Introduction to the Theory of Relativity

Introduction to the Theory of Relativity

Introduction to the Theory of Relativity

Introduction to the Theory of Relativity

Excerpt

Almost all the laws of physics deal with the behavior of certain objects in space in the course of time. The position of a body or the location of an event can be expressed only as a location relative to some other body suitable for that purpose. For instance, in an experiment with Atwood's machine, the velocities and accelerations of the weights are referred to the machine itself, that is, ultimately to the earth. An astronomer may refer the motion of the planets to the center of gravity of the sun. All motions are described as motions relative to some reference body.

We imagine that conceptually, at least, a framework of rods which extends into space can be rigidly attached to the reference body. Using this conceptual framework as a Cartesian coördinate system in three dimensions, we characterize any location by three numbers, the coördinates of that space point. Such a conceptual framework, rigidly connected with some material body or other well-defined point, is often called a frame of reference.

Some bodies may be suitable as reference bodies, others may not. Even before the theory of relativity was conceived, the problem of selecting suitable frames of reference played an important part in the development of science. Galileo, the father of post-medieval physics, considered the choice of the heliocentric frame to be so important that he risked imprisonment and even death in his efforts to have the new frame of reference accepted by his contemporaries. In the last analysis, it was the choice of the reference body which was the subject of his dispute with the authorities.

Later, when Newton gave a comprehensive presentation of the physics of his time, the heliocentric frame of reference had been generally accepted. Still, Newton felt that further discussion was necessary. To show that some frames of reference were more suitable for the description of nature than others, he devised the famous pail experiment: He filled a pail with water. By twisting the rope which supported the pail, he made it rotate around its axis. As the water gradually began to participate in the rotation, its surface changed from a plane to a paraboloid. After the water had gained the same speed of rotation as the . . .

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