomy, and a unit introduced here, the solid degree (with 360 solid degrees in a hemisphere) could be used with benefits that are similar to those of the degree when it is used as the unit for plane angle. The article, which is suitable for students at A-level and introductory undergraduate level, also shows how solid angles can provide a gentle introduction to crystal structure, spherical trigonometry and non-Euclidean geometry.

read more at arxiv.org/abs/2108.05226

## Modeling Transport of SARS-CoV-2 Inside a Charlotte Area Transit System (CATS) Bus

**Gregory McGowan, Jeffrey Feaster, Andy Jones, Lucas Agricola, Matthew Goodson, William Timms, Mesbah Uddin**

We present in this paper a model of the transport of human respiratory particles on a Charlotte Area Transit System (CATS) bus to examine the efficacy of interventions to limit exposure to SARS-CoV-2, the virus that causes COVID-19. The methods discussed here utilize a commercial Navier-Stokes flow solver, RavenCFD, run using a massively parallel supercomputer to model the flow of air through the bus under varying conditions, such as windows being open or the HVAC flow settings. Lagrangian particles are injected into the RavenCFD predicted flow fields to simulate the respiratory droplets from speaking, coughing, or sneezing. These particles are then traced over time and space until they interact with a surface or are removed via the HVAC system. Finally, a volumetric Viral Mean Exposure Time (VMET) is computed to quantify the risk of exposure to the SARS-CoV-2 under various environmental and occupancy scenarios. Comparing the VMET under varying conditions should help identify viable methods to reduce the risk of viral exposure of CATS bus passengers during the COVID-19 pandemic.

read more at

## Green’s functions in quantum mechanics courses

William J. Herrera, Herbert Vinck-Posada, Shirley Gomez Paez

Green’s functions in Physics have proven to be a valuable tool for understanding fundamental concepts in different branches, such as electrodynamics, solid-state and many -body problems. In quantum mechanics advanced courses, Green’s functions usually are explained in the context of the scattering problem by a central force. However, their use for more basic problems is not often implemented. The present work introduces Green’s Function in quantum mechanics courses with some examples that can be solved with essential tools. For this, the general aspects of the theory are shown, emphasizing the solution of different fundamental issues of quantum mechanics from this approach. In particular, we introduce the time-independent Green’s functions and the Dyson equation to solve problems with an external potential. As examples, we show the scattering by a Dirac delta barrier, where the reflection and transmission coefficients are found. In addition, the infinite square potential well energy levels, and the local density of states, are calculated.

read more at https://arxiv.org/abs/2107.14104