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Just when you thought invisibility cloaks couldn't get any weirder, researchers come up with this: a way to make one object look like any other.

Invisibility cloaks work by steering light around a region of space, making any object inside that region invisible. In effect, an invisibility cloak creates the illusion of free space. This is possible because of a new generation of artificial materials called metamaterials that can, in principle at least, steer light in any way imaginable. Indeed, various teams have built real invisibility cloaks that hide objects from view in both the microwave and optical bands.

Now Che Chan and pals from the Hong Kong University of Science and Technology say that metamaterials could be used for an even more exotic effect: for cloaks that create the illusion that a different object is present.

The illusion is a two-step process, and to see how it works, imagine making a mouse look like an elephant. The first step involves an idea that these guys came up with about six months ago in which they described a way of cloaking objects at a distance.

The trick is to create a material in which the permittivity and permeability are complementary to the values in a nearby region of space containing the mouse wee want to hide. "Complementary" means that the material cancels out the effect that the mouse has on a plane lightwave passing through. So a plane wave would be bent by the mouse but then bent back into a plane as it passes through the complementary material, making the mouse disappear.

The second step is to then distort this plane wave in the way that an elephant would. This means creating transformational material that distorts a plane lightwave in the same way as an elephant. So anybody looking at this mouse would instead see an elephant.

An invisibility cloak is just a special case of this, when the mouse is simply replaced by the illusion of free space, say Chan and co.

The researchers have even found a mind-boggling application. Their idea is to create the illusion that a wall has a hole in it, and then use the hole to look through the wall.

That's not quite as bonkers as it sounds. The wall has to be pretty thin, and what the new device does is allow light to tunnel through the wall in a way that would not ordinarily be possible. Amazing, if it works.

There's no telling where this kind of thinking will lead. But surely metamaterials can't do anything weirder than this?