The speed of light
It is natural of ask whether the concept of Newtonian relativity and the Galilean addition law for velocities in mechanics also apply to Electricity, magnetism, and optics. Recall that Maxwell in the 1860s showed that the the speed of light in free speed was given by ……
Physicists of the late 1800s were certain that light waves (like familiar sound and water waves) required a definite medium in which to move to move, called the ether, and that the speed of light was c only with respect to the ether or a frame fixed in the ether called the ether frame. In any other frame moving at a frame fixed in the ether called the ether frame, the Galilean addition law was expected to hold. Thus, the speed of light in this other frame was expected to be c – v for light traveling in the same direction as the frame, c + V for light traveling opposite to the frame, and in between these two values for light moving in an arbitrary direction with respect to the moving frame.
Because the existence of the ether and a preferred ether frame would show that light was similar to other classical waves (in requiring a medium), considerable importance was attached to establishing the existence of the special ether frame. Because the speed of light is enormous, experiments involving light traveling in media moving at then attainable laboratory speeds had not been capable of detecting small changes of the size of …….. prior to the late 1800s.