'History editor' takes invisibility cloaks to fourth dimension
Scientists have taken invisibility cloaks into the fourth dimension with the creation of a proof-of-concept design for a cloak that conceals events. Writing in the Journal of Optics, they explain that their 'spacetime cloak' or 'history editor' could prove useful in signal processing and computing. It could even be used to create the illusion of a Star Trek transporter, in which objects disappear from one spot and pop up elsewhere instantaneously, the researchers speculate.
The novel design is based on metamaterials which can be engineered to distort light or sound waves. Scientists have already created invisibility cloaks that can bend light around objects, effectively rendering them invisible. The spacetime cloak described in this study does not bend light in space but alters the speed at which it travels through the material so that the light splits in two, opening up a kind of corridor of invisibility.
'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 the lead author of the paper, Professor Martin McCall of Imperial College London in the UK.
As a light beam hits the cloak, the leading part of the light speeds up, while the trailing part slows down. This creates a gap in the light; any event that takes place in this gap is not illuminated and so is invisible to any observer. The gap can be closed by slowing down the leading part of the light and speeding up the trailing part. Anyone observing the light beam would simply see a continuous beam of light.
'[The spacetime cloak] is unlike ordinary cloaking devices because it does not attempt to divert light around an object,' explains Alberto Favaro of Imperial College London. 'Instead it pulls apart the light rays in time, as if opening a theatre curtain - creating a temporary corridor through which energy, information, and matter can be manipulated or transported undetected.'
Mr Favaro provides a useful analogy based on cars driving along a busy road. 'Imagine computer data moving down a channel to be like a highway full of cars,' he says. '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.'
Once the pedestrian has crossed the road, the leading cars slow down briefly, while the trailing cars speed up, closing the gap. 'Meanwhile an observer down the road would only see a steady stream of traffic,' Mr Favaro adds.
The device has a number of potential applications, in particular in signal processing and computing. For example, one data channel could be interrupted to carry out a priority calculation on a parallel channel while the cloak is operating. Afterwards, it would appear to external parts of the circuit as though the original channel had never stopped operating, creating an 'interrupt-without-interrupt' system.
Another novel idea exploits the gap or corridor in the light in a different way. '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,' comments Professor McCall. 'So, theoretically, this person might be able to do something and you wouldn't notice!'
The team gives the example of a safe-cracker entering a scene, opening a safe, removing its contents, closing the door and leaving, while a nearby surveillance camera continues to transmit images of a closed safe.
'Although this sounds like science fiction, the lesson from metamaterials research in the last decade has taught us that, within certain restrictions, such speculations are not fantasy,' the team writes. 'We here show how the magic of editing history can be achieved by introducing the concept of the spacetime cloak.'
The team concludes: 'We are sure that there are many other possibilities that are opened up by our introduction of the concept of the spacetime cloak. Whilst the camera is said to never lie, it does not always record the whole truth.'
Data Source Provider: Imperial College London; University of Salford; Journal of Optics
Document Reference: McCall, M. W., et al. (2010) A spacetime cloak, or a history editor. Journal of Optics 13: 024003.DOI: 10.1088/2040-8978/13/2/024003.
Subject Index: Information and communication technology applications ; Materials Technology; Scientific Research; Other Technology