Cloaking technology takes big step forward

MIT scientists have discovered a new cloaking technology that can hide an object as large as a peppercorn.

Rather than the synthetic materials used in other research efforts, the Singapore-MIT Alliance for Research and Technology (SMART) team used a common mineral called calcite — a crystalline form of calcium carbonate.

“Very often, the obvious solution is just sitting there,” says MIT mechanical-engineering professor George Barbastathis.

The object to be hidden is placed on a flat, horizontal mirror, and a layer of calcite crystal — made up of two pieces with opposite crystal orientations, glued together — is placed on top. When illuminated by visible light and viewed from a certain direction, the object under the calcite layer ‘disappears’.

The team placed the MIT logo upside-down on the vertical wall behind the apparatus, positioned so that one of the letters could be viewed directly via the mirror, while the other two were behind a two-millimeter-high wedge and its concealing layer of calcite. Then the whole setup was submerged in liquid.

The logo appeared normal, as though there was no wedge but a flat mirror piece, when illuminated with visible green light. With blue or red illumination, the cloaking was still effective, but with a little misalignment.

In principle, Barbastathis says, the same method could be used in real-life situations to conceal an object from view — and the only limitation on the size of the hidden object is the size of the calcite crystal that’s available.

The team paid about $1,000 for the small crystal it used, he says, but much larger ones could be used to conceal much larger objects. The largest known natural crystal of calcite measures 21 feet square.

For now, the system is essentially two-dimensional, limiting the cloaking effect to a narrow range of angles – but Barbastathis says he has some ideas about how to make the effect work in three dimensions. He also hopes to eliminate the need for immersing the system in liquid and make it work in air.

Coincidentally, another independent research team, from the UK’s University of Birmingham, has also published a paper this month describing a similar calcite-based method.

The MIT and Birmingham results “are two beautiful experiments. I particularly like their simplicity,” says Ulf Leonhardt, chair in theoretical physics at Scotland’s University of St Andrews.