Rigid Body and Space, an Easy Introducntion to Physics
This article is based on a very good essay in Harvard University in the department of physics.
From the very beginning of theoretical physics until the recent development of the theory of relativity, the concepts of Space and time were universally accepted by physicists. These concepts, which were taken to be intuitively clear, enter in a most fundamental fashion into all physical theories. They are at the very basis of classical hydrodynamics and elasticity, of gravitational and electromagnetic theory; any revision of them, such as is contemplated in the theory of relativity, involves a recasting of physics as a whole and must, therefore, be undertaken only after very careful consideration.
Intimately connected with any idea we may have of Space is the intuitive notion of a rigid body. The relation of congruence between two such bodies is the specific concept upon which is built a theory of distance, and thence of angles. It is in this sense that, from a physical point of view, EucHdean geometry is a science which is deducible by direct postulational methods from the notion of a rigid body. If the portion of space which we consider contains a number of rigid bodies, it is natural to prefer one of them with reference to which position is specified. The body which is thus preferred forms what is known as a frame of reference, or reference body. In referring the location of other bodies to the chosen one, ä particular space is determined. This is the classical Situation. In physical experiment the earth is generally made to serve as a body of reference, while in astronomy it is convenient to refer to a body at rest with respect to the fixed stars.
From the psychological point of view it appears inevitable that the first approximate analysis of experience should yield space and geometry in this way.
