A UFO supposedly can travel at speeds of thousands of miles an hour and shatter the sound barrier without making any noise because the G-field would create a kind of protective envelope around the saucer. But if the G-field breaks down for any reason, so that the protective envelope is opened, then the ionized moving air hits ordinary static air and creates the thunderous detonation produced by some UFOs. (Even with an intact G-field, a boundary or gradient would always exist somewhere between the air that was dragged along by the saucer and the air that was not. A thunderous impact would certainly occur at this barrier.) The “invisible” UFOs supposedly become so by using the G-field to bend or deflect rays of light. (It is true that starlight passing near the sun’s gravitational field suffers a deflection that makes the star appear slightly displaced from its actual position on the celestial sphere, but a shift in apparent location does not dim a star and does not make it invisible. Furthermore the amount of deflection is only 1.75 _seconds_ of arc, less than half of a thousandth of a degree! To produce even this small deflection, a covey of saucers would have to be able to increase its mass to equal that of the sun: 1.97 times 10^{33} grams! What this increase in mass would do to the rest of the solar system doesn’t bear thinking of.) Angel hair (see _Chapter_ XI) is supposed to be a waste product from the operation of the G-field. The ionization of the air inside the G-field allegedly would create heavy atoms that reacted chemically with the atoms in ordinary air to produce a kind of precipitate that falls to the ground and disappears as the ionization decreases. (In the physics laboratory, ionization means taking an electron away from an electrically neutral atom. The resulting atom would not be heavier. The contact between ordinary air and that in the ionized trail of a meteor has never yet produced “angel hair.” No laboratory has ever reported that isotopes of oxygen, hydrogen, nitrogen, and other elements in the atmosphere can react with their normal analogues to produce precipitates. A change in ionization cannot make a chemical compound disappear.) The envelope of air enclosed by the G-field is supposed to allow a UFO to accelerate or change direction instantaneously, even when flying at enormous speeds, because the UFO would not encounter atmospheric friction. (Vehicles moving in the earth’s gravitational field are also surrounded by a cushion of air, but they still must overcome friction.)