© Universal/The Kobal CollectionImagine what the Invisible Man - memorably played by Claude Rains (above) - could do if his actions were invisible in spacetime...
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 today1 - would be an upgraded version of the 'invisibility cloak' that was first proposed in 20062,3 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.
"I realized that it may be possible to use metamaterials to bend light rays in both space and time, not just in space," says Martin McCall, an optical physicist at Imperial College London who led the team behind the new proposal. "This would add a new dimension to the invisibility cloak - literally.
Bypassing Time
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.
"You could imagine a burglar using a space - time cloak to create an invisible corridor leading to a safe," says McCall. With the cloak turned on, the burglar could run through this corridor, open the safe, steal the contents, shut the safe and escape, while any security camera trained on the safe would just show a continuous image of a locked door at every point in time, explains McCall. "The dastardly event would have been edited from history," he says. By contrast, putting a conventional invisibility cloak over the safe would hide the object from observers - but a security camera would spot that the once-visible safe had been hidden.
Security guards do not need to worry, however. McCall and his colleagues have calculated the precise properties of the metamaterial needed to build a space - time cloak that would be perfectly invisible, and there are fundamental problems that prevent it from being constructed. The theoretical calculations work only in a vacuum, and to create a space - time void of even a few minutes would require a cloak bigger than Earth because of the space required to recombine the accelerated leading edge and slowed trailing edge of the light wave. Worst of all, the theory requires the metamaterial to boost light rays beyond the fundamental speed of light.
Fibre-Optic Void
Undeterred, the team has designed a less efficient version to be built from optical fibres - inside which light can be accelerated and slowed without breaking the fundamental speed limit. Lasers would be used to control the fibres' refractive indices, opening and closing the temporal void. The fibre-optic cloak could hide events only from observers standing directly ahead of the oncoming light waves, and it could not fully block all reflections from light travelling through the cloak while it is turned on, so some light might bleed out. A distant observer looking down the optical fibre would not spot the hidden event, but they would notice the background light getting brighter and dimmer.
McCall hopes that a fibre-optic cloak creating a space - time void around 30 centimetres long, to hide actions taking place over a few nanoseconds, could be built within the next year. It could be tested by firing a separate illuminating laser signal into the region of the void - the light should disappear when the cloak is turned on and reappear when it is switched off, he says.
However, Ulf Leonhardt, a physicist at the University of St Andrews, UK, who made one of the original proposals for an invisibility cloak, is sceptical that the design could be implemented with current technology, because of the difficulty involved in dramatically manipulating the refractive index of optical fibres to create a void for even a few nanoseconds. "It is, however, a very inspiring theoretical idea, so it could motivate others to come up with a more practical design," he says.
Ortwin Hess, a researcher into metamaterials at the University of Surrey in Guildford, UK, believes that the design might have practical applications for the developing field of quantum computing. Physicists are struggling to build quantum computers, because any accidental observation of fragile quantum properties can disrupt calculations before they have been completed. "This cloak could be used to shield quantum systems from being observed, long enough for calculations to be done," he says.
References
1. McCall, M. W., Favaro, A., Kinsler, P. & Boardman, A. J. Opt. 13, 024002 (2010).
2. Leonhardt, U. Science 312, 1777-1780 (2006).
3. Pendry, J. B. , Shurig, D. & Smith., D.R. Science 312, 1780-1782 (2006).
quantum computers is really what we are after. The current ones we have can only transfer info either in optical and/or audio form, it took us decades to make them and they are still unstable due mainly to the fact that electronics are unstable.
The problem with quantum physics is even bigger since it deals with in-between stages of on/off electricity. However once mastered , I don't think anyone of us will be around to see them working, quantum computers can transfer material, even human beings. There's one very big problem though, especially in transferring humans.They need 100% accuracy. One small mistake and you 'll find yourself with half your body parts missing. Well, we have other things to worry about for the time being.