The paper is dedicated to one of the greatest breakthroughs in the classical physics at the beginning of the 20-th century – the appearance of the special theory of relativity of Albert Einstein in 1905. In it, by the help of the rotary theory, a new proof of the most famous formula in the world – the equation giving the connection between the energy and the mass of the bodies, is presented. Rotary theory appeared in 1998, trying to explain the electromagnetic phenomena from another point of view and to answer to series of questions connected with the basic electromagnetic laws, reaching the same results but giving simpler and direct answers compared with the classical electromagnetic theory of Maxwell. In it, by the help of the method of moments, the vector of the magnetic field intensity and the vector of the magnetic flux density are presented as moments of the vector of the current density of the tangential displacement current , claiming in this way that the magnetic field is a form of rotating electric field. The final result is a set of electromagnetic equations in fully electrical form, depicting all the electromagnetic phenomena from another point of view.