Cloaking device

A cloaking device is a hypothetical or fictional stealth technology that can cause objects, such as spaceships or individuals, to be partially or wholly invisible to parts of the electromagnetic (EM) spectrum. Fictional cloaking devices have been used as plot devices in various media for many years.

Developments in scientific research show that real-world cloaking devices can obscure objects from at least one wavelength of EM emissions. Scientists already use artificial materials called metamaterials to bend light around an object. However, over the entire spectrum, a cloaked object scatters more than an uncloaked object.

Cloaks with magical powers of invisibility appear from the earliest days of story-telling. Since the advent of modern Science fiction, many variations on the theme with proposed basis in reality have been imagined. Star Trek screenwriter Paul Schneider, inspired in part by the 1958 film Run Silent, Run Deep, and in part by The Enemy Below, which had been released in 1957, imagined cloaking as a space-travel analog of a submarine submerging, and employed it in the 1966 Star Trek episode "Balance of Terror", in which he introduced the Romulan 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.) Another Star Trek screenwriter, D.C. Fontana, coined the term "cloaking device" for the 1968 episode "The Enterprise Incident", which also featured Romulans.

Star Trek placed a limit on use of this device: a space vessel cannot fire weapons, employ defensive shields, or operate transporters while cloaked; thus it must "decloak" to fire. 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 the B-2 bomber cannot avoid becoming 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, including Doctor Who, Star Wars, and Stargate.

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 (usually infrared), 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.

Optical metamaterials 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. Using transformation 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.

These spatially varying optical parameters do not correspond to any natural material, but may be implemented using metamaterials. There are several theories of cloaking, giving rise to different types of invisibility. 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.