L’esame di un passo della Vita di Platone laerziana (III 2) – e di luoghi paralleli riferibili alla medesima tradizione – permette di mettere a fuoco un motivo mitico cruciale nei racconti sulla concorrenza tra essere umano e divinità nel gamos con una mortale: il divieto, che il nume impone al ‘rivale’ di inferiore natura, di congiungersi con la donna che reca in grembo il suo seme, finché l’eroina non abbia portato a termine la gravidanza. Le storie di personaggi femminili (...) colpevoli di duplice gamos – prima con il dio e poi, a breve distanza di tempo, con il mortale – si focalizzano sulle conseguenze nefaste del secondo contaminante connubio: uno per tutti il caso di Coronide madre di Asclepio (e forse il motivo era associato anche ad altre note madri mitiche, come Arianna, Semele e Alcmena). In questi esempi, tuttavia, in genere non è esplicitato il tema della proibizione divina, come invece accade nel bios di Diogene Laerzio. Qui infatti Apollo, dopo aver concepito Platone con Perictione, appare ad Aristone per vietargli il rapporto sessuale con la parthenos. (shrink)

We give an account of the appearance and first developments of the statistical model of atoms proposed by Thomas and Fermi, focusing on the main results achieved by Fermi and his group in Rome. Particular attention is addressed to the unknown contribution to this subject by Majorana, anticipating some important results reached later by leading physicists.

Spin-half fermions are considered to be limited in a spherical potential well with periodic boundary conditions. The whole system is treated like a relativistic Fermi Gas. Solving the corresponding Dirac equation, the density of states, the Fermi energy, the average energy, the density of states of nucleons and the total energy of the ground-state are obtained.

Expert reasoning combines voluminous domain‐specific knowledge with more general factual and strategic knowledge. Whereas expert system builders have recognized the need for specificity and problem‐solving researchers the need for generality, few attempts have been made to develop expert reasoning engines combining different kinds of knowledge at different levels of generality. This paper reports on the FERMI project, a computer‐implemented expert reasoner in the natural sciences that encodes factual and strategic knowledge in separate semantic hierarchies. The principled decomposition of knowledge (...) according to type and level of specificity yields both power and cross‐doman generality, as demonstrated in FERMI's ability to apply the same principles of invariance and decomposition to solve problems in fluid statics, DC‐circuits, and centroid location. Hierarchical knowledge representation and problem‐solving principles are discussed, and illustrative problem‐solving traces are presented. (shrink)

Fermi-Dirac statistics are one of two kinds of statistics exhibited by !identical quantum particles, the other being !Bose-Einstein statistics. Such particles are called fermions and bosons respectively (the terminology is due to Dirac [1902-1984] [1]). In the light of the !spin-statistics theorem, and consistent with observation, fermions are invariably spinors (of half-integral spin), whilst bosons are invariably scalar or vector particles (of integral spin). See !spin.

We consider the quantum electrodynamic problem of the velocity of transmission of an excitation from one atom to another, which was posed by Fermi in 1932 and, in an improved version, by Ferretti in 1968. The problem is solved using Heisenberg operators. The transmission velocity is shown to exceed the light velocity. We argue that this acausal result is to be explained by the inadmissibly idealized description of the signal source used by Fermi and Ferretti.

We present a Gedankenexperiment that leads to a violation of detailed balance if quantum mechanical transition probabilities are treated in the usual way by applying Fermi’s “golden rule”. This Gedankenexperiment introduces a collection of two-level systems that absorb and emit radiation randomly through non-reciprocal coupling to a waveguide, as realized in specific chiral quantum optical systems. The non-reciprocal coupling is modeled by a hermitean Hamiltonian and is compatible with the time-reversal invariance of unitary quantum dynamics. Surprisingly, the combination of (...) non-reciprocity with probabilistic radiation processes entails negative entropy production. Although the considered system appears to fulfill all conditions for Markovian stochastic dynamics, such a dynamics violates the Clausius inequality, a formulation of the second law of thermodynamics. Several implications concerning the interpretation of the quantum mechanical formalism are discussed. (shrink)

The Fermi energy at 0°K is evaluated for electrons confined to cubical and spherical rigid-walled boxes of equal volume, respectively, in the Sommerfeld approximation. Due primarily to large differences in single-particle degeneracies, Fermi energies compared for equal numbers of particles in these two configurations are found to be unequal. Approximate expressions of the Fermi energy in the large particle-number limit for the spherical case reveal that it agrees in form with the Fermi energy for the cubical (...) configuration. The finite cylindrical box is also examined and it is found that for fixed number of particles and fixed volume, the Fermi energy varies as the two characteristic lengths of the box are changed. Experimental corroboration of the theory is suggested through measurement of the work function for equal-volume micrometallic samples in the different geometries. (shrink)

We present a reformulation of loop quantum gravity with a cosmological constant and no matter as a Fermi-liquid theory. When the topological sector is deformed and large gauge symmetry is broken, we show that the Chern–Simons state reduces to Jacobson’s degenerate sector describing 1+1 dimensional propagating fermions with nonlocal interactions. The Hamiltonian admits a dual description which we realize in the simple BCS model of superconductivity. On one hand, Cooper pairs are interpreted as wormhole correlations at the de Sitter (...) horizon; their number yields the de Sitter entropy. On the other hand, BCS is mapped into a deformed conformal field theory reproducing the structure of quantum spin networks. When area measurements are performed, Cooper-pair insertions are activated on those edges of the spin network intersecting the given area, thus providing a description of quantum measurements in terms of excitations of a Fermi sea to superconducting levels. The cosmological constant problem is naturally addressed as a nonperturbative mass-gap effect of the true Fermi-liquid vacuum. (shrink)

In this review of Milan Ćirković’s The Great Silence: Science and Philosophy of Fermi’s Paradox, we attempt to reconstruct the logic of Fermi’s paradox as understood by the author, and we critically examine the reasoning that leads to the paradox. We show that there is no plausible solution to Fermi’s paradox that can satisfy all of Ćirković’s proposed desiderata, which in turn suggests that the author’s standards for hypothesis adjudication need to be revised.

A rational approach to the Fermi-Walker transport equation is proposed by deriving it from a condition of “non-rotation”. First, the condition is applied to a tetrad basis and then generalized to an arbitrary space-time four-vector. The method is conceptually simple and apart from the use of tetrad bases in four-dimensional space-time, does not require the effort of visualizing abstract geometrical constructs in spaces of more than three dimensions. The argument develops in the context of the flat space-time of special (...) relativity theory but the final result can be immediately extended to curved space-time. (shrink)

The generally covariant Lagrangian densityG = ℛ + 2K ℒmatter of the Hamiltonian principle in general relativity, formulated by Einstein and Hilbert, can be interpreted as a functional of the potentialsg ikand φ of the gravitational and matter fields. In this general relativistic interpretation, the Riemann-Christoffel form Γ kl i = kl i for the coefficients г kl i of the affine connections is postulated a priori. Alternatively, we can interpret the LagrangianG as a functional of φ, gik, and the (...) coefficients г kl i . Then the г kl i are determined by the Palatini equations. From these equations and from the symmetry г kl i = г lk i for all matter fields with δℒ/δΓ=0 the Christoffel symbols again result. However, for Dirac's bispinor fields, δℒ/δΓ becomes dependent on the Dirac current, essentially with a coupling factor ∼Khc. In this case, the Palatini equations define a new transport rule for the spinor fields, according to which a second universal interaction results for the Dirac spinors, besides Einstein's gravitation. The generally covariant Dirac wave equations become the general relativistic nonlinear Heisenberg wave equations, and the second universal interaction is given by a Fermi-like interaction term of the V-A type. The geometrically induced Fermi constant is, however, very small and of the order 10−81erg cm3. (shrink)

La humildad copernicana establece que el Universo es un lugar homogéneo en el que las leyes aplican indistintamente en cualquier lugar, es decir, no existe un centro o lugar privilegiado. En ese sentido el lugar que ocupa la Tierra en el Universo es como cualquier otro, nada especial y mucho menos relacionado al favor de un Dios omnipotente cuya creación favorita sea el ser humano. Siendo esto así, sería entonces factible pensar la existencia de muchos otros planetas que como la (...) Tierra alberguen vida y sustenten civilizaciones tan o más avanzadas como la nuestra. Sin embargo, ¿por qué aún no hemos contactado con otras civilizaciones? Esto último se conoce como la paradoja de Fermi. ¿Por qué si la Tierra es solo un punto más en el Universo parece que no hay otras Tierras y otros seres inteligentes con quienes contactar? Una posible solución que intentamos discutir en el presente texto es la Hipótesis de la Tierra Rara, según la cual, en efecto, el planeta Tierra es una cosa extraordinaria en el universo. Más allá de discutir la plausibilidad de esta hipótesis frente a otras, interesa puntualizar algunos corolarios que se siguen de la misma, y su implicación para el copernicanismo desde un punto de vista filosófico. (shrink)

In analogy with Planck’s construction of fundamental quantities in gravitation, we construct fundamental quantities associated with (1) theories of electrodynamics in which the electromagnetic field has a maximum value (e.g. Born-Infeld theory), and (2) the Fermi interaction. This gives us a maximum intensity of the electromagnetic field, and also reveals a close relationship between the fundamental lengths associated with the gravitational and weak interactions, supporting the connection between these two interactions.