arXiv subjects
Publications and source records attributed to Nuno Barros e Sá.
We lay down a set of requirements for a field theory to produce a covariant conservation law out of Noether's second theorem, and show that neither local invariance implies a covariant conservation law, nor the existence of a covariant conservation law necessarily stems from the local invariance of the theory. We illustrate our results with the examples of well-known theories.
The Earth's magnetic field can be decomposed into spherical harmonics, and the exact coefficients of the decomposition can be determined through a few measurements of its value at different locations. Using measurements from a magnetometer on board the International Space Station, we computed the multipolar moments of an expansion in spherical harmonics to order 3, calculated the location of the magnetic dip poles, and produced an intensity map of the magnetic field across the globe. The accuracy of the results was evaluated by comparison with data from the International Geomagnetic Reference Field.
We provide a simultaneous derivation of the Dirac bracket and of the equations of motion for second-class constrained systems when the constraints are time-dependent. The necessity of time-dependent gauge-fixing conditions is shown in the example of parameterized mechanics and is illustrated geometrically.
We present a proof that the currents arising from Noether's first theorem in a physical theory with local invariance can always be decomposed into two terms, one of them vanishing on-shell, and the other having an off-shell vanishing divergence, or that they are improper, using the original terminology of Noether. We also prove that, when there is a current which is covariantly conserved, it differs from the canonical current by an improper current. Both proofs are performed in the most general case, that is, for arbitrary maximal order of the derivatives of the dynamical fields of the theory in the Lagrangian, and for arbitrary maximal order of the derivatives of the parameters of the symmetry transformations present in the infinitesimal transformations of the fields and spacetime coordinates. Both proofs are made using only elementary calculus, making them accessible to a large number of physicists.
We construct the algebra of operators acting on the Hilbert spaces of Quantum Mechanics for systems of $N$ identical particles from the field operators acting in the Fock space of Quantum Field Theory by providing the explicit relation between the position and momentum operators acting in the former spaces and the field operators acting on the latter. This is done in the context of the non-interacting Klein-Gordon field. It may not be possible to extend the procedure to interacting field theories since it relies crucially on particle number conservation. We find it nevertheless important that such an explicit relation can be found at least for free fields. It also comes out that whatever statistics the field operators obey (either commuting or anticommuting), the position and momentum operators obey commutation relations. The construction of position operators raises the issue of localizability of particles in Relativistic Quantum Mechanics, as the position operator for a single particle turns out to be the Newton-Wigner position operator. We make some clarifications on the interpretation of Newton-Wigner localized states and we consider the transformation properties of position operators under Lorentz transformations, showing that they do not transform as tensors, rather in a manner that preserves the canonical commutation relations.