It's been the decade of metamaterials, with breakthroughs toward an invisibility cloak occurring every few months.   With conventional materials, light typically travels along a straight line, but with metamaterials, scientists can exploit additional flexibility to create blind spots by deflecting certain parts of the electromagnetic spectrum.  Basically, an image can be altered or made to look like it has disappeared. 

An optical invisibility cloak is still far in the future but now a team of researchers has extended that idea (mathematically) from a cloak that conceals objects to one that conceals events.   Yes, an invisibility cloak for the fourth dimension which would create a 'temporal void', allowing undetectable moments of invisibility.

Light normally slows down as it enters a material but they theorize by locally manipulating the speed of light of an initially uniform light distribution it will be possible to manipulate the light rays so that some parts speed up and others slow down.   When light is 'opened up' in that way, rather than being curved in space, the leading half of the light speeds up and arrives before an event, while 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.

space-time invisibility cloak metamaterials
Graphic showing how the proposed "space-time" cloak would work.  Credit: Imperial College London

"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," says Professor Martin McCall of Imperial College London. "So, theoretically, this person might be able to do something and you wouldn't notice!"

So while knowledge of physics may not yet allow the creation of a transporter which can dematerialize objects and then rematerialize them elsewhere, it is at least suggested that an object could move from one region of space to another completely unseen by anyone watching.

So it's a killer app for future thieves, right?   There are serious applications also, like quantum computing, which depends on the manipulation of light for the safe transmission of vast amounts of data.

Alberto Favaro, who also worked on the project, explains: "Imagine computer data moving down a channel to be like a highway full of cars. 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 material whose properties varied in both space and time, allowing the cloak to be formed.

Citation: Martin W McCall, Alberto Favaro, Paul Kinsler and Allan Boardman, 'A spacetime cloak, or a history editor', 2011 J. Opt. 13 024003 doi: 10.1088/2040-8978/13/2/024003 (free to read)