The value of stealth in combat has been known since ancient times. Indeed, the earliest human hunters must have discovered the advantage of approaching their prey without being seen — especially if that prey might have attacked them if it spotted them.
Likewise, from ancient times, creative imagination has evoked the ultimate form of stealth: not only being inconspicuous but invisible. Harry Potter’s invisibility cloak is a recent and familiar example of stealth capability achieved through magic.
But only in recent decades has stealth emerged as a technology, or family of technologies, designed to make aircraft or other military assets difficult for an enemy to detect. Which raises an interesting question: Could advances in stealth technology, not magic spells, make aircraft or even individual soldiers invisible?
When Stealth Took to the Air
Soldiers can conceal themselves in underbrush, and submarines can dive beneath the waves. But stealth is more difficult for aircraft, since the open sky has no good hiding places.
Night can help, and during World War II, the forerunner of modern stealth aircraft made its debut: the Northrop P-61 night fighter. As the National Air and Space Museum recounts, the P-61 was nicknamed the Black Widow for its color and deadliness.
The war ended and the jet age began. Stealth technology was de-emphasized; the new jets relied on their sheer blink-and-you-missed-it speed for survivability in combat. But improved radar and antiaircraft weapons made even the fastest jet vulnerable.
Hiding From the Spotlight
With that, the Defense Advanced Research Projects Agency (DARPA) — the same organization that created DARPAnet, the forerunner of the internet — started looking again at stealth technology. The emphasis was on concealment from enemy radar, which at its most basic level amounts to a radio frequency spotlight that shines on its target, reflecting the radio waves back to a receiver.
Stealth technology seeks to defeat radar by one of two means: absorbing the radio waves, or reflecting them in a direction that does not go back to the receiver. Absorbing radio waves depends on special materials. Reflecting it away from the receiver depends on shape, which is why the B-2 Spirit stealth bomber has its distinctive “Batplane” appearance.
Radar is not the only threat that stealth technology protects against. Like its predecessor the Black Widow, the B-2 is painted black to make it harder to see at night. And its jet engines are positioned so that their exhaust — invisible to the naked eye, but bright in an infrared scope — cannot be seen from below.
Is a Real Invisibility Cloak Next?
No existing stealth technology makes an aircraft invisible, whether to radar, infrared scopes or the eye. Current stealth technology simply makes it harder to “see.” The B-2 is a large airplane, but its radar image is hundreds of times less conspicuous than a normal plane its size. If the faint blip is noticed at all, it may be mistaken for the background “noise” that any radar set generates.
But technology never stands still. Improved radar can detect fainter blips. And stealth aircraft are less survivable in daylight and clear skies, when a sharp-eyed observer might spot them with ordinary binoculars.
Various technological means of achieving invisibility or at least near-invisibility have been proposed over the years. The surface of a real invisibility cloak might take on the coloration of whatever is behind it. (Some animals, such as chameleons, do this to a degree.) Or light might be bent around an object, providing the illusion of seeing right through it — and thus not seeing it at all.
Invisibility Goes Beyond Stealth
Scientists are exploring multiple avenues to achieving these effects. According to William Harris and Robert Lamb at howstuffworks, one technology uses carbon nanotubes to produce a mirage-like effect just above a surface that would allow us to see around corners. Carbon nanotubes also have other interesting properties.
Also being examined for invisibility potential, writes Alyssa Danigelis at Seeker, are metamaterials. These substances have layers whose atoms are interlinked to form “meta-atoms” that can bend both light and sound in ways that no natural material can match. Beyond military stealth, invisibility and near-invisibility have a host of potential uses, some of which are — so to speak — not at all obvious. Companies like Northrop Grumman hire world-class experts to conduct basic research in these fields to ensure the nation’s security into the future.
For example, invisible structural materials would allow automakers to eliminate the blind spot for drivers, or even make the entire cockpit and nose of an aircraft transparent from the inside, giving pilots an unrestricted view. Beyond these practical uses, materials that play with light could find countless aesthetic uses from hiding unsightly structures to providing visual effects with complementary or contrasting colors.
Much of this work is still at the laboratory stage of development. But as Nancy Owano reported at phys.org, demonstrations have been made with a form of metamaterials technology, named quantum stealth, by its developers. Other experts are skeptical of this work; one said that he would believe it when he didn’t see it. But that is the remarkable thing about stealth technology. Who knows what we won’t see tomorrow?
Harry Potter might not be so fictional after all.