Theory of the Optically Induced Forces

On the basis of physical laws that require no assumption about kinds of optically induced forces and the quantity of these kind, we have shown that the momentum of light in matter is greater by n times than that of the same light in free space. Here n is the refractive index of the matter. If the momentum of a light pulse is considered, that it consists of two components. This is the electromagnetic one that is equal to 1/n2 part of the total momentum and the mechanical component that is equal to (1-1/n2) part of the total momentum. An alternative treatment for the momentum of a light pulse is possible. A propagation of a light pulse the momentum of which is greater by n times than that of the same light pulse in free space is accompanied by two additional optically induced forces applied to the regions of the matter where the leading and trailing edges of the light pulse are propagating. These forces transmit a part of the total momentum into the mechanical component that is located between the edges and id propagating together with the light pulse at speed c/n. The mechanical component is connected with the mechanical motion of the matter. An origin of the optically induced forces arising at a change of the momentum density flux of a continuous light wave is explained by the Maxwell-like kind of force that acts on a dielectric located in an electrical field. An origin of the force arising at propagation of a light pulse in a homogeneous optical medium is explained by action of two different kinds of forces. The first kind can be called by the Ampere-like force that acts on a conductor with electrical current located in a magnetic field. Although a current of free charges is absent in a dielectric, a change of the polarization of the dielectric in time can be considered as the same current. The name of the second kind is absent till present day. This kind is dual to the Ampere-like kind. The force arises at an interaction between the "magnetic current" due to the alternate magnetization and an electrical field. Magnetic free charges are absent in the nature and, therefore, the kind of force that is dual to the Ampere force does not exist. However, magnetic dipoles are presented in the nature and, therefore, the kind of force that is dual to the Ampere-like force does exist. At a steady-state when the intensity of light is not changed in time, a sum of the Ampere-like and Ampere-dual forces averaged over a period of oscillations is equal to zero There is a perfectly different kind of optically induced force arising at compression of the optical medium due to the electrostriction pressure. A magnitude of this kind of force is equal to the gradient of the electrostriction pressure and is comparable with the Maxwell-like force. However, forces of this kind are produced by inner forces rather than a change of the momentum of light. Thus, this kind of force can not be calculated by means of an analysis of a change of the momentum of light. On the basis of presented notions about optically induced forces, the Abraham-Minkowski controversy is absent because the initial notions about the momentum of light inmate are modified. In the same time it is shown that that the approach to calculation of optically induced forces on the basis of the Lorentz force that is considered at present as a main instrument for calculation of optically induced force is erroneous. The presented theory enables us to analyze debates about explanation of results of the recent experiment regarding measurement of the momentum of light in matter. We have shown that a generally accepted notions about kinds of optically induced force optically induced is absent at present. We hope that our book will eliminate this drawback.