F C Santos

A simple way of understanding the nonadditivity of van der Waals dispersion forces

Using a semiclassical model for three interacting fluctuating dipoles we introduce a simple scenario in which the nonadditivity of the van der Waals dispersion forces arises in a very transparent way. For simplicity, we illustrate our model in the case of nonretarded dispersion forces. The argument can be straightforwardly generalized to the case of N interacting fluctuating dipoles. For pedagogical reasons, we also give a brief review of some basic points concerning van der Waals forces.

A simple model for the nonretarded dispersive force between an electrically polarizable atom and a magnetically polarizable one

It is known that the consideration or not of retardation effects on the dispersive van der Waals force between two electrically polarizable atoms leads to power laws for the forces that differ only by one power: 1/r8 for the retarded force and 1/r7 for the nonretarded one. On the other hand, for the case of an electrically polarizable atom and a magnetically polarizable one, while the retarded force is still proportional to 1/r8, the nonretarded one is proportional to 1/r5. Here, employing the fluctuating dipole model for both atoms, we reobtain this quite unexpected behavior for the nonretarded force mentioned above. A physical interpretation is also given for such a result.

The motion of two masses coupled to a finite mass spring

We discuss the classical motion of a finite mass spring coupled to two pointlike masses fixed at its ends. A general approach to the problem is presented and some general results are obtained. Examples for which a simple elastic function can be inferred are discussed and the normal modes and normal frequencies obtained. An approximation procedure to the evaluation of the normal frequencies in the case of uniform elastic function and mass density is also discussed.

On the force between an electrically polarizable atom and a magnetically polarizable one

In this paper we re-obtain the retarded as well as the non-retarded force between an electrically polarizable atom and a magnetically polarizable one in a very simple and suggestive way. Using a perturbative approach, we show that while the retarded force between these two atoms is proportional to 1/r8, the non-retarded force is proportional to 1/r5, in agreement with Feinberg and Suchers result (Feinberg G and Sucher J 1970 Phys. Rev. A 2 2395). This rather surprising result should be contrasted with the 1/r7 behaviour of the London–van der Waals force between two electrically polarizable atoms. Our approach permits us to give a physical interpretation for such a result.

Confined Maxwell Field and Temperature Inversion Symmetry

We evaluate the Casimir vacuum energy at finite temperature associated with the Maxwell field confined by a perfectly conducting rectangular cavity and show that an extended version of the temperature inversion symmetry is present in this system.

QED vacuum between a conducting and a permeable plate

We consider the photon field between an unusual configuration of infinite parallel plates, namely: a perfectly conducting plate () and an infinitely permeable one (µ). After quantizing the vector potential in the Coulomb gauge, we obtain explicit expressions for the vacuum expectation values of field operators of the form EiEj0 and BiBj0. These field correlators allow us to re-obtain the Casimir effect for this set-up and to discuss the light velocity shift caused by the presence of plates (Scharnhorst effect: Scharnhorst (1990 Phys. Lett. B 236 354), Barton (1990 Phys. Lett. B 237 559), Barton and Scharnhorst (1993 J. Phys. A: Math. Gen. 26 2037)) for both scalar and spinor QED.We consider the photon field between an unusual configuration of infinite parallel plates: a perfectly conducting plate (ǫ → ∞) and an infinitely permeable one μ → ∞). After quantizing the vector potential in the Coulomb gauge, we obtain explicit expressions for the vacuum expectation values of field operators of the form < ÊiÊj >0 and < B̂iB̂j >0. These field correlators allow us to reobtain the Casimir effect for this set up and to discuss the light velocity shift caused by the presence of plates (Scharnhorst effect [1, 2, 3]) for both scalar and spinor QED. PACS numbers: 11.10.-z, 11.10.Mn

The electrostatic field of a point charge and an electrical dipole in the presence of a conducting sphere

We evaluate the electrostatic potential and the electrostatic field created by a point charge and an arbitrarily oriented electrical dipole placed near a grounded perfectly conducting sphere. Induced surface charge distributions and possible variants of the problem are also discussed.

The speed of light in confined QED vacuum: faster or slower than c?

Abstract We consider the propagation of light in the QED vacuum between an unusual pair of parallel plates, namely: a perfectly conducting one ( ϵ →∞) and an infinitely permeable one ( μ →∞). For weak fields and in the soft photon approximation we show that the speed of light for propagation normal to the plates is smaller than its value in unbounded space (in contrast to the original Scharnhorst effect [K. Scharnhorst, Phys. Lett. B 236 (1990) 354, G. Barton, Phys. Lett. B 237 (1990) 559, G. Barton, K. Scharnhorst, J. Phys. A 26 (1993) 2037]).

Confined quantum fields under the influence of a uniform magnetic field

We investigate the influence of a uniform magnetic field on the zero-point energy of charged fields of two types, namely, a massive charged scalar field under Dirichlet boundary conditions and a massive fermion field under MIT boundary conditions. For the first, exact results are obtained, in terms of exponentially convergent functions, and for the second, the limits for small and for large masses are analytically obtained also. Coincidence with previously known, partial results serves as a check of the procedure. For the general case in the second situation—a rather involved one—a precise numerical analysis is performed.

A note on the Lorentz force, magnetic charges and the Casimir effect

We show that in order to account for the repulsive Casimir effect in the parallel-plate geometry in terms of the quantum version of the Lorentz force, it is possible to introduce virtual surface densities of magnetic charge and currents. The quantum version of the Lorentz force expressed in terms of the correlators of the electric and magnetic fields for planar geometries then yields the Casimir pressure correctly.