In Star Trek IV: The Voyage Home, the crew of the Enterprise hijacks a Klingon battle cruiser. In case you’re not a die-hard trekkie, that was quite a feat for a Federation starship. Usually the Klingon ships are the ones ambushing with impunity, thanks to a secret “cloaking device” that renders them invisible to light or radar.
Is such a device really feasible? Invisibility has long been one of the marvels of science fiction and fantasy, from H.G. Wells’s The Invisible Man to the The Lord of the Rings to the Harry Potter series. Yet physicists have doggedly dismissed such vanishing acts as impossible, claiming that escaping detection violates the laws of optics and does not conform to the known properties of matter.
But today the impossible may become possible. New advances in “metamaterials”—man-made materials that can, in a sense, control the movement of light—are forcing a major revision of optics textbooks. Working prototypes of such materials have actually been built in laboratories, sparking intense interest from the media, industry, and the military, eager to know how the visible could someday become invisible.
Modern optics truly began with work done by James Clerk Maxwell, a Scottish physicist, in the mid-nineteenth century. At Cambridge, where Isaac Newton had worked two centuries earlier, Maxwell excelled as a student of what would now be called mathematical physics. Calculus—an invention of Newton’s that uses equations to describe how objects move in space and time—armed Maxwell with the mathematical tools to explore the nature of electromagnetism.
Maxwell began with physicist Michael Faraday’s discoveries that electricity could generate magnetism, that magnetism could generate electricity, and that each could be thought of as a force field. Maxwell rewrote Faraday’s depictions of force fields in the precise language of calculus, producing eight fierce-looking differential equations—one of the most important series of equations in modern science. (Every physicist and engineer in the world has to sweat over them when mastering electromagnetism in graduate school.)
Next, Maxwell asked himself a fateful question: if changing magnetic fields create electric fields and vice versa, what happens if those fields are constantly generating each other in a never-ending pattern? Maxwell found that electric-magnetic fields behave much like ocean waves—for example, in the way they undulate through space. He calculated the speed of the waves and to his astonishment, found it to be the speed of light! Upon discovering this fact in 1864, he wrote prophetically: “This velocity is so nearly that of light that it seems we have strong reason to conclude that light itself . . . is an electromagnetic disturbance.”
It was perhaps one of the greatest discoveries in human history. For the first time the secret of light was revealed. Maxwell suddenly realized that the brilliance of the sunrise, the blaze of the setting sun, the dazzling colors of the rainbow, and the stars in the firmament could all be explained in terms of waves. Today we realize that the entire electromagnetic spectrum—from radio waves, including broadcast frequencies and radar, through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays—can all be described by Maxwell’s wave theory of light.
Maxwell’s theory of light, paired with the insight that all matter is made up of atoms, provides simple explanations for the phenomena of optics and lays the foundation for invisibility. Most solids, for example, are opaque because light rays, traveling as waves, cannot pass through the dense matrix of atoms. Many liquids and gases, by contrast, are transparent because the wavelengths of visible light can pass more readily through the larger spaces between their loosely arranged atoms. Diamonds and other crystals are something of an exception: they are both solid and transparent. That’s because the atoms of a crystal, while tightly packed, are arrayed in a precise lattice structure that offers many straight pathways for a light beam to take.