An introduction to the classical three-body problem

Lagrange’s periodic solution with three bodies at vertices of equilateral triangles. The constant ratios of separations
are functions of the mass ratios alone

Govind S. Krishnaswami, Himalaya Senapati
The classical three-body problem arose in an attempt to understand the effect of the Sun on the Moon’s Keplerian orbit around the Earth. It has attracted the attention of some of the best physicists and mathematicians and led to the discovery of chaos. We survey the three-body problem in its historical context and use it to introduce several ideas and techniques that have been developed to understand classical mechanical systems.
Read more at https://arxiv.org/pdf/1901.07289.pdf

 

John Archibald Wheeler: A Biographical Memoir

John Wheeler as a postdoc of Niels Bohr in
Copenhagen, 1934

Kip S. Thorne
John Archibald Wheeler was a theoretical physicist who worked on both down-to-earth projects and highly speculative ideas, and always emphasized the importance of experiment and observation, even when speculating wildly. His research and insights had large impacts on nuclear and particle physics, the design of nuclear weapons, general relativity and relativistic astrophysics, and quantum gravity and quantum information. But his greatest impacts were through the students, postdocs, and mature physicists whom he educated and inspired.
He was guided by what he called the principle of radical conservatism, inspired by Niels Bohr: base your research on well established physical laws (be conservative), but push them into the most extreme conceivable domains (be radical). He often pushed far beyond the boundaries of well understood physics, speculating in prescient ways that inspired future generations of physicists.
After completing his PhD with Karl Herzfeld at Johns Hopkins University (1933), Wheeler embarked on a postdoctoral year with Gregory Breit at NYU and another with Niels Bohr in Copenhagen. He then moved to a three-year assistant professorship at the University of North Carolina (1935-37), followed by a 40 year professorial career at Princeton University (1937-1976) and then ten years as a professor at the University of Texas, Austin (1976-1987). He returned to Princeton in retirement but remained actively and intensely engaged with physics right up to his death at age 96.
Read more at https://arxiv.org/ftp/arxiv/papers/1901/1901.06623.pdf

How the periodic table went from a sketch to an enduring masterpiece

150 years ago, Mendeleev perceived the relationships of the chemical elements

In Danish physicist Niels Bohr’s 1922 version of the periodic table, adapted from a table by Danish chemist Julius Thomsen, elements with similar properties occupy horizontal rows connected by lines. The empty box on the right marks the expected occurrence of a group of elements that are chemically similar to the rare earth elements (numbers 58–70) in the preceding column.


Every field of science has its favorite anniversary.
For physics, it’s Newton’s Principia of 1687, the book that introduced the laws of motion and gravity. Biology celebrates Darwin’s On the Origin of Species (1859) along with his birthday (1809). Astronomy fans commemorate 1543, when Copernicus placed the sun at the center of the solar system.
And for chemistry, no cause for celebration surpasses the origin of the periodic table of the elements, created 150 years ago this March by the Russian chemist Dmitrii Ivanovich Mendeleev…
Read more at https://www.sciencenews.org/article/periodic-table-history-chemical-elements-150-anniversary

A crowd that flows like water

Dynamic response and hydrodynamics of polarized crowds
Nicolas Bain, Denis Bartolo
Modeling crowd motion is central to situations as diverse as risk prevention in mass events and visual effects rendering in the motion picture industry. The difficulty of performing quantitative measurements in model experiments has limited our ability to model pedestrian flows. We use tens of thousands of road-race participants in starting corrals to elucidate the flowing behavior of polarized crowds by probing its response to boundary motion. We establish that speed information propagates over system-spanning scales through polarized crowds, whereas orientational fluctuations are locally suppressed. Building on these observations, we lay out a hydrodynamic theory of polarized crowds and demonstrate its predictive power. We expect this description of human groups as active continua to provide quantitative guidelines for crowd management.

Read more at http://science.sciencemag.org/content/363/6422/46

Two Notions of Naturalness

Porter Williams
My aim in this paper is twofold: (i) to distinguish two notions of naturalness employed in BSM physics and (ii) to argue that recognizing this distinction has methodological consequences. One notion of naturalness is an “autonomy of scales” requirement: it prohibits sensitive dependence of an effective field theory’s low-energy observables on precise specification of the theory’s description of cutoff-scale physics. I will argue that considerations from the general structure of effective field theory provide justification for the role this notion of naturalness has played in BSM model construction. A second, distinct notion construes naturalness as a statistical principle requiring that the values of the parameters in an effective field theory be “likely” given some appropriately chosen measure on some appropriately circumscribed space of models. I argue that these two notions are historically and conceptually related but are motivated by distinct theoretical considerations and admit of distinct kinds of solution.
Read more at https://arxiv.org/pdf/1812.08975.pdf

NASA and the Search for Technosignatures

Technosignature axes of merit, illustrating some of the considerations that go into developing a good search strategy for technosignatures.

NASA Technosignatures Workshop Participants
This report is the product of the NASA Technosignatures Workshop held at the Lunar and Planetary Institute in Houston, Texas, in September 2018. This workshop was convened by NASA for the organization to learn more about the current field and state of the art of searches for technosignatures, and what role NASA might play in these searches in the future. The report, written by the workshop participants, summarizes the material presented at the workshop and incorporates additional inputs from the participants. Section 1 explains the scope and purpose of the document, provides general background about the search for technosignatures, and gives context for the rest of the report. Section 2 discusses which experiments have occurred, along with current limits on technosignatures. Section 3 addresses the current state of the technosignature field as well as the state-of-the-art for technosignature detection. Section 4 addresses near-term searches for technosignatures, and Section 5 discusses emerging and future opportunities in technosignature detection.

Read more at https://arxiv.org/ftp/arxiv/papers/1812/1812.08681.pdf

Noether’s Theorem and Symmetry

A.K. Halder, Andronikos Paliathanasis, P.G.L. Leach
In Noether’s original presentation of her celebrated theorm of 1918 allowance was made for the dependence of the coefficient functions of the differential operator which generated the infinitesimal transformation of the Action Integral upon the derivatives of the depenent variable(s), the so-called generalised, or dynamical, symmetries. A similar allowance is to be found in the variables of the boundary function, often termed a gauge function by those who have not read the original paper. This generality was lost after texts such as those of Courant and Hilbert or Lovelock and Rund confined attention to point transformations only. In recent decades this dimunition of the power of Noether’s Theorem has been partly countered, in particular in the review of Sarlet and Cantrijn. In this special issue we emphasise the generality of Noether’s Theorem in its original form and explore the applicability of even more general coefficient functions by alowing for nonlocal terms. We also look for the application of these more general symmetries to problems in which parameters or parametric functions have a more general dependence upon the independent variables
Read more at https://arxiv.org/pdf/1812.03682.pdf