Once the New SI is approved by the General Conference on Weights and Measures (CGPM), all base units of the international metric system of units (SI) will be defined in terms of physical constants and atomic properties. In this paper, we consider the rationale and the direction of the possible further evolution of the SI. The idea is to define all base units exclusively in terms of fundamental physical constants, with no reference to specific phenomena, physical theories or properties of material entities (including properties of atoms and elementary particles), so that those definitions would not have to be altered or amended following advancement in our understanding of the structure of matter, emergence of new physical theories or due to the technological progress. New developments in science and technology would then affect only the mise en pratique (realization) of base units, rather than their definitions. Furthermore, we point out the need for including base units for the weak interaction and the strong interaction into the SI and propose a way to do it. The structure of the fundamental-constants--based system of units (the FC SI) is discussed and prerequisites for the implementation of the FC SI are considered.

This letter examines the structure of draft definitions of the New SI base units, which are posted on the BIPM website. It is argued that the new definitions of base units should be free from references to derived units and should not comprise conditional phrases; names of base units should be clearly distinguished from their symbols. To alleviate the identified problems, relevant modifications of the draft definitions are proposed.

This letter calls for reconsidering the new definition of the mole, which has been proposed in the draft chapter 2 for the SI Brochure, posted on the BIPM website. It is argued that the list of particles should be deleted from the new definition of the mole. The necessary elements of the definition are summarized and another wording is suggested. Proposals to rename ‘amount of substance’ are briefly discussed.

The frequency-sensitive extremum principle for propagation of light rays in the geometrical optics regime is used to develop a nonperturbative method for tracing light rays in a transparent refractive medium in the general relativistic environment. The general formulation of the theory is given first; it allows for the numerical analysis of a nonlinear superposition of gravitational and refractive lensing, when neither of the two effects can be treated as a small perturbation. The scope of the general theory is gradually narrowed to the Schwarzschild field, the spherical symmetry of the refractive properties of the medium, and the small deflection regime approximation. A simple, analytically solvable example of deflection of light rays by a mass embedded in a refractive medium is considered in detail; in a vacuum, deflection of light rays by the Sun is reproduced.PACS Nos.: 42.15-i, 04.20-q, 98.90+s

We consider spontaneous thermalization of free photon field~in a vacuum! due to self-interaction, mediated by the virtual e Ϫ e ϩ field at ultra-high concentrations of the electromagnetic energy. That nonlinear, attractive, short-range interaction between photons triggers spontaneous evolution of the initial, low-frequency spectrum toward the maximumentropy spectral distribution peaking in the gamma range of frequencies. Collimation and the total power of the photon beam~pulse! are hardly affected by the process of spontaneous thermalization. We estimate the threshold intensity that triggers spectral evolution of the photon field, the necessary power of the laser beam, the minimum size of the interaction region, parameters of the fully thermalized photon field, and discuss the near-threshold behavior of the electromagnetic field. Possible applications of thermalized photon beams are suggested, for example, they can serve as the pump field to attain gamma-lasing or facilitate ignition in the fusion pellet.