‘Space-time’ Cloak Could Hide History

Theoretical physicists have taken the idea of creating an invisibility cloak and broadened it to include the object's movement. Such a "space-time cloak" could be used to hide events, they suggest.

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. Such artificially engineered materials can distort light or sound waves to create undetectable blind spots by deflecting certain parts of the electromagnetic spectrum. While the object is still there, its image is altered to make it look like it has disappeared.

In the new study, reported this week in the Journal of Optics, Imperial College London researchers led by physics professor Martin McCall mathematically extend the cloak idea to include 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,” said McCall.

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.

Such a space-time cloak would open up a temporary corridor through which energy, information and matter could be manipulated or transported undetected. “If you had someone moving along the corridor, it would appear to a distant observer as if they had relocated instantaneously, creating the illusion of a Star-Trek transporter, McCall said. “So, theoretically, this person might be able to do something and you wouldn’t notice!”

While using the space-time cloak to make people move undetected is still science fiction, there are many serious applications for the new research, which was funded by the Engineering and Physical Sciences Research Council (EPSRC), the UK's main agency for funding engineering and physical sciences research, and the Leverhulme Trust. Co-author Dr Paul Kinsler developed a proof of concept design using customized optical fibers, which would enable researchers to use the event cloak in signal processing and computing. A given data channel could for example be interrupted to perform a priority calculation on a parallel channel during the cloak operation. Afterwards, it would appear to external parts of the circuit as though the original channel had processed information continuously, so as to achieve ‘interrupt-without-interrupt.’

Illustration of the spacetime cloak concept. (Image: Imperial College London)

“Imagine computer data moving down a channel to be like a highway full of cars," said Alberto Favaro, who also worked on the project. "You want to have a pedestrian crossing without interrupting the traffic, so you slow down the cars that haven’t reached the crossing, while the cars that are at or beyond the crossing get sped up, which creates a gap in the middle for the pedestrian to cross. Meanwhile an observer down the road would only see a steady stream of traffic.”

One issue that cropped up during their calculations was to speed up the transmitted data without violating the laws of relativity. Favaro solved this by devising a clever material whose properties varied in both space and time, allowing the cloak to be formed.

“We’re sure that there are many other possibilities opened up by our introduction of the concept of the space-time cloak," said McCall, “but as it’s still theoretical at this stage we still need to work out the concrete details for our proposed applications.”

The paper "A spacetime cloak, or a history editor," can be found in the Nov. 16 issue of Journal of Optics.

For more information, visit: www.imperial.ac.uk