Until recently, every invisibility cloak developed has operated at the microscopic scale, hiding objects that were already too small to see with the naked eye. If invisibility cloaks for invisible things seem disappointing, take heart: This year two separate teams stepped up with the first devices capable of concealing objects as big as a paper clip, bringing a practical invisibility cloak within reach.
In both cases, the researchers carved out a hiding place in a crystal of the mineral calcite. Physicist Shuang Zhang and colleagues at the University of Birmingham in England hid a paper clip, while MIT engineer George Barbastathis and his team chose a rolled-up piece of paper. Both objects appeared to vanish.
The real magic is an optical property of calcite called anisotropy. The mineral splits up light rays and reflects them in such a way that it renders the chamber invisible. Of course, there is a catch. Concealment depends on light waves oscillating in the same direction, an effect that spoils most practical applications. Zhang, however, is hopeful; he believes the technology might eventually be capable of hiding humans.
