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

The black hole fifty years after: Genesis of the name

Ann Ewing’s article in 1964 where the term Black Hole is published for the first time

Carlos A. R. Herdeiro, José P. S. Lemos
Black holes are extreme spacetime deformations where even light is imprisoned. There is an extensive astrophysical evidence for the real and abundant existence of these prisons of matter and light in the Universe. Mathematically, black holes are described by solutions of the field equations of the theory of general relativity, the first of which was published in 1916 by Karl Schwarzschild.
Another highly relevant solution, representing a rotating black hole, was found by Roy Kerr in 1963. It was only much after the publication of the Schwarzschild solution, however, that the term black hole was employed to describe these objects. Who invented it?
Conventional wisdom attributes the origin of the term to the prominent North American physicist John Wheeler who first adopted it in a general audience article published in 1968. This, however, is just one side of a story that begins two hundred years before in an Indian prison colloquially known as the Black Hole of Calcutta.
Robert Dicke, also a distinguished physicist and colleague of Wheeler at Princeton University, aware of the prison’s tragedy began, around 1960, to compare gravitationally completely collapsed stars to the black hole of Calcutta. The whole account thus suggests reconsidering who indeed coined the name black hole and commends acknowledging its definitive birth to a partnership between Wheeler and Dicke.
Read more at https://arxiv.org/pdf/1811.06587.pdf

The Science and Legacy of Richard Feynman

Avinash Dhar, Apoorva D. Patel, Spenta R. Wadia
This year is the 100th birth anniversary of Richard Philips Feynman. This article commemorates his scientific contributions and lasting legacy.

… He influenced the way physicists think about physics, especially physical processes whose description requires the quantum theory. Feynman’s approach to physics was to show how the solution to a problem unravels, aided by a visual language that encapsulates complicated mathematical expressions. James Gleick put this very succinctly, “Feynman’s reinvention of quantum mechanics did not so much explain how the world was, or why it was that way, as to tell how to confront the world. It was not knowledge of or knowledge about. It was knowledge how to.” He went to the heart of the problem he was working on, built up the solutions from simple ground rules in a step by step nuts and bolts way, articulating the steps as he built up the solution, keeping in mind that science is highly constrained by the fact that it is a description of the natural world. He laid bare the strategy of the solution, and was explicit about the various difficulties that need to be surmounted, perhaps now or in the next attempt to solve the problem: “In physics the truth is rarely perfectly clear.” Feynman’s attitude to ‘fundamental physics’ is well put in the collection, ‘The Pleasure of Finding Things Out’: “People say to me, ‘Are you looking for the ultimate laws of physics?’ No, I’m not, I’m just looking to find out more about the world, and if it turns out there is a simple ultimate law which explains everything, so be it, that would be very nice to discover.” …

Read more at https://arxiv.org/pdf/1810.07409.pdf

Who discovered positron annihilation?

Positron annihilatioTim 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

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

The Gibbs Paradox

gibs

The Gibbs setup. In (a) the membrane MA is permeable to A, impermeable to B, whilst MB is permeable to B, impermeable to A; the pistons are allowed to expand; In (b) the gases are the same and a partition is removed. The pressures and temperatures in both chambers are the same.


Simon Saunders
The Gibbs Paradox is essentially a set of open questions as to how sameness of gases or fluids (or masses, more generally) are to be treated in thermodynamics and statistical mechanics. They have a variety of answers, some restricted to quantum theory (there is no classical solution), some to classical theory (the quantum case is different). The solution offered here applies to both in equal measure, and is based on the concept of particle indistinguishability (in the classical case, Gibbs’ notion of ‘generic phase’). Correctly understood, it is the elimination of sequence position as a labelling device, where sequences enter at the level of the tensor (or Cartesian) product of one-particle state spaces. In both cases it amounts to passing to the quotient space under permutations. ‘Distinguishability’, in the sense in which it is usually used in classical statistical mechanics, is a mathematically convenient, but physically muddled, fiction.
Read more at https://arxiv.org/ftp/arxiv/papers/1808/1808.01953.pdf

Aside

Temporal relationalism

Lee Smolin
Because of the non-locality of quantum entanglement, realist approaches to completing quantum mechanics have implications for our conception of space. Quantum gravity also is expected to predict phenomena in which the locality of classical spacetime is modified or disordered. It is then possible that the right quantum theory of gravity will also be a completion of quantum mechanics in which the foundational puzzles in both are addressed together. I review here the results of a program, developed with Roberto Mangabeira Unger, Marina Cortes and other collaborators, which aims to do just that. The results so far include energetic causal set models, time asymmetric extensions of general relativity and relational hidden variables theories, including real ensemble approaches to quantum mechanics. These models share two assumptions: that physics is relational and that time and causality are fundamental.
Read more at https://arxiv.org/pdf/1805.12468.pdf