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Oberth's version of this G-Field theory - insofar as he has made it available - is less comprehensive than Plantier's. Oberth seems not to have incorporated either a separate hypothetical factor into his artificial gravity field, or supposed the existence of an entirely separate field to complete his model. Perhaps such an explanation will be forthcoming. There is a quite different theoretical concept which seems to satisfy the three major requirements mentioned: first, control over inertia; second, control over terrestrial gravity; and third, translational capability, the capacity for level flight. This theory (we might tag it the "space valve" theory) makes use of the relationship between gravity and inertia. In order to explain it, we must define inertia closely. According to Newton's famous first two laws of motion, a mass at rest, or moving at a constant speed, will continue at rest or at its same speed, unless a force is applied to it. Further, a mass will move in the direction of an applied force with an acceleration proportional to that force. The world is a world of inertia. It is the curse and condition of our being. We can hardly conceive of existence without it. All our experience with our own bodies and things external to us reflects it. We swim in an inertial sea, almost unaware of it because it is so commonplace. It was once assumed that gravity and inertia were different and unrelated. The Austrian physicist Ernst Mach first hinted at a relationship. Newton seemed to regard inertia as a kind of given property of the universe, something of an ultimate, ill-defined category he called absolute space. Mach, in criticizing Newton's view, suggested that the final reference of physics was not absolute space but the fixed stars - we would say the nebulae. Here was the germ of an idea about inertia. It remained, however, for Einstein to formulate it. Einstein noted that it would be impossible for a person in an elevator to be sure whether he was resting securely on the surface of the Earth, or actually deep in space, away from any large masses, and accelerating at a value equal to the Earth's "g." In other words, both gravity and inertia can produce the same effect, with no detectable difference. Gravity and inertia are thus observationally equivalent But General Relativity and the Principle of Equivalence at its heart went a step further: inertia and gravity might be identical also in their origins. The inertia we know may merely be the gravitational resultant of the entire mass of the universe. Thus, to go back to our man sitting in his easy chair, imagine that the forces of inertia and gravity playing upon him can be represented by rubber bands. One very large rubber band pulls him and his chair down toward the surface of the Earth. But around him is a vast multitude of little rubber bands connecting him with the millions of stars throughout the universe. The large rubber band we call gravity, the great multitude of other "star bands" account for inertia. The man and the chair have a tendency to remain in a given piece, or proceed in straight line motion at a constant rate, because of a multitude of distant gravitational sources. Inertia is therefore, the General Theory of Relativity suggests, only the physical influence of the rest of matter in the universe. Since, as we have noticed, gravity and inertia are numerically equal, the man his original publications are hard to obtain in this country.