Wednesday, July 13, 2011

Invisibility, Cloak, Cloaking Space and Time Now Possible

Cloaking space–time

From Harry Potter cloaks to our childhood attempts to write with invisible ink, invisibility science is something that captures most people's imagination. But the quest for invisibility has made real progress in recent years.

Most existing invisibility cloaks are designed to hide objects from view. But as Martin McCall and Paul Kinsler explain, it could also be possible to make "space–time" cloaks that allow selected events to go undetected – perfect for the ultimate bank heist.

Our view of the world is determined by what our eyes see, our ears hear and our noses smell, or what the philosopher Bertrand Russell termed "sense data". But we know from simple optical illusions that our eyes can be fooled – things are not necessarily always what they seem. However, the techniques that physicists have recently developed to manipulate the path taken by light and other electromagnetic radiation are not mere tricks of the eye: they are real advances that can result in some fascinating and useful effects.
By making specially engineered "metamaterials", we can now create primitive versions of Harry Potter's invisibility cloak. After diverting light around an object – like water flowing round a tree stump in a river, or cars parting to either side of a traffic island – we can seamlessly reintegrate it afterwards. Our senses are subverted, not by trickery, but because the light reaching our eyes is the same as if the object were not there. By changing the paths of light rays through space to hide an object at a selected location, we are able to make what is called a "spatial cloak".

But imagine if we could make a cloak that operates not only in space but in time as well. To understand how such a "space–time" cloak might work, consider a bank housing a money-filled safe. Initially, all incoming light continuously scatters off the safe and its surroundings, revealing the rather dull scene of an undisturbed safe visible to surveillance cameras. But imagine, near some specified time, splitting all the light approaching the safe into two parts: "before" and "after", with the "before" part sped up, and the "after" part slowed down. This would create a brief period of darkness in the stream of illuminating photons. If the photons were a stream of cars on a motorway, it is as if the leading cars were to speed up and those trailing behind were to decelerate, creating a gap in the traffic edged by bunches of cars (a dark period with bright edges) 

Now imagine that during the moment of darkness, a safe-cracker enters the scene and steals the money, being careful to close the safe door before he leaves. With the safe-cracker gone, the process of speeding up and slowing down the light is reversed, leading to an apparently untouched, uniform illumination being reconstituted. As far as the light reaching the surveillance cameras is concerned, everything looks the same as it did beforehand, with the safe door firmly shut. The dark interval when the safe was cracked has literally been edited out of visible history.
To complete our motorway analogy, it is as if the cars have acted to first open up and then close a gap in traffic, leaving no disturbance in the flow of vehicles. There is now no evidence of that temporary car-free interlude, during which the proverbial chicken may even have crossed the road without getting squashed. So by manipulating how light travels in time around a region of space, we can, at least in principle, make a space–time cloak that can conceal events – an "event cloak", if you will.

Both space and space–time cloaks use a general method called "transformation optics", whereby cloak designers decide what route they want light to take before calculating what sort of material the light should pass through to achieve that aim. The point is that light rays travel along paths that can be mathematically altered – for example from straight lines to curves. However, to create the desired distortions of the ray paths, we need our material to be carefully designed, a process that is usually expressed in terms of coordinate transformations. We can then use Einstein's "principle of covariance", which says that all physical theories are independent of the coordinates used, to calculate the material properties that will produce the desired light trajectories. Whereas regular (i.e. spatial) invisibility cloaks apply this principle only in space (figure 1a), an event cloak applies it in space–time (figure 1b) – after all, time is as much a coordinate as space, with both appearing in Maxwell's equations for the electromagnetic field.
What is remarkable is that the event cloak leaves the light rays undeviated from their path from source to detector – they do not curve in space, instead they curve in space–time. It is their speed, not direction, that changes as a function of both position and time. But because our proposal is based on speeding light up in some places and slowing it down in others, we have to ensure that the average speed of the light in our material is less than it would be in a vacuum. After all, since nothing can travel faster than light in a vacuum, our method, which involves speeding up part of the light, would otherwise not work. Another important detail is ensuring that the cloaking light rays do not point towards the past. The simple circular space–time cloak of figure 1b, although ideal for explanatory purposes, unfortunately does include such rays. Thankfully, the design can be modified to remove such features.

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