Questions. What is the physical origin of this inertia that keeps an object moving forever unless it is acted upon by a force? What is so special about motion in a straight line with constant speed?

Newton thought of inertia as an intrinsic property of bodies, a property they would have no matter what surrounded them. Newton also held a concept of absolute space, a space that existed independent of objects that might be placed within it. He conceived of the inertial motion of an object as relative to absolute space.

The Austrian physicist and philosopher Ernst Mach, 1838 - 1916, the man after whom the concept of mach number is named, held a very different point of view. For him, there was no such thing as absolute space. Space was defined by the relationship between bodies, and the inertial properties of a body were caused by an interaction of this body with the totality of all fixed stars. This would have to be an interaction different from the universal gravitational interaction.

Attempts to build a consistent theory of inertia on such a new kind of interaction have not succeeded, so far.