Recently, science writers have introduced us to life's third certainty, right behind death and taxes: anyone who reports on invisible technology must mention Harry Potter.
Having fulfilled that obligation, I now disclose—with no small bit of pride—that I have never read any of the books starring this young wizard. But apparently the lad owns a cloak that renders him invisible, and the aforementioned writers enjoy pondering whether this fantastical character is playing by science's factual rules. I prefer to know when we can whisk Master Potter out of sight and, therefore, out of mind.
If I want to hide an object—say, a popular fantasy book in the center of a display table—I have some options. I can steal it when I think no one's looking. Or, if I prefer my apartment to prison, I can cover the book with some sort of cloak so that the table simply appears empty.
To do this, I have to manipulate light, which streams atop the table as though traveling along a checkered traffic grid. Stopping light entirely would be rather difficult. Instead, I can re-route this grid and change the path light takes—and in the process change what it illuminates.
Think of light as a car driving on one of the lines in this traffic grid. Its goal is to get from one end of the table to the other. When it reaches the middle, it illuminates the book.
Now suppose someone plops a traffic circle in the center of the grid. In this case, our light-car must detour around the center, missing the book. In this scenario, light would still reach the other end of the table, but it would fail to hit the bestselling wizard in the middle.
Altering the path of light, though, is a bit trickier than making a car swerve. Electromagnetic waves, such as light, rigidly follow the original, checkered traffic grid. Materials capable of altering light's path don't exist in nature, with few exceptions. But with new technology, engineers can create tiny traffic cops, called metamaterials, which bend light in abnormal directions. Right now, these metamaterials take the form of tiny metal coils and rods.
From here, the blueprint for designing an invisibility cloak is clear. Step one: assemble these metamaterials with an opening in the center. Step two: place the desired book inside this opening. Step three: see—or don't see—light swirl right around the bespectacled phenom.
No matter where a person watches from, the effect holds true: once light completes its circuitous route around the cloak, it resumes a normal grid-like path and appears as though it had never strayed.
Scientists have tested this idea by placing an object inside such a cloak and firing microwave light in its direction. When they collected spatial data on the microwaves, the information createsd a picture that looked as though the light had continued unimpeded along its path.
Here, however, we encounter a bit of frustration. Microwave light can't detect anything smaller than its wavelength—about an inch—such as metamaterials. But people don't see in microwaves; we see colors with much smaller wavelengths, on the scale of nanometers. So concealing an object from human vision would require metamaterials dramatically smaller than their present size.
The problem gets worse. For light to travel around the cloak and resume its original path, it must, for a brief instant, move faster than the speed of light. Scientists can achieve this boost along a single light frequency, but the system breaks down when several colors are involved. So, while it might be possible to mask some yellow in young Potter's striped scarf, the red would regrettably remain.
Finally, diverting light around a cloak takes precise placement of metamaterials. That's fine if we want to disguise a stationary object, but makes it extremely hard to keep a moving object invisible—a problem given how quickly those books fly off the shelf.
So we're faced with an unfortunate Catch-22 (a book we'd never dare cloak): We can hope that invisible technology becomes more efficient, but if it does, we must accept the inevitable science articles making reference to you know who.
The real Wishful Thinker behind this column was engineer David R. Smith of Duke University, whose greatest act of invisibility might be the way he circumvents the question of when we'll have a fully operational cloak.
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