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The (fictional) Langston Field generator creates a spherical shield around an object (usually a spacecraft, but cities have been known to deploy them) which absorbs energy. Since the shield absorbs all wavelengths of light, it is completely black in normal use, like a black hole. It is necessary to raise sensors through the Field to see anything, much like a periscope. These sensors can be destroyed since they are outside the Field, so there are usually many backups on a warship. As the Field absorbs energy it reradiates it over its entire surface, acting as a perfect black body; the colour of the Field moves from infrared to red and through the visible spectrum to blue as the amount of reradiated energy increases. (However, in The Mote in God's Eye the field is also described as glowing green or violet, which is not possible for a true black body. It has been suggested that the Langston Field's behavior is a tribute to the ray screens featured in Doc Smith's novel The Skylark of Space, whose color also climbs the spectrum as the energy load increases.) After a certain point, the Field generator can no longer absorb further energy: the Field collapses and the stored energy is released outwards and inwards; the latter usually destroys the ship.
Expanding Langston Field
In the novel The Mote in God's Eye, the Moties did not realize that the Langston Field was possible until humans demonstrated it. The Moties were inconceivably superior to humanity in the development of existing technologies, but lacked the ability (or perhaps the inclination) to carry out primary research. Even if they had had the ability, however, it is possible that they would never have developed Langston Field technology; for Humanity it was a purely accidental discovery which did not result from a natural progression of research. Nonetheless, Motie Watchmakers proved able to repair, maintain, and even improve it.
When a human starship was infested by Watchmakers, every one of the ship's systems was modified and enhanced. The most notable improvement was to the Field generator, which produced a Field which expanded as it absorbed energy. This produces more surface area to disperse absorbed energy, and when the ship was attacked, the Field expanded to many times its original size, increasing in diameter (and hence surface area able to radiate away the absorbed energy) as increasing firepower was brought to bear. Only when the field expanded almost too far for it to be generated at all did it stop expanding, allowing it to be overloaded, a process that still took many times more firepower than would have previously been necessary.
Almost as soon as human vessels with operating Fields jumped into the Mote system, Motie engineers were aware of the Field's existence and quickly built Fields of their own. Motie engineers are incredibly effective at building and using tools, but they display a rather marked lack of original thinking and creativity, which may partly explain why they never developed the Field in the first place. Given that an expanding Field is obviously far more effective at protecting a ship than a static one, all Motie Field generators were designed to expand as necessary.
Though this would have made Motie starships incredibly formidable in combat, the expanding Field actually proved fatal to Motie ships that left their system. The endpoint of the original Alderson point in the Mote system opens within the photosphere of Murcheson's Eye, a red giant star. Survival in such a hostile environment requires a Langston Field, but in this case, an expanding Field is counterproductive. While inside a star, the Field has no opportunity to radiate heat, as it is constantly absorbing heat from all directions. A standard, static field will absorb heat at a fixed rate and will eventually overheat and explode if the ship does not leave the star regularly, but the absorption is slow enough to allow any ship with such a Field to survive for many hours before needing to exit the star to cool off. Unlike in normal space where extra surface area means extra radiated heat, inside a star increased surface area radically increases the rate at which heat is absorbed, causing a ship with an expanding Field to explode within minutes. Often this occurs before the Moties onboard recover from the side effects of traversing the Alderson tramline.