## String Theory or Loop Quantum Gravity?

**David Gross vs Carlo Rovelli**

String theory has dominated discussions at the frontiers of physics for decades, especially in the attempts to build a quantum theory of gravity. But does it deserve its exalted status? Nobel Prize winner and String Theory pioneer David Gross debates Carlo Rovelli, one of the founding fathers of Loop Quantum Gravity. The discussion is lively; full of insights, insults, backhanded compliments and even some common ground as to the nature of physics.

Timecodes 0:00 Introduction 3:00 David Gross early years, 4:00 Carlo Rovelli early years 5:22 David on string theory 20:22 Carlo on string theory 31:08 David&Carlo on string theory 53:20 Loop Quantum Gravity 1:00 David&Carlo on LQG 1:21 Black Holes 1:30 Predictions and the Scientific Method

## Chirality: A Scientific Leitmotif

**Frank Wilczek**

Handedness, or chirality, has been a continuing source of inspiration across a wide range of scientific problems. After a quick review of some important, instructive historical examples, I present three contemporary case studies involving sophisticated applications of chirality at the frontier of present-day science in the measurement of the muon magnetic moment, in topological physics, and in exploring the “chirality” of time. Finally, I briefly discuss chirality as a source of fertile questions.

Read more at https://arxiv.org/abs/2112.06927

## 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