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Simulation of a hypothetical cloaking device. Normally, incident light waves on an object are absorbed or reflected, causing the object to appear visible.

With the cloaking device active, light is 'deflected' around the object to make it appear as if it did not exist, rendering it invisible.

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^[1] 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.^[2] However, over the entire spectrum, a cloaked object scatters more than an uncloaked object.^[3]

Fictional origins

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;^[4] thus it must "decloak" to fire—essentially like a submarine needing to "surface" in order to launch torpedoes.^[5]

Writers and game designers have since incorporated cloaking devices into many other science-fiction narratives, including Doctor Who, Star Wars, and Stargate.

Scientific experimentation

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.^[6]

Metamaterial research

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.^[7]^[8]

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.^[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

Active camouflage (or adaptive camouflage) is a group of camouflage 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 the British Army has tested an invisible tank.^[14]

Plasma stealth

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.^[15] The Defense Technical Information Center is also following up research on plasma reducing RCS technologies.^[16] A plasma cloaking device was patented in 1991.^[17]

Metascreen

A prototype Metascreen is a claimed cloaking device, which is just few micrometers thick and to a limited extent can hide 3D 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 the University of Texas, Austin by Professor Andrea Alù.^[18]

The metascreen consisted of a 66 micrometre thick polycarbonate film supporting an arrangement of 20 micrometer thick copper strips that resembled a fishing 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.^[18] The device only cancelled out the scattering of microwaves in the first order.^[18] The same researchers published a paper on "plasmonic cloaking" the previous year.^[19]

Howell/Choi cloaking device

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 the optical pathway.^[20]

Cloaking in mechanics

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.^[21]

References

^ John Schwartz (October 20, 2006). "Scientists Take Step Toward Invisibility". The New York Times.
^ Sledge, Gary. "Going Where No One Has Gone Before", Discovery Channel Magazine #3. ISSN 1793-5725
^ Monticone, F.; Alù, A. (2013). "Do Cloaked Objects Really Scatter Less?". Phys. Rev. X. 3 (4): 041005. arXiv:1307.3996. Bibcode:2013PhRvX...3d1005M. doi:10.1103/PhysRevX.3.041005. S2CID 118637398.
^ Okuda, Michael; Okuda, Denise (1999). The Star Trek Encyclopedia. Simon and Schuster. ISBN 9781451646887.
^ Sopan Deb (November 12, 2017). "Star Trek: Discovery, Season 1, Episode 9: Sloppy Showdowns". The New York Times. The Klingons have to decloak to fire
^ Service, Robert F.; Cho, Adrian (17 December 2010). "Strange New Tricks With Light". Science. 330 (6011): 1622. Bibcode:2010Sci...330.1622S. doi:10.1126/science.330.6011.1622. PMID 21163994.
^ Pendry, J.B.; Schurig, D.; Smith, D.R. (2006). "Controlling electromagnetic fields" (PDF). Science. 312 (5781): 1780–1782. Bibcode:2006Sci...312.1780P. doi:10.1126/science.1125907. PMID 16728597. S2CID 7967675. Archived (PDF) from the original on 2016-10-06.
^ Leonhardt, Ulf; Smith, David R. (2008). "Focus on Cloaking and Transformation Optics". New Journal of Physics. 10 (11): 115019. Bibcode:2008NJPh...10k5019L. doi:10.1088/1367-2630/10/11/115019.
^ ^a ^b Inami, M.; Kawakami, N.; Tachi, S. (2003). "Optical camouflage using retro-reflective projection technology" (PDF). The Second IEEE and ACM International Symposium on Mixed and Augmented Reality, 2003. Proceedings. pp. 348–349. CiteSeerX 10.1.1.105.4855. doi:10.1109/ISMAR.2003.1240754. ISBN 978-0-7695-2006-3. S2CID 44776407. Archived (PDF) from the original on 2016-04-26.
^ Alù, A.; Engheta, N. (2008). "Plasmonic and metamaterial cloaking: physical mechanisms and potentials". Journal of Optics A: Pure and Applied Optics. 10 (9): 093002. Bibcode:2008JOptA..10i3002A. CiteSeerX 10.1.1.651.1357. doi:10.1088/1464-4258/10/9/093002. Archived from the original on 2016-04-20.
^ Gonano, C.A. (2016). A perspective on metasurfaces, circuits, holograms and invisibility (PDF). Politecnico di Milano, Italy. Archived (PDF) from the original on 2016-04-24.
^ Smith, David R. (25 July 2014). "A cloaking coating for murky media". Science. 345 (6195): 384–385. Bibcode:2014Sci...345..384S. doi:10.1126/science.1256753. PMID 25061192. S2CID 206559590.
^ Schittny, Robert et cl. (25 July 2014). "Invisibility cloaking in a diffuse light scattering medium". Science. 345 (6195): 427–429. Bibcode:2014Sci...345..427S. doi:10.1126/science.1254524. PMID 24903561. S2CID 206557843.
^ Clark, Josh. "Is the army testing an invisible tank?" Archived 2012-03-01 at the Wayback Machine, HowStuffWorks.com, 3 December 2007. accessed 22 February 2012.
^ Plasma cloaking: Air chemistry, broadband absorption, and plasma generation backup Archived 2009-08-02 at the Wayback Machine, February 1990.
^ Gregoire, D. J. ; Santoru, J. ; Schumacher, R. W.Abstract Archived 2009-08-02 at the Wayback Machine Electromagnetic-Wave Propagation in Unmagnetized Plasmas Archived 2009-08-02 at the Wayback Machine, March 1992.
^ Roth, John R. "Microwave absorption system" U.S. patent 4,989,006
^ ^a ^b ^c Tim Wogan (28 March 2013). "Ultrathin "metascreen" forms latest invisibility cloak". PhysicsWorld.com. Archived from the original on 17 August 2013.
^ http://iopscience.iop.org/1367-2630 New Journal of Physics, March 2013.
^ "Cloaking' device uses ordinary lenses to hide objects across range of angles". Science Daily. 29 September 2014. Archived from the original on 2014-10-01. Retrieved 15 August 2021.
^ Bückmann, Tiemo (2014). "An elasto-mechanical unfeelability cloak made of pentamode metamaterials". Nature Communications. 5 (4130): 4130. Bibcode:2014NatCo...5.4130B. doi:10.1038/ncomms5130. PMID 24942191.

Rincon, Paul (19 October 2006). "Experts test cloaking technology". BBC News. Retrieved 5 August 2008.
Mosnews - Dr Oleg Gadomsky Cloaking Device Article^[usurped]
"MSNBC: Can objects be turned invisible?"
Optical Camouflage by the Tachi Lab in Japan
Space Daily - Engineers Create Optical Cloaking Design For Invisibility, April, 2007

External links

University of Texas at Austin, Cockrell School of Engineering, Researchers at UT Austin Create an Ultrathin Invisibility Cloak, 26 March 2013.
New Journal of Physics, "Demonstration of an ultralow profile cloak for scattering suppression of a finite-length rod in free space", by JC Soric, PY Chen, A Kerkhoff, D Rainwater, K Melin, and Andrea Alù, March 2013.
New Journal of Physics, "Experimental verification of three-dimensional plasmonic cloaking in free-space", by D Rainwater, A Kerkhoff, K Melin, J C Soric, G Moreno and Andrea Alù, January 2012.
Physical Review X, "Do Cloaked Objects Really Scatter Less", by Francesco Monticone and Andrea Alù, October 2013.