Just before Christmas I finally got to see an invisibility cloak. I’ve written a whole book about invisibility—its myths, magic, and technology—but never before had I seen in the flesh one of the new devices that promise to make things vanish.
We all know what invisibility is, don’t we? Now you see it, now you don’t. Well, this invisibility cloak certainly wasn’t like that. It was housed in a Plexiglass box, and there was an on-off switch on the side. I was told to look through a viewing screen, through which I saw, in a gold-tinted mirror, the slightly distorted reflection of a toy panda. The reflection had a chunk missing where a couple of small triangular prisms, glued back to back and placed in front of the mirror, obscured the view. When the switch was flipped “on,” that missing part sprang back into view, as though the prisms had become wholly transparent.
That was all. This, I had to admit, was a strange sort of invisibility.
Was it as underwhelming as it sounds? I hadn’t arrived with high expectations, because I knew enough about these “cloaks” not to anticipate something out of Harry Potter. I knew that the cloak itself would not be some fabulous garment, but would pretty much resemble what it was: small prisms of transparent crystal made from a mineral called calcite. They blocked the reflection in the mirror because they had mirror-coated rear faces themselves, off which the light from the panda bounced out of my line of sight.
When my host, Jensen Li—a very smart young Chinese physicist from the University of Birmingham in England—hit the switch, the light now appeared to travel in a straight line right through the prisms, despite their mirrored facets, and was reflected off the mirror behind them and through the viewing screen to reach my eye.
What, then, had vanished? Well, it’s complicated. Bear with me.
We were in the Metamaterials Laboratory of the university’s physics department —a room dominated by a huge steel “optics table,” in which all manner of lenses and mirrors had been positioned to send laser beams along carefully arranged routes. I’d seen this kind of kit before; it’s like a gigantic Erector set. But all that stuff wasn’t the point. The action was taking place inside the plastic box set up in the corner.
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