Nature News | 15 Nov 2010
'Space-time cloak' to conceal events revealed in new studyProposed device could edit actions out of history.
It would be the perfect hiding place: a hole carved out of space–time. Optical physicists have created blueprints for a cloak that generates a pocket in reality in which actions can be concealed. In practice, the proposed design can be built only inside the special environment of an optical fibre. But even this constrained space–time cloak could have useful effects, such as assisting quantum computing.
The ideal space–time cloak — the theory behind which is published in the Journal of Optics today — would be an upgraded version of the 'invisibility cloak' that was first proposed in 2006 and has since been built for some wavelengths of light. Such standard invisibility cloaks are made from metamaterials — substances with a complex internal structure that allows them to channel light around objects like water flowing around a rock in a river. A distant observer perceives the light as if it has travelled in a straight line without ever hitting an intervening object.
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The key feature of the proposed space–time cloak is that its refractive index — the optical property that governs the speed of light within a material — is continually changed, pulling light rays apart in time. When the leading edge of a light wave hits the cloak, the material is manipulated to speed up the light, but when the trailing edge hits, the light is slowed down and delayed. "Between these two parts of the light, there will be a temporal void — a space in which there will be no illuminating light for a brief period of time," explains McCall.
Events and actions inside this void in space–time, after the leading edge of the light has passed through but before the trailing edge has arrived, would be shielded from the view of anyone outside. After the hidden action has been completed, the cloak would be turned off. The delayed trailing portion of light would then recombine with the leading edge, reconstructing a full light wave that would travel on; any observers would be unaware that it had been postponed.
Imperial College London | via EurekAlert | 15 Nov 2010
A spacetime cloak, or a history editor - MW McCall et alCloak allows objects to move undetected, according to a paper in the Journal of Optics
The study, by researchers from Imperial College London, involves a new class of materials called metamaterials, which can be artificially engineered to distort light or sound waves. With conventional materials, light typically travels along a straight line, but with metamaterials, scientists can exploit a wealth of additional flexibility to create undetectable blind spots. By deflecting certain parts of the electromagnetic spectrum, an image can be altered or made to look like it has disappeared.
Previously, a team led by Professor Sir John Pendry at Imperial College London showed that metamaterials could be used to make an optical invisibility cloak. Now, a team led by Professor Martin McCall has mathematically extended the idea of a cloak that conceals objects to one that conceals events.
"Light normally slows down as it enters a material, but it is theoretically possible to manipulate the light rays so that some parts speed up and others slow down," says McCall, from the Department of Physics at Imperial College London. When light is 'opened up' in this way, rather than being curved in space, the leading half of the light speeds up and arrives before an event, whilst the trailing half is made to lag behind and arrives too late. The result is that for a brief period the event is not illuminated, and escapes detection. Once the concealed passage has been used, the cloak can then be 'closed' seamlessly.
- Journal of Optics 13 024004 (2011) (online 16 Nov 2010) DOI: 10.1088/2040-8978/13/2/024003 (pdf)
New Scientist | Physics & Math | 16 Nov 2010
