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Acloaking device is a hypothetical or fictionalstealth technology that can cause objects, such asspaceships or individuals, to be partially or whollyinvisible to parts of theelectromagnetic (EM) spectrum. Fictional cloaking devices have been used asplot devices in various media for many years.
Developments in scientific research[1] show that real-world cloaking devices can obscure objects from at least onewavelength of EM emissions. Scientists already use artificial materials calledmetamaterials to bend light around an object.[2] However, over the entire spectrum, a cloaked object scatters more than an uncloaked object.[3]
Cloaks with magical powers of invisibility appear from the earliest days of story-telling. Since the advent of modernScience fiction, many variations on the theme with proposed basis in reality have been imagined.Star Trek screenwriterPaul Schneider, inspired in part by the 1958 filmRun Silent, Run Deep, and in part byThe Enemy Below, which had been released in 1957, imagined cloaking as a space-travel analog of asubmarine submerging, and employed it in the 1966Star Trek episode "Balance of Terror", in which he introduced theRomulan species, whose space vessels employ cloaking devices extensively. (He likewise predicted, in the same episode, that invisibility, "selective bending of light" as described above, would have an enormous power requirement.) AnotherStar Trek screenwriter,D.C. Fontana, coined the term"cloaking device" for the 1968 episode "TheEnterprise Incident", which also featured Romulans.
Star Trek placed a limit on use of this device: a space vessel cannot fire weapons, employ defensiveshields, or operatetransporters while cloaked;[4] thus it must "decloak" to fire.[5] At this point, Trek parted ways with the submarine analogy (subs don't "surface" to launch torpedoes), instead anticipating a significant limitation of Stealth technology: aircraft like theB-2 bomber cannot avoidbecoming un-stealthy during the time their bomb bay doors are open.
Writers and game designers have since incorporated cloaking devices into many other science-fiction narratives, includingDoctor Who,Star Wars, andStargate.
An operational,non-fictional cloaking device might be an extension of the basic technologies used by stealth aircraft, such as radar-absorbing dark paint, optical camouflage, cooling the outer surface to minimize electromagnetic emissions (usuallyinfrared), or other techniques to minimize other EM emissions, and to minimize particle emissions from the object. The use of certain devices to jam and confuse remote sensing devices would greatly aid in this process, but is more properly referred to as "active camouflage". Alternatively, metamaterials provide the theoretical possibility of making electromagnetic radiation pass freely around the 'cloaked' object.[6]
Opticalmetamaterials have featured in several proposals for invisibility schemes. "Metamaterials" refers to materials that owe their refractive properties to the way they are structured, rather than the substances that compose them. Usingtransformation optics it is possible to design the optical parameters of a "cloak" so that it guides light around some region, rendering it invisible over a certain band of wavelengths.[7][8]
These spatially varying optical parameters do not correspond to any natural material, but may be implemented usingmetamaterials. There are severaltheories of cloaking, giving rise to different types of invisibility.[9][10][11]In 2014, scientists demonstrated good cloaking performance in murky water, demonstrating that an object shrouded in fog can disappear completely when appropriately coated with metamaterial. This is due to the random scattering of light, such as that which occurs in clouds, fog, milk, frosted glass, etc., combined with the properties of the metamaterial coating. When light is diffused, a thin coat of metamaterial around an object can make it essentially invisible under a range of lighting conditions.[12][13]
Active camouflage (oradaptive camouflage) is a group ofcamouflage technologies which would allow an object (usually military in nature) to blend into its surroundings by use of panels or coatings capable of changing color or luminosity. Active camouflage can be seen as having the potential to become the perfection of the art of camouflaging things from visual detection.
Optical camouflage is a kind of active camouflage in which one wears a fabric which has an image of the scene directly behind the wearer projected onto it, so that the wearer appears invisible. The drawback to this system is that, when the cloaked wearer moves, a visible distortion is often generated as the 'fabric' catches up with the object's motion. The concept exists for now only in theory and in proof-of-concept prototypes, although many experts consider it technically feasible.
It has been reported that theBritish Army has tested an invisible tank.[14] Mercedes demonstrated an invisible car using LED and camera in 2012.[15]
Plasma at certain density ranges absorbs certain bandwidths of broadband waves, potentially rendering an object invisible. However, generating plasma in air is too expensive and a feasible alternative is generating plasma between thin membranes instead.[16] TheDefense Technical Information Center is also following up research on plasma reducingRCS technologies.[17] A plasma cloaking device was patented in 1991.[18]
A prototype Metascreen is a claimed cloaking device, which is just fewmicrometers thick and to a limited extent can hide3D objects from microwaves in their natural environment, in their natural positions, in all directions, and from all of the observer's positions. It was prepared at theUniversity of Texas at Austin by ProfessorAndrea Alù.[19]
The metascreen consisted of a 66 micrometre thick polycarbonate film supporting an arrangement of 20 micrometer thick copper strips that resembled afishing net. In the experiment, when the metascreen was hit by 3.6 GHz microwaves, it re-radiated microwaves of the same frequency that were out of phase, thus cancelling out reflections from the object being hidden.[19] The device only cancelled out the scattering of microwaves in the first order.[19] The same researchers published a paper on "plasmonic cloaking" the previous year.[20]
University of Rochester physics professor John Howell and graduate student Joseph Choi have announced a scalable cloaking device which uses common optical lenses to achieve visible light cloaking on the macroscopic scale, known as the "Rochester Cloak". The device consists of a series of four lenses which direct light rays around objects which would otherwise occlude theoptical pathway.[21]
The concepts of cloaking are not limited to optics but can also be transferred to other fields of physics. For example, it was possible to cloak acoustics for certain frequencies as well as touching in mechanics. This renders an object "invisible" to sound or even hides it from touching.[22]
The Klingons have to decloak to fire
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