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This notion of Newton's was never satisfactory and in the last part of the I9th Century the Austrian physicist and philosopher Ernst Mach turned his critical mind to it. Mach, whom we all know for his Mach numbers of aerodynamics, was also < a | forerunner of the Vienna Circle which developed logical positivism. To him anything beyond observation - such as absolute space - was unreal. Hence he proposed that inertia was a reference to all the matter in the universe. By all the matter in the universe he meant all the fixed stars, or in our day, when we realize that the cosmos is made up of vast numbers of stars collected in vast numbers of galaxies, to all the galaxies. For Mach an object subject to the laws of inertia was relative to all the stars, or as we would say today, all the nebulae. Yet Mach's principle, as Einstein called it, had a difficulty. It did not supply any physical link between the stars and an inertial system. Mach just substituted the universe for Newton's absolute pace as a system of coordinates in which objects existed and moved. He did not take us any further down the road to showing what inertia is, or why it works the way it does. Perhaps we should say, rather, that he took us a little way and he took Albert Einstein a very long way. In 1916 Einstein proposed his General Theory of Relativity. In effect it was a theory of universal gravitation and inertia. Einstein reduced the two forces to the same thing and expressed this in his famous Principle of Equivalence: gravitational and inertial forces are indistinguishable and equal. His illustration of this is a man in an elevator deep in space. The man is away from any large objects. If the elevator is moving uniformly at any constant speed, from a very small one to a very large one, the man will seem quite weightless. He will sense no motion nor any gravity. However, if the elevator speeds up, if it is pulled by its cosmic cable along the direction of the man's height at an increasing speed, the man will begin to feel as if gravity is acting upon him. When a certain acceleration is reached, equivalent in earth's measurements to 32 feet per second, the man will imagine that he is back on the earth and is being pulled down by the earth's gravity just the way he was before he left earth. Actually, of course, he is not. His false impression is merely the result of inertia and the acceleration of his elevator. There is no gravitation or, more correctly we should say, there is no large object in his vicinity. Thus Einstein illustrated the fact that inertia and gravity have exactly the same effects on the observer and cannot be distinguished on the basis of local observations. He went further. He sought to explain gravity and inertia in the same physical terms. While the weight of objects on a large celestial body like the earth is caused by the latter's gravitational attraction, the inertial behavior of objects is explained by the gravitational attraction of all matter everywhere. To use a simple analogy, the pipe resting on the table in front of me remains