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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: OFFl@IAL l!ISl!!! 9HLY A B C Figure 18. Wave Packets are Made by Combining Waves With Different Frequencies. The picture shows the simplest example: two waves (A and B) that add up to the wave packet (C) . Suppose that the phase velocity varies for different frequencies, what is called dispersion. In this case, the wave group made by the constructive interference of the single-frequency waves moves at a different speed than the phase velocity: group and phase velocities differ. So, in dispersive materials, the phase velocity may approach infinity without violating the principles of relativity, but only for a single frequency, because otherwise the group velocity would tend to infinity as well. The cloaking of electromagnetic waves of fixed frequency is possible, as the successful demonstration of the microwave-cloaking device has confirmed, but the cloaking of wave packets carrying information is impossible. It turns out31 that the group velocity actually tends to zero at the inner lining of such cloaking devices; wave packets would get stuck there instead of traveli ng around. Turn ing invisibility from a tantalizing idea into a practical device requires a new paradigm. 32 Curved Space Light rays are curved in materials with varying refractive index . In conventional cloaking devices, the rays are curved because the material performs a transformation to curved coordinates. However, the curvature of a space does not depend on coordinates; curved coordinates create the illusion of curvature , but the space they describe is still flat. A flat space obeys the axioms of Euclidean geometry, in particular the parallel axiom: through each point outside out of straight line goes exactly one parallel line; parallels never meet. The light rays focused by a lens clearly violate the parallel axiom, because parallel light rays meet at the focus of the lens. Optical materials establish non-Euclidean geometries in general; the Euclidian geometries of cloaking devices are rather the exceptions. The advantage of Euclidean spaces is that one can easily visualize them; curved space is difficult to comprehend, in particular three-d imensional curved space. However, two-dimensional curved spaces can be visua lized as surfaces of three-d imensional curved objects. These surfaces are the virtual spaces that are implemented, by the optical material, in physical space . The simplest example is the sphere. On the surface of the sphere, the equivalent of straight lines, the geodesic lines, are the great circles. The great circles originating from one point meet again at the antipodal points, which shows that the surface of the sphere establishes a non -Euclidean geometry. To implement this geometry in the two- UNCLASSIFIED/ /EAR OFFICIO! I 155 ONL¥ 17
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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.