At this point the whole G-field myth falls apart. One of the fundamental laws established by Newton, to which no exception has ever been found in the laboratory, states that a moving object will continue to move in a straight line unless it encounters an applied force. Let us suppose, for the moment, that a gravity shield could suddenly be interposed between a spacecraft and the earth, and thus make the craft reverse its direction of flight. The occupants would still be subject to the law of inertia. They would be hurled against the wall of the craft with a violence far greater than that experienced by a plane crashing to earth from an altitude of 30,000 feet. There could be no cushioning of the blow. Such dreams demonstrate an almost contemptuous disregard for reality. Physicists admittedly do not yet understand the basic nature of gravity, but they do know a great deal about how it acts. Gravity is the force that holds the universe together. It exerts a pull on all objects in the physical world--the earth, the moon, the planets, our sun, the distant stars, and even the stars in other galaxies. All these bodies without exception move according to the law of universal gravitation as formulated by Newton and refined by Einstein: Every particle in the universe attracts every other particle with a force that is proportional to the product of their masses, and inversely proportional to the square of the distance between them. The magnitude of the force depends only on the masses of the bodies and on their distances from each other. It does not depend at all on the nature of the medium that separates them. It operates unchanged through stone, metal, water, air, or empty space. With a metal shield we can reduce electrical forces to zero; with a soft-iron shield we can weaken magnetic forces; but no substance existing in nature can act as a shield to shut out the force of gravitation. _Electricity, Magnetism, and Gravity_ No responsible scientist would assert that man has found out all there is to know about the universe, and few would insist that some kind of a shield for gravity is an absolute impossibility. As yet, however, no laboratory has detected any phenomenon that might be a clue to “negative gravity.” In recent years nuclear physicists have occasionally caught fleeting glimpses of what has been called “anti-matter,” electrons with positive charges and protons with negative charges--the reverse of their charges in the normal world. Some investigators have speculated on the gravitational properties of anti-matter, and have wondered whether it might exert a force that would repel instead of attract.