Scale Relativity: A Fractal Matrix for Organization in Nature

In this review paper, we recall the successive steps that we have followed in the construction of the theory of scale relativity. The aim of this theory is to derive the physical behavior of a nondifferentiable and fractal space-time and of its geodesics (to which waveparticles are identified), under the constraint of the principle of relativity of all scales in nature. The first step of this construction consists in deriving the fundamental laws of scale dependence (that describe the internal structures of the fractal geodesics) in terms of solutions of differential equations acting in the scale space. Various levels of these scale laws are considered, from the simplest scale invariant laws to the log-Lorentzian laws of special scale relativity. The second step consists in studying the effects of these internal fractal structures on the laws of motion. We find that their main consequence is the transformation of classical mechanics in a quantum-type mechanics. The basic quantum tools (complex, spinor and bi-spinor wave functions) naturally emerge in this approach as consequences of the nondifferentiability. Then the equations satisfied by these wave functions (which may themselves be fractal and nondifferentiable), namely, the Schrödinger, Klein-Gordon, Pauli and Dirac equations, are successively derived as integrals of the geodesics equations of a fractal space-time. Moreover, the Born and von Neumann postulates can be established in this framework. The third step consists in addressing the general scale relativity problem, namely, the emergence of fields as manifestations of the fractal geometry (which generalizes Einstein’s identification of the gravitational field with the manifestations of the curved geometry). We recall that gauge transformations can be identified with transformations of the internal scale variables in a fractal space-time, allowing a geometric definition of the charges as conservative quantities issued from the symmetries of the underlying scale space, and a geometric construction of Abelian and non-Abelian gauge fields. All these steps are briefly illustrated by examples of application of the theory to various sciences, including the validation of some of its predictions, in particular in the domains of high energy physics, sciences of life and astrophysics. c © Electronic Journal of Theoretical Physics. All rights reserved.