Catálogo de publicaciones - revistas

<jats:p> Although physical laws or theories are often invoked in debates over “causality” and “determinism,” our best current understanding of physics assigns only a limited (though still very broad) validity to these concepts. It may be, thus, helpful (particularly when having to deal with the challenges posed by quantum mechanics) to think of them as prejudices, extrapolated from our experience with a limited (essentially classical) set of phenomena and/or theoretical models. This paper discusses how, over time, different physical theories have either reinforced or challenged these prejudices, focusing specifically on conservative “Laplacian” mechanics, dissipative mechanics (thermal physics), and quantum mechanics. </jats:p>

<jats:p> We describe a pandemic-inspired, modern physics distance lab course, focused both on engaging undergraduate physics majors in scientific research from their homes and on building skills in scientific paper reading and writing. To introduce the experimental and analytic tools, students are first asked to complete a traditional lab assignment in which collections of Hexbugs™, randomly moving toy automatons, are used to model gas molecules and to confirm the ideal gas law. Subsequently, after consulting the literature, students propose and implement semester-long experiments using Hexbugs™, smartphones, and materials commonly found at home to model various concepts in statistical mechanics and electrical conduction. A sample project focused on the Drude model, in which Hexbugs™ on a tilted plane are used to model electrical conduction, is described in detail. Alongside the research projects, students write formal, peer-reviewed scientific papers on their work, modeling the professional publication process as closely as possible. Somewhat paradoxically, we found that the pandemic-inspired exigency of reliance on simple, home-built experiments enabled an increased focus on developing experimental research skills and achieving the laboratory learning objectives recommended by the American Association of Physics Teachers. </jats:p>

<jats:p>[Media: see text]</jats:p><jats:p> The pioneering work of Taylor on the turbulent dispersion of aerosols is one century old and provides an interesting way to introduce both diffusive processes and turbulence at an undergraduate level. Low mass particles transported by a turbulent flow exhibit a Brownian-like motion over time scales larger than the velocity correlation time. Aerosols and gases are, therefore, subjected to an effective turbulent diffusion at large length scales. However, the case of a source of pollutant much smaller than the integral scale is not completely understood. Here, we present experimental results obtained by undergraduate students in the context of the COVID-19 pandemic. The dispersion of a fog of oil droplets by a turbulent flow is studied in a wind tunnel designed for pedagogical purposes. It shows a ballistic-like regime at short distance, followed by Taylor's diffusive-like regime, suggesting that scale-free diffusion by the turbulent cascade process is bypassed. Measurements show that the dispersion of CO<jats:sub>2</jats:sub> emitted when breathing in a natural, indoor air flow is not isotropic but rather along the flow axis. The transverse spread is ballistic-like, leading to the concentration decaying as the inverse-squared distance to the mouth. The experiment helps students understand the role of fluctuations in diffusive processes and in turbulence. A Langevin equation governing aerosol dispersion based on a single correlation time allows us to model the airborne transmission risk of pathogens, indoors and outdoors. The results obtained in this study have been used to provide public health policy recommendations to prevent transmission in shopping malls. </jats:p>

<jats:p> The quantum version of the free fall problem is a topic often skipped in undergraduate quantum mechanics courses, because its discussion usually requires wavepackets built on the Airy functions—a difficult computation. Here, on the contrary, we show that the problem can be nicely simplified both for a single particle and for general many-body systems by making use of a gauge transformation that corresponds to a change of reference frame from the laboratory frame to the one comoving with the falling system. Using this approach, the quantum mechanics problem of a particle in an external gravitational potential reduces to a much simpler one where there is no longer any gravitational potential in the Schrödinger equation. It is instructive to see that the same procedure can be used for many-body systems subjected to an external gravitational potential and a two-body interparticle potential that is a function of the distance between the particles. This topic provides a helpful and pedagogical example of a quantum many-body system whose dynamics can be analytically described in simple terms. </jats:p>

<jats:p> In Supplee's submarine paradox, a naive argument based on Lorentz contraction leads to a contradiction that a fast submarine should sink in the water's reference frame but float in the submarine's reference frame. Due to the submarine's rigidity constraints, it is not easy to resolve the paradox in a manifestly covariant form. To simplify the problem, we consider a version of the paradox in which one fluid moves through another fluid. An analysis of ideal relativistic fluids in a weak gravitational field shows that the moving fluid has a larger pressure and, hence, sinks, in agreement with known results for the rigid submarine. </jats:p>

<jats:p> Long-term changes in the tilt of the Earth's axis, relative to the plane of its orbit, are of great significance to long-term climate change, because they control the size of the arctic and Antarctic circles. These “Milankovitch cycles” have hitherto been calculated by classical perturbation methods or by direct numerical integration of Newton's equations of motion. This paper presents an approximate calculation from simple considerations of angular momentum using similar methods to those used to study the precession of a spinning top. It is an instructive exercise in classical mechanics and gives a simple explanation of the phenomenon in terms of angular momentum. It is shown that the main component of “Milankovitch cycles” has a period of 41,000 yr and is due to one of the modes of precession of the Earth-Venus system. The other mode of this system produces a component of period 29,500 yr, and a third component of period 54,000 yr results from the influence of the precession of the orbits of Jupiter and Saturn. These results agree closely with several of the numerical simulations in the literature and strongly suggest that some other results in the literature are incorrect. </jats:p>