Abstract

According to the principle of special relativity, the laws of nature are the same for systems at rest and for systems in uniform and rectilinear motion: for the formulation of the laws of nature, the body C and the body C´, which, relative to the former, is in uniform and rectilinear motion, are equivalent. Galileo, in the Dialogue on the Two Greatest Systems of the World, developed the following argument in favor of such a principle: two bodies C and C´, which are in uniform and rectilinear motion in relation to a third body C´´, with the same direction and speed, are at rest in relation to one another; therefore, what is applicable to rest, is applicable to uniform and rectilinear motion. Einstein, in On the Influence of Gravity in the Propagation of Light, generalizes the principle of relativity: all reference bodies C, C´, etc., are equivalent for the formulation of the laws of nature, whatever their state of motion is. The argument is analogous to that of Galileo: two bodies in free fall are at rest in relation to each other; therefore, what is applicable to rest, is applicable to constant acceleration. I argue that those arguments are invalid: to say that rest is shared motion does not mean that rest is, simply, movement. Therefore, one can only conclude that the laws of nature are the same for systems at rest and for systems with shared motion.