Invisible channels offer optical collusion.
A prescription for wormholes that transmit light invisibly between remote regions has been devised by a team of researchers. These ‘electromagnetic wormholes’ are the light-based equivalent of space-time wormholes, the staples of science fiction that permit time travel and create short cuts across intergalactic space. They could one day be used in three-dimensional (3D) video-display units wired up with invisible light-carrying cables.
In the electromagnetic wormholes described by Allan Greenleaf of the University of Rochester in New York state and his colleagues, light would disappear in one place and reappear in another, conveyed along channels that cannot be seen from the outside. They remain theoretical at this stage, but the technologies for making them already exist. They draw on the same ideas and methods as those used recently to make invisibility shields. Greenleaf’s team envisages a slew of possible applications for their wormholes, in areas ranging from information technology to biomedicine.They would, for example, provide perfectly insulated ‘optical cables’ that totally shield the world outside from the electromagnetic field of the light within.
“You don’t see that two parts of space are connected by the wormhole until you look through it.”
“It’s a cool idea,” says physicist Ulf Leonhardt at the University of St Andrews in Scotland, who specializes in this kind of manipulation of light. The implication, he says, is that “you don’t see that two parts of space are connected by the wormhole until you look through it” — rather like looking into your coffee cup and seeing the street outside.
The wormholes would be made from ‘metamaterials’, structures engineered to interact with light in odd ways. The building blocks of metamaterials — the equivalent of atoms in normal materials — are little electrical circuits, typically made from loops and coils of wire on circuit boards. These interact with the electromagnetic field of a light beam to create unusual optical effects, such as a negative refractive index — the light is bent the ‘wrong’ way as it passes through.
Last year, Leonhardt and a team led by David Smith of Duke University in Durham, North Carolina, independently proposed that metamaterials might be used to make an invisibility shield that bends light smoothly around an object placed inside it. Smith’s group subsequently made such a shield that works at microwave frequencies.
The shielding in that case wasn’t perfect, and worked only within a flat plane and for a very narrow frequency range. Moreover, making an invisibility shield for visible light is harder, because the components of the metamaterial have to be much smaller.
Greenleaf and his colleagues say that such a shield can be regarded as ‘blowing up a point’: expanding an infinitely small — and thus invisible — point in space while ‘moulding’ the way light interacts with it so as to sustain the invisibility. Their wormholes, in contrast, are like ‘blowing up a line’ — in essence, rather like making a tubular shield. They have worked out a prescription for the properties a metamaterial tube would need to have in order for light from outside to bend around it while light inside bounces along the channel as if along an optical fibre. In this way, light entering one end of the wormhole would be visible at the other end — albeit with some odd distortions. If the wormhole is very short, it acts rather like a fisheye lens, they say.
The researchers believe that it should be possible to make such devices with the same microwave metamaterials as those used by Smith’s team. But Leonhardt cautions that this remains “very far in the future”. Making a 3D invisibility shield is already a big challenge, he says, and a wormhole would be harder still.
Smith is optimistic, however. “A version of these wormholes at microwave frequencies could definitely be feasible, although over a very narrow frequency range,” he says. “I’m hoping to demonstrate lots of interesting optical structures using metamaterials in the near future, and we may add this one to the list.”