Imagine peering into a hole, only to see a distant object as though it were right next to you. No cameras, no elaborate periscopes are involved — instead you are gazing through an electromagnetic “wormhole” created in a specially designed material.
A proposal for such a wormhole comes from Yaroslav Kurylev at University College London in the UK and colleagues in the US and Finland, who came up with the idea by building on the mathematical theory that gave us the invisibility cloak — a device that was realized for microwaves last year. Whereas in an invisibility cloak rays of light are guided around a cylindrical or spherical volume like water flowing around a stone, a wormhole would have light guided around a more elaborate, tubular shape. The device would appear solid at most wavelengths of light, but at cloaking wavelengths it would disappear, and light entering the tube at one end would emerge at the other with no visible tunnel in-between (Phys. Rev. Lett. 99 183901).
In empty space, which has a uniform refractive index, light travels in straight lines according to cartesian co-ordinates. The trick to bending light around an invisibility cloak or a wormhole is to design a material with a non-uniform refractive index that transforms these cartesian co-ordinates into curved co-ordinates. Handily, the mathematics required to produce such arbitrary co-ordinate transforms can be found in the geometry underpinning Einstein’s theory of general relativity, which can be combined with Maxwell’s equations for describing the propagation of electromagnetic waves.
Since results for the wormhole are parallel to those for cloaking, progress in cloaking can be transferred to wormholes
Kurylev and colleagues call the transformation for an invisibility cloak “blowing up a point” because it is essentially stretching an infinitesimally small region into a sphere. To make a wormhole, therefore, all one needs to do is “blow up a curve”. And, as with the invisibility cloak, the researchers say the device could be made by creating metamaterials — exotic, manmade materials with strong electromagnetic properties — that have the necessary non-uniform refractive index profile.
“What they propose is definitely an interesting idea,” Ulf Leonhardt, one of the physicists who first dreamt up the invisibility cloak, told physicsworld.com. But he added that, since only cylindrical invisibility cloaks have actually been created, the next step will be to create a truly 3D cloak. “Making a wormhole is even more complicated,” he said.
Still, were their idea to be realized, Kurylev and colleagues have a list of potential applications including “optical cables” for measuring electromagnetic fields without disturbing them, or making a 3D video display. By placing a bar magnet close to one end of a wormhole, the magnetic field would emerge seemingly from nowhere at the other end and become a magnetic monopole, although Kurylev does not know what it could be used for.
The researchers also admit that, because the optical properties of current metamaterials change rapidly as a function of wavelength, a practical wormhole would probably only work at a narrow range of wavelengths. This means that, as with invisibility cloaks, a wormhole working over the entire visible light spectrum is some way off. “There are already first results on invisibility in the optical scale,” said Kurylev. “Since results for the wormhole are parallel to those for cloaking, progress in cloaking can, in principle, be transferred to wormholes.”