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This Defense Intelligence Reference Document, dated 2 March 2010 and produced by the Defense Intelligence Agency under its Advanced Aerospace Weapon System Applications (AAWSA) Program, is a technical survey of invisibility. It covers camouflage, including stealth aircraft and optical camouflage, then transparency, cloaking by coordinate transformation, metamaterials and non-Euclidean broadband cloaking. It concludes that perfect cloaking is impossible but imperfect microwave cloaks are within reach of present technology. Whether cloaking will work at visible wavelengths remains unclear.
UNCLASSIFIED/ ;raA 8FFI~Ie ■ 1!55 0111 Y dimensional plane, one can use the stereographic projection, the central ingredient of the Mercator projection in cartography that is used to map the surface of a round object, the Earth, onto a flat sheet of paper. A line drawn from the North Pole of the sphere through a point on the surface intersects the equatorial plane at one point. This point is the stereographic projection of the point on the sphere. Figure 19 shows that the stereographic projection of a circle on the sphere is a circle on the plane with a different radius. The stereographic projection distorts the measure of space, but the distortion around any given point is the same in all directions, because otherwise circles would be deformed. Therefore, an optical material that implements the geometry of the sphere via the stereographic projection must be isotropic. One can read off the required refractive index from the drawing as follows: in virtual space, on the sphere, light propagates at the speed of light in vacuum from a point to its infinitesimally close neighbor; in physical space the distance between the two infinitesimal neighbors is modified-the speed of light is changed by the refractive index that is given by the ratio between infinitesimal distances in virtual space and the corresponding distances in physical space. For the stereographic projection, the refractive index is smaller than 1 for points on the northern hemisphere and larger than 1 on the southern hemisphere. This device is known as Maxwell's fish- eye lens. 33 In this lens, light follows the great circles, light goes around in circles, and light rays meet at antipodal points; the fish-eye makes a perfect lens (although a fairly near-sighted one). It is possible to extend these ideas to three-dimensional curved spaces. For example, the surface of the four-dimensional sphere is a three-dimensional curved space, and the device implementing this hyperspace object is just a three- dimensional fish-eye. Hyperspace is not out of this world; it can be built, and it turns out to be practically useful for invisibility. 34 .. -_ . . .. . . . .. Figure 19. Stereographic Projection 18 UNCLASSIFIED/;«F8R 8FFI@Itllt ~!JI! 8HLY
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 29 pages are in the text index: search them above, or from the library's search.