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.
What was actually being hidden in there? Well you see, there was a little sliver of metal inserted behind the calcite prisms. It was masked by their mirrored rear surface when the switch was off. But when the prisms became apparently transparent, the sliver should have come into view obscuring part of the panda’s reflection. Yet it didn’t.
So “invisibility” here meant failing to reveal something that was already hidden. The air of farce was compounded by the fact that I was witnessing all of this for a radio documentary. It’s hard enough trying to convey any sort of invisibility trick on the radio. But how was I to describe, let alone explain, this complicated arrangement of prisms, mirrors, screens and, um, pandas … and to make it sound fabulous?
A novice witnessing this piece of physics magic could be forgiven for thinking she was being hoodwinked—as if she’d been ushered into a circus sideshow to see “wonders” that are all too obviously monkey torsos crudely stitched onto fish tails.
I use that metaphor advisedly. Invisibility is a magical idea, and for many centuries the only way we have been able to achieve it is with magic. It was a specialty of the stage magician, dating back to the earliest accounts of sleight-of-hand in Roman times, when prestidigitators performed a variant of the old balls-and-cups trick in which the balls seemed to vanish from one place and reappear in another.
It’s sometimes said that there are only two real magic tricks in the entire repertoire: making things vanish, and making them appear. Prince Charles, the heir to the British throne, gained a coveted place in the UK’s Magic Circle—the high priesthood of stage magicians—in 1977 by demonstrating his own competence at that ancient trick. In the music halls and theaters of the Victorian golden age of magic, the performers would use mirrors to create disembodied heads, or women with no legs or who entered cabinets and vanished.
In a sense, Jensen Li’s invisibility cloak is part of this same tradition, since it too relies on sending light rays along unexpected paths so as to cloak an object from sight. It emerged from research on a remarkable new class of materials known as optical metamaterials, which can transform and defy the normal laws of optics.