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.
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.
These materials are arrays of so-called “meta-atoms”: little manufactured devices that can act as antennas to receive and transmit light. The meta-atoms are designed and configured in ways that let them pass on the light’s energy along unusual paths from one element to the next. In the classic metamaterials invisibility cloak, unveiled in 2006, the light is guided gently around the cloaked object and then restored to its original path on the other side, rather like water in a stream flowing around a rock protruding from the surface. The cloak is a ring of metamaterial with the hidden object in its center. The light hits the ring on one side, is guided around the central hole, and is restored to its path. To an observer on the far side, it looks as though it has just travelled in a straight line all along, with nothing in its way.
The catch was that the 2006 cloak only worked with microwaves, which are the same kind of electromagnetic radiation as light but with a longer wavelength: These are the rays with which radar “sees” objects, which are detected when microwaves bounce back. The cloak itself was eminently visible: a series of concentric rings made from strips of plastic circuit board, on which the meta-atoms were little loops and rings of copper foil etched into shape. The researchers chose to work with microwaves because the meta-atom structures have to be about the same size as the wavelength of the light they’re manipulating: If you want to work with visible light, the structures have to be a lot smaller, which means they’re much tougher to make.
The scientists were totally upfront about this. All the same, for many who read news reports claiming that scientists had invented Harry Potter-style invisibility cloaks, the reality seemed anticlimactic to say the least.
The theory used to figure out what the metamaterial had to look like in order to bend light in the required manner is called transformation optics. It is partly the brainchild of John Pendry, a physicist at Imperial College in London, who collaborated with researchers at Duke University in North Carolina to make the microwave cloak. Jensen Li was Pendry’s postdoctoral student, and in 2008 the two of them figured out how to achieve cloaking in a simpler fashion, which didn’t have to rely on metamaterials at all. They outlined plans for a “carpet cloak”: A structure with a concave base that, when placed over an object on a surface, will bend reflected light so that it looks as if the reflected beam has simply bounced off the flat surface on which it rests, as if the cloak and the object beneath it were not there.
Using this principle, one can make a cloak from prism-shaped pieces of calcite. The trick takes advantage of the fact that calcite has the unusual property of birefringence: Light travels through it at slightly different speeds in different directions. The birefringence means that objects seen through a piece of calcite look “doubled,” the two images superimposed imperfectly on one another. In 2009 a team at Birmingham led by Shuang Zhang made a calcite carpet cloak that could effectively hide an object within a shallow cavity at the base of the prisms as they stand on a surface. Jensen was demonstrating to me a variant of that device.
Jensen exemplifies the Chinese-born researchers who are setting the pace in the field of transformation optics. Xiang Zhang at the University of California, Berkeley, is another. Zhang recently reported a device that represents the closest Jensen’s “carpet cloak” concept has yet come to being made into a traditional cloak that can genuinely be laid over a bump so as to make it vanish.
Zhang imprinted an array of microscopic gold patches—the cloak’s meta-atoms—on a thin film of material so that they reflect light in just the way needed to cancel out the distortions introduced by the bump, making the cloak-wrapped surface look flat. It’s a cumbersome procedure, though, because the shapes and patterns of the gold reflectors have to be calculated and tailored to every differently-shaped bump—there’s no “one size fits all” yet.
But back in the Birmingham Metamaterials Lab we struggled, despite Jensen’s best efforts, to put the blocky calcite cloak into words. I haven’t even told you what the switch does, and I don’t think it would help. (It’s to do with the reflected light beams being polarized, and the screen being a polarizing filter.) Was it proper to call this invisibility at all?
Days later I went to the headquarters of the Magic Circle to watch their Christmas show. Doves appeared out of silk handkerchiefs, cards vanished and reappeared in impossible places, a woman was levitated, all the traditional illusions of magic were mustered to eye-popping effect. And I had to wonder: does science set itself up for a fall by appropriating the trappings of myth, legend, magic, or science fiction? Does anyone want, or even know how to interpret, compromised forms of immortality, teleportation, time travel, and so on? Or should we accept that science produces wonders only on its own terms? Goodness knows, they’re often wonderful enough.
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