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arXiv2018

The unsustainable legacy of the Nuclear Age

It is seldom acknowledged the tremendous burden that the Nuclear Age leaves on future generations, and the environment, for an extremely long time. Nuclear processes, and products, are activated at energies millions of times higher than the energies of chemical processes, and consequently they cannot be eliminated by the natural environment on Earth. So it turns out that hundreds of nuclear tests performed in the atmosphere left a huge radioactive contamination; Rosalie Bertell estimated 1,300 millions victims of the Nuclear Age; civil nuclear programs have produced enormous quantities of radioactive waste, whose final disposal has not been solved by any country; decommissioning of tens of shut down nuclear plants shall involve costs which were underestimated in the past; spent nuclear fuel accumulates in decontamination pools, or in dry cask storage, but no final storage has been carried out yet; radioactivity of spent fuel will last for tens of thousand years; military nuclear programs leave, besides almost 15,000 nuclear warheads, approximately 1,300 metric tons of plutonium; even mining of natural uranium was, and is, carried out mainly by poor and exploited populations, which suffer serious health consequences; paradoxically enough (or maybe not), French territory itself is widely contaminated. All these facts have been downplayed during the whole history of the Nuclear Age. Future generations shall not be grateful.

physics.pop-ph↗

arXiv2018

Of Naturalness and Complexity

It is argued that the occurrence of disproportionately ("un-natural") large (or small) numbers, as well as deep cancellations, are comparatively natural traits of the way Nature is geared to operate in most complex systems. The idea is illustrated by means of two outstanding and over-resilient problems in theoretical physics: fluid turbulence and the computation of ground-states of quantum many-body fermion systems. Potential connections with the issue of Naturalness, or lack thereof, in high-energy physics are sketched out.

physics.hist-ph↗

arXiv2018

An exposition of special relativity without appeal to "constancy of speed of light" hypotheses

We present the theory of special relativity here through the lens of differential geometry. In particular, we explicitly avoid any reference to hypotheses of the form "The laws of physics take the same form in all inertial reference frames" and "The speed of light is constant in all inertial reference frames", or to any other electrodynamic phenomenon. For the author, the clearest understanding of relativity comes about when developing the theory out of just the primitive concept of time (which is also a concept inherent in any standard exposition) and the basic tenets of differential geometry. Perhaps surprisingly, once the theory is framed in this way, one can predict existence of a "universal velocity" which stays the same in all "inertial reference frames". This prediction can be made by performing much more basic time measurement physical experiments that we outline in these notes, rather than experiments of an electrodynamic nature. Thus, had these physical experiments been performed prior to Michelson-Morley type experiments (which, in principle, could have been done in any period with precise enough time keeping instruments), the Michelson-Morley experiments would simply give us an example of a physical entity, i.e., light, which enjoys this special "universal" status.

physics.hist-ph↗

arXiv2018

Julian Schwinger: A Centennial Celebration at Harvard

A biographical profile of the theoretical physicist, Julian Schwinger, and a discussion of "The Memories of Julian" Centennial Celebration (held at Harvard University on February 12, 2018) is presented herein.

physics.hist-ph↗

arXiv2018

QBism and the Ithaca Desiderata

In 1996, N. David Mermin proposed a set of desiderata for an understanding of quantum mechanics, the "Ithaca Interpretation". In 2012, Mermin became a public advocate of QBism, an interpretation due to Christopher Fuchs and Ruediger Schack. Here, we evaluate QBism with respect to the Ithaca Interpretation's six desiderata, in the process also evaluating those desiderata themselves. This analysis reveals a genuine distinction between QBism and the IIQM, but also a natural progression from one to the other.

quant-ph↗

arXiv2018

Chronic Incompleteness, Final Theory Claims, and the Lack of Free Parameters in String Theory

String theory has not even come close to a complete formulation after half a century of intense research. On the other hand, a number of features of the theory suggest that the theory, once completed, may be a final theory. It is argued in this chapter that those two conspicuous characteristics of string physics are related to each other. What links them together is the fact that string theory has no dimensionless free parameters at a fundamental level. The paper analyses possible implications of this situation for the long term prospects of theory building in fundamental physics.

physics.hist-ph↗

arXiv2018

Neutrino, parity violaton, V-A: a historical survey

A concise story of the rise of the four fermion theory of the universal weak interaction and its experimental confirmation, with a special emphasis on the problems related to parity violation.

physics.hist-ph↗

arXiv2019

Fueling Peter's Mill: Mikhail Lomonosov's Educational Training in Russia and Germany, 1731-1741

This article, the second in a series about the Russian scientist Mikhail Lomonosov (1711-1765), traces his education from his arrival in Moscow in 1731 to study at the Slavo-Greco-Latin Academy, through his admission to the St. Petersburg Academy of Sciences in 1736, to his trip abroad to complete his educational studies from 1736 to 1741. Lomonosov's story during this time opens a vista on the introduction of modern physics and modern science into Russia. Michael D. Gordin has argued that Peter the Great's plans to Westernize Russia were more broadly conceived than he is usually credited, with ambitions that exceeded mere utilitarian and pragmatic goals. Lomonosov's career trajectory is a good example, illustrating how different aspects of the Petrine vision intersected with and reinforced each other. The article ends with Lomonosov's return to Russia from Germany in 1741, an important landmark in the growth of the Academy and of Russian science.

physics.hist-ph↗

arXiv2019

John Archibald Wheeler: A Biographical Memoir

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.

physics.hist-ph↗

arXiv2019

Quantum States of a Time-Asymmetric Universe: Wave Function, Density Matrix, and Empirical Equivalence

What is the quantum state of the universe? Although there have been several interesting suggestions, the question remains open. In this paper, I consider a natural choice for the universal quantum state arising from the Past Hypothesis, a boundary condition that accounts for the time-asymmetry of the universe. The natural choice is given not by a wave function (representing a pure state) but by a density matrix (representing a mixed state). I begin by classifying quantum theories into two types: theories with a fundamental wave function and theories with a fundamental density matrix. The Past Hypothesis is compatible with infinitely many initial wave functions, none of which seems to be particularly natural. However, once we turn to density matrices, the Past Hypothesis provides a natural choice---the normalized projection onto the Past Hypothesis subspace in the Hilbert space. Nevertheless, the two types of theories can be empirically equivalent. To provide a concrete understanding of the empirical equivalence, I provide a novel subsystem analysis in the context of Bohmian theories. Given the empirical equivalence, it seems empirically underdetermined whether the universe is in a pure state or a mixed state. Finally, I discuss some theoretical payoffs of the density-matrix theories and present some open problems for future research.

quant-ph↗

arXiv2019

Information-theoretic foundations of thermodynamics in general probabilistic theories

We study the informational underpinnings of thermodynamics and statistical mechanics, using an abstract framework, general probabilistic theories, capable of describing arbitrary physical theories. This allows one to abstract the informational content of a theory from the concrete details of its formalism. In this framework, we extend the treatment of microcanonical thermodynamics, namely the thermodynamics of systems with a well-defined energy, beyond the known cases of classical and quantum theory, formulating two necessary requirements for a well-defined thermodynamics. We adopt the recent approach of resource theories, where one studies the transitions between states that can be accomplished with a restricted set of physical operations. We formulate three different resource theories, differing in the choice of the restricted set of physical operations. To bridge the gap between the objective dynamics of particles and the subjective world of probabilities, one of the core issues in the foundations of statistical mechanics, we propose four information-theoretic axioms. They are satisfied by quantum theory and more exotic alternatives, including a suitable extension of classical theory where classical systems interact with each other creating entangled states. The axioms identify a class of theories where every mixed state can be modelled as the reduced state of a pure entangled state. In these theories it is possible to introduce well-behaved notions of majorisation, entropy, and Gibbs states, allowing for an information-theoretic derivation of Landauer's principle. The three resource theories define the same notion of resource if and only if, on top of the four axioms, the dynamics of the underlying theory satisfy a condition called "unrestricted reversibility". Under this condition we derive a duality between microcanonical thermodynamics and pure bipartite entanglement.

quant-ph↗

arXiv2019

Reality and the Probability Wave

Effects associated in quantum mechanics with a divisible probability wave are explained as physically real consequences of the equal but opposite reaction of the apparatus as a particle is measured. Taking as illustration a Mach-Zehnder interferometer operating by refraction, it is shown that this reaction must comprise a fluctuation in the reradiation field of complementary effect to the changes occurring in the photon as it is projected into one or other path. The evolution of this fluctuation through the experiment will explain the alternative states of the particle discerned in self interference, while the maintenance of equilibrium in the face of such fluctuations becomes the source of the Born probabilities. In this scheme, the probability wave is a mathematical artifact, epistemic rather than ontic, and akin in this respect to the simplifying constructions of geometrical optics.

quant-ph↗

arXiv2019

Demystifying Gauge Symmetry

Gauge symmetries are often highlighted as a fundamental cornerstone of modern physics. But at the same time, it is commonly emphasized that gauge symmetries are not a fundamental feature of nature but merely redundancies in our description. We argue that this paradoxical situation can be resolved by a proper definition of the relevant notions like "local", "global", "symmetry" and "redundancy". After a short discussion of these notions in the context of a simple toy model, they are defined in general terms. Afterwards, we discuss how these definitions can help to understand how gauge symmetries can be at the same time fundamentally important and purely mathematical redundancies. In this context, we also argue that local gauge symmetry is not the defining feature of a gauge theory since every theory can be rewritten in locally invariant terms. We then discuss what really makes a gauge theory different and why the widespread "gauge argument" is, at most, a useful didactic tool. Finally, we also comment on the origin of gauge symmetries, the common "little group argument" and the notoriously confusing topic of spontaneous gauge symmetry breaking.

physics.hist-ph↗

arXiv2019

The Metaphysics of Machian Frame-Dragging

The paper investigates the kind of dependence relation that best portrays Machian frame-dragging in general relativity. The question is tricky because frame-dragging relates local inertial frames to distant distributions of matter in a time-independent way, thus establishing some sort of non-local link between the two. For this reason, a plain causal interpretation of frame-dragging faces huge challenges. The paper will shed light on the issue by using a generalized structural equation model analysis in terms of manipulationist counterfactuals recently applied in the context of metaphysical enquiry by Schaffer (2016) and Wilson (2017). The verdict of the analysis will be that frame-dragging is best understood in terms of a novel type of dependence relation that is half-way between causation and grounding.

physics.hist-ph↗

arXiv2018

Pion Interactions and the Standard Model at High Precision

Pions were predicted by H. Yukawa as force carriers of the inter-nucleon forces, and were detected in 1947. Today they are known to be bound states of quarks and anti-quarks of the two lightest flavours. They satisfy Bose statistics, and are the lightest particles of the strong interaction spectrum. Determination of the parameters of the Standard Model, including the masses of the lightest quarks, has only recently reached high precision on the lattice. Pions are also known to be pseudo-Goldstone bosons of spontaneously broken approximate axial-vector symmetries, and a probe of their properties and interactions at high precision tests our knowledge of the strong interactions. While also being a probe of the solution of the strong interactions on the computer, which is known as lattice gauge theory. Despite their long history, there are significant experimental and theoretical challenges in determining their properties at high precision. Examples include the lifetime of the neutral pion, and the status of their masses and decay widths in effective field theories. Pion-pion scattering has been studied for several decades using general methods of field theory such as dispersion relations based on analyticity, unitarity and crossing. Knowledge from these theoretical methods are used to confront high precision experimental data, and to analyze them to extract information on their scattering and phase shift parameters. This knowledge is crucial for estimating the Standard Model contributions to the anomalous magnetic moment of the muon, which is being probed at Fermilab in ongoing experiments. Other sensitive tests include the rare decay of the eta meson into three pions, which represents an isospin violating decay. The present article briefly reviews these important developments.

physics.hist-ph↗

arXiv2019

Statistical Mechanics and the Ghosts of Departed Quantities

Quantum mechanics appears to contain ghosts from both classical statistical mechanics and special relativity. On one hand, both the Dirac and Schrödinger equations have classical analogs that emerge directly from classical statistical mechanics, unimpeded by major problems of interpretation. On the other hand, the formal analytic continuation that takes these classical equations to the quantum version introduces a velocity dependent phase. However, among classical theories, only in relativistic mechanics does one find path-dependent phase in the form of relativistic time dilation. This paper explores the idea that if we start with statistical mechanics and special relativity we can discover a version of the quantum algorithm and show that at least some of the resulting ghosts are direct descendants of those connected with the birth of the differential calculus.

physics.hist-ph↗

arXiv2019

La cosmolog\'ıa y los matemáticos (Cosmology and mathematicians)

Free translation of the original abstract in Spanish: Some of the most relevant milestones due to, or instigated by, mathematicians concerning the creation, development and advances of Cosmology as a scientific discipline are presented and discussed. In particular, the close relationship between Cosmology and Mathematics, derived from the geometrization of the former performed by Einstein after his General theory of Relativity, is described in detail with a thorough analysis of the contributions and collaborations by distinguished XX-th century mathematicians.

math.HO↗

arXiv2019

Neutrinos and Particle Physics Models

As in Greek mythology, the neutrino was born in the mind of Wolfgang Pauli to salvage a fundamental principle. Its existence met with universal skepticism by a scientific community used to infer particles from experiment. Its detection in 1956 brought particle physics acceptance; its chirality explained maximal parity violation in $β$ decay; its (apparent) masslessness led theorists to imagine new symmetries. Neutrinos are pioneers of mutli-messenger astronomy, from the Sun, from SNA1987, and now through IceCube's blazar. The discovery of neutrino masses opened a new era in particle physics aswell as unexplored windows on the universe. -Tiny neutrino masses suggest new physics at very short distances through the Seesaw. - Neutrinos and quarks, unified by gauge structure, display different mass and mixing patterns: small quark mixing angles and two large neutrino mixing angles. This difference in mass and mixings in the midst of gauge unification may be an important clue towards Yukawa unification. - Neutrino mixings provide a new source of CP-violation, and may solve the riddle of matter-antimatter asymmetry. We present a historical journey of these "enfants terribles" of particle physics and their importance in understanding our universe.

physics.hist-ph↗