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This Defense Intelligence Reference Document, dated 2 March 2010, was prepared by the Defense Intelligence Agency's Defense Warning Office under the Advanced Aerospace Weapon System Applications program. It reviews invisibility through camouflage, transparency, and cloaking, covering metamaterials, transformation optics, and non-Euclidean broadband cloaking designs. It concludes that perfect cloaking is impossible, but imperfect devices could be made. Microwave cloaking is within reach of present technology, while visible-light invisibility remains uncertain and depends mainly on new theoretical research.
From the source:Release of 2026-09-18 Incident: 3/2/10, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys the theory and early experiments behind invisibility cloaking, describing several ways an object might be hidden from visual or sensor detection, including camouflage, transparency effects, and optical cloaking that bends light around an object. It focuses mainly on metamaterials, negative refraction, and transformation optics, and reviews experiments that had already demonstrated limited cloaking at microwave frequencies. The report argues that “imperfect” cloaking is physically achievable in some parts of the electromagnetic spectrum, especially for microwaves, but that “perfect” cloaking is not practical because it would require material properties that conflict with the underlying physics. Its overall conclusion is that cloaking is a scientific field with plausible narrow applications, but that useful visible-light cloaking depends more on future theoretical breakthroughs than on conventional advances in materials science.
UNCLASSIFIED/ /FOR OFFICIAL USE ONLY B I I I I I I I I I I I I ! I I I I ~.,.. ' I -.-_ , I V j I I I' I ~ I I I "', I I 'j I 'I I I 'I I - ·- - ... . 'I - I j I --- --- ...- - \ I I 'I I\. - 1-- , --- -i--_,_,_ I ,_ ... I .,, i,... I I ..>-- I I ~·... • I I I I I I I I I I I I I I I I I I I I I I I I I I I Figure 17. Fundamental Problem of Transformation-Based Cloaking Devices30 The device creates the illusion of the empty virtual space A where light travels along straight lines, whereas in reality light rays are curved by the coordinate transformation from virtual space to real space B. If the light waves are indistinguishable from light propagating through empty space, the speed of light in the cloaking device must be larger than the speed of light in the surrounding material-air, for instance-to make up for the longer path on the detour through the cloak. To make matters worse, the speed of light must be infinitely large at the inner lining of the invisibility cloak. To understand this, consider a light ray that just straddles the red point in the virtual space shown in A. In real space, B, this po int is enlarged to a finite volume that contains the hidden core of the cloaking device. Now, if for light propagation virtual space and real space are indistinguishable, the light ray should pass the extended path along the inner lining in precisely the time it takes to pass a single point, zero time. Consequently, the speed of light must approach infinity near the core of the cloaking device. The following argument shows that this is possible in principle, but also that such devices would be completely useless as a cloaking device in practice. In wave propagation, one distingu ishes between the phase velocity and the group velocity. The phase velocity is the velocity at which the phase fronts of waves appear to move. For light, the wave fronts are the features of oscillations across space and time; by themselves they do not transport energy or information. On the other hand, the phase fronts are orthogonal to the paths of light rays; if they are ti lted, rays are refracted. Therefore, the refraction of light, the bending of light rays, is controlled by the phase velocity. The refractive index that enters Fermat's principle of the shortest optical path is the phase index, the ratio between the speed of light in vacuum and the phase velocity in the material. The group velocity is the speed at which wave packets, pulses, and most information travels; it is the velocity of a wave group. Such a group consists of a range of single-frequency waves that, by their interference, establ ish the group, t he wave packet, as Figure 18 below shows. UNCLASSIFIED/ /FOR. OFFI@IAL l:t!H!! 9HL'I" 16
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Official release, from the pursue 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.