L’artigiano e il rivoluzionario verso una teoria delle particelle elementari: influenze culturali, divergenze e rincontri

Nel loro cammino verso una teoria delle particelle elementari, Sin-Itiro To- monaga, abile e pragmatico artigiano della matematica e ‘conservatore non reazionario’ - come Shoichi Sakata ebbe a definirlo affettuosamente - e il suo amico e compagno di studi Hideki Yukawa, rivoluzionario e audace ‘desi- gner’ - prendendo a prestito la definizione di Yoichiro Nambu - si sono posti in una feconda tensione dialettica. Attraverso di essa, questi due protagonisti della fisica teorica dell’ultimo secolo si sono vicendevolmente plasmati, arric- chiti e completati, arrivando da ultimo a una risonanza di punti di vista. In questo contributo ripercorreremo le due avventure intellettuali e illustreremo come, in modi diversi, abbiano attinto alla tradizione del pensiero giapponese

Taking Approximations Seriously: The Cases of the Chew and the Nambu-Jona-Lasinio Models

In this article, we offer a detailed study of two important episodes in the early history of high-energy physics, namely the development of the Chew and the Nambu-Jona-Lasinio models. Our study reveals that both models resulted from the combination of an old Hamiltonian, which had been introduced by earlier researchers, and two new approximation methods developed by Chew and by Nambu and Jona-Lasinio. These new approximation methods, furthermore, were the key component behind the models’ success. We take this historical investigation to support two philosophical theses about the manner in which scientific modelling operates in high-energy physics. Both of these theses run counter to a view that is commonly accepted among philosophers of science: the view that all approximations can be embedded within an equivalent idealized system, and that whatever role the former might play in scientific modelling is therefore parasitic on the much more substantial work performed by the latter. Our first thesis, which we call “Distinctness,” states that approximation methods constitute an independent category of theoretical output from idealized systems. We thus believe that approximations and idealized systems constitute two independent types of objects, both of which are essential to the practice of modelling. Our second, more radical thesis is called “Content Determination.” Our claim here is that approximation methods can in fact be essential to assigning determinate physical content to the idealized systems with which they jointly operate. As we show, this is due to the fact that quantum field theory allows for a very thin characterization of idealized systems only, making the use of approximations necessary to supply additional content. We conclude the paper with a few reflections about the manner in which our two theses can be used to articulate David Kaiser’s views on the “vanishing of scientific theory” in physics after WWII

Kondo Effect in a Neutral and Stable All Organic Radical Single Molecule Break Junction

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin-orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics

Finite element models of piezoelectric actuators for active flow control

A numerical procedure, based on the finite element method, capable to simulate the interaction of active structures with an incompressible fluid flow is discussed. In particular the active functionality of such structures is demanded to piezoelectric type actuators. The development of this interaction is connected to the study of problems that involve an active flow control for different potential applications as drag reduction, noise abatement, separation control, mixing enhancement, etc. Two kind of finite element models, one for the electromechanical field and the other for the fluid dynamic field, are built. The analyses are performed with a coupled iterative solver and they are based on the Arbitrarian Lagrangian-Eulerian (ALE) description. A Reynolds Averaged Navier-Stokes Equations (RANS) formulation for the model of turbulent fluid is adopted. The results of some numerical analyses are correlated to an experimental benchmark case founded in literature with the aim to validate the procedure. A sample application to control of separated flow from a backward-facing step is described, in which a piezoelectric unimorph actuator is patched on a Euler-Bernoulli beam installed at the upper corner of the step. The numerical model describes the displacement of the incoming shear layer and the velocity perturbation produced by the periodic oscillations of the actuator and how these parameters are related each other. In order to produce sensible amplitude for the oscillations, the actuator is driven near its natural frequency. A preliminary response analysis to examine the effects of the fluid on the resonant behaviour of the structure is done

Light-induced propulsion of graphene-on-grid sails in microgravity

Light sailing is the only existing in-space propulsion technology that could allow us to visit other star systems in a human lifespan. In order to best harness radiation pressure, light sails need to be highly reflective, lightweight and mechanically robust. This is traditionally achieved by the use of nanometer-thin reflective layers supported by a micrometer-thick substrate that endows them with the necessary sturdiness. This combination usually results in a sail mass that is too high to be efficiently used for extrasolar exploration. Here, we propose a potentially scalable sail design that combines a hollow substrate with an atomically-thin 2D material which, thanks to its ultimately low surface density, allows reducing the mass contribution of the substrate. To demonstrate the potential of such sails, we have studied the laser-induced displacement of graphene-on-copper sails in vacuum and in microgravity. In these conditions, 0.25 mg samples are accelerated by using lasers of different wavelengths (450 and 655 nm) and power ( W). The measured thrust is one order of magnitude larger than the theoretical calculations for radiation pressure alone. This calls for further theoretical studies and attracts interest on graphene as light-sail material

Taking approximations seriously: The cases of the Chew and Nambu-Jona-Lasinio models

In this article, we offer a detailed study of two important episodes in the early history of high-energy physics, namely the development of the Chew and the Nambu-Jona-Lasinio models. Our study reveals that both models resulted from the combination of an old Hamiltonian, which had been introduced by earlier researchers, and two new approximation methods developed by Chew and by Nambu and Jona-Lasinio. These new approximation methods, furthermore, were the key component behind the models’ success. We take this historical investigation to support two philosophical theses about the manner in which scientific modelling operates in high-energy physics. Both of these theses run counter to a view that is commonly accepted among philosophers of science: the view that all approximations can be embedded within an equivalent idealized system, and that whatever role the former might play in scientific modelling is therefore parasitic on the much more substantial work performed by the latter. Our first thesis, which we call “Distinctness,” states that approximation methods constitute an independent category of theoretical output from idealized systems. We thus believe that approximations and idealized systems constitute two independent types of objects, both of which are essential to the practice of modelling. Our second, more radical thesis is called “Content Determination.” Our claim here is that approximation methods can in fact be essential to assigning determinate physical content to the idealized systems with which they jointly operate. As we show, this is due to the fact that quantum field theory allows for a very thin characterization of idealized systems only, making the use of approximations necessary to supply additional content. We conclude the paper with a few reflections about the manner in which our two theses can be used to articulate David Kaiser’s views on the “vanishing of scientific theory” in physics after WWII

Water distribution in GDL near optimal humidification

In order to better understand the two phase properties of the porous GDL, the liquid water pattern and local saturation in the Toray 060 gas diffusion layer (GDL) is visualized in-situ in cells close to the optimal performance point at 80°C by X-ray tomographic microscopy. A recording time of about 10 s is achieved. At the optimal performance point (dew point of reactant gases 74 °C) the water volume fraction observed depends on the rib width. A variation of ± 8 °C of the feed gas dew point leads to significant saturation and performance changes. For different current densities the water volume fraction and permeability of the liquid phase of the cathode are determined and analyzed