Accessory to War

by Neil DeGrasse Tyson

  Newton, however, was smart enough to make no assumptions. By directing a ray of sunlight through a glass prism, which caused the visible spectrum to emerge from it, and then reversing the sequence, sending the spectrum back through the prism, whereupon white light emerged, he convincingly demonstrated that white light is indeed composed of multiple colors. Although each color in the spectrum shades gradually into its neighbor, Newton, a proponent of cosmic orderliness and the mystically significant number 7,3 declared there were not six colors, as most of us today might list, but seven, slotting indigo between blue and violet to round out the set.

  As early as the summer of 1672, decades before publishing his great work Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light, Newton sent a letter to the Royal Society with a list of questions about light and color that could be properly answered only through experiments. Two of his earliest queries were “Whether rays, which are endued with particular degrees of refrangibility, when they are by any means separated, have particular colours constantly belonging to them . . . ?” and “Whether a due mixture of rays, indued with all variety of colours, produces Light perfectly like that of the Sun, and which hath all the same properties . . . ?”4 His prism experiment would answer yes to both.

  Might Newton also have wondered, just once, even for a moment, whether there might exist some other, adjacent bands of light that our eyes could not see? He had noticed that red, on one end of the visible spectrum, and violet, on the other end, both just faded away to darkness.5 He had raised the further possibility of there being “other original Properties of the Rays of Light, besides those already described.”6 Perhaps most important, he was comfortable with the idea of hidden attributes. Yet Opticks offers no clear evidence that he ventured there. In any case, a century would come and go before anyone conceived an answer to that unstated query.

  Turned out, there were multiple answers. One came early in 1800, when the English astronomer William Herschel—the man who had discovered the planet Uranus two decades earlier—explored the relation between sunlight, color, and heat.

  As Newton had so often done, Herschel began by placing a prism in the path of a sunbeam, but took it a step further. To determine whether each color had a different temperature, he placed thermometers in the various regions of the rainbow cast by the prism. And, like any good scientist conducting a well-designed experiment, he placed a control thermometer outside the color range—adjacent to the red side of the spectrum—to measure the ambient air temperature, unaltered by the warmth of the sunbeam. Herschel did indeed discover that different colors register different temperatures, but that turned out to be the second most interesting result of his experiment. The control thermometer, sitting in darkness, registered an even higher temperature than any of the thermometers placed within the rainbow. Only invisible rays could have caused that warming.

  Sir William had discovered “infra” red light, the band just “below” red. His finding was the astronomy equivalent of geologists discovering the colossal Nubian Aquifer beneath the sands of the eastern Sahara. Behold his account of it:

  By several experiments . . . it appears that the maximum of illumination has little more than half the heat of the full red rays; and from other experiments, I likewise conclude, that the full red falls still short of the maximum of heat; which perhaps lies even a little beyond visible refraction. In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentum as to be unfit for vision.7

  The following year, 1801, Johann Wilhelm Ritter, a German scientist whose main interest was the intersection of electricity and chemistry, picked up where Herschel left off. Philosophically attracted to the concept of polarity in nature, Ritter assumed that infrared must have a companion just off the other side of the visible spectrum. Rather than use thermometers to demonstrate its presence, he used silver chloride, a substance known to decompose and darken at different rates when exposed to different colors of light. Ritter’s experiment, like Herschel’s, was both simple and smart: he placed a small mound of silver chloride in each visible color as well as in the unlit area alongside the violet, then awaited the results. As expected, the pile in the unlit patch darkened even more than the pile in the violet patch. What’s more violet than violet? Ultraviolet.

  Detecting without seeing was now a scientific reality.

  Skywatching didn’t change overnight, though. The first telescope capable of detecting wavelengths outside the slim visible portion of the electromagnetic spectrum wasn’t built for another 130 years, well after the German physicist Heinrich Hertz had shown that the only real difference among the different kinds of light is the amount of energy they carry. And that would ultimately include it all: radio waves, microwaves, infrared, ROY G BIV, ultraviolet, X-rays, and gamma rays. In other words, he figured out that there is such a thing as an electromagnetic spectrum—a symphony of vibrating waves, each with a unique wavelength, frequency, and energy. To the astrophysicist, it’s all energy, it’s all radiation, it’s all light.

  Sometimes light behaves like particles, which we call photons. Sometimes—in fact, most times in our daily lives—light behaves like waves. Whether light should be conceptualized as waves or particles is an old disagreement: Democritus argued with Aristotle about it, Newton argued with Huygens, and quantum physics says it’s both. Hence we’re stuck with the phrase “wave–particle duality,” even though our brains have a hard time wrapping themselves around the concept. Unfortunately, the word “wavicle” never caught on.

  For now, think of light—electromagnetic radiation—as made up of waves made of particles. The word “wavelength” obviously applies to waves. It’s the simple measure of length from crest to crest or trough to trough. Gamma-ray wavelengths are shorter than the diameter of an atom; on the far end of the radio band, wavelengths can be longer than the diameter of Earth.8 The shorter the wavelength, the higher the energy and, broadly speaking, the greater the danger to life as we know it. And whether we’re exploiting the electromagnetic spectrum for saintly or nefarious reasons, the shorter the wavelength, the higher the density of information that can be carried by the light beam.

  Without technological assistance, garden-variety humans see only the tiniest fraction of the full electromagnetic spectrum, ranging from violet light—with a wavelength of about four hundred nanometers—to red light, with a wavelength not quite twice as long, about seven hundred nanometers. When you consider that the bands of the electromagnetic spectrum we’ve measured so far span more than a dozen powers of ten in wavelength, our span of barely a single power of two is just plain lame. Crucially for us, the peak of the Sun’s energy output lies smack in the middle of the visible part of the spectrum. Since we’re daytime creatures, it’s evolutionarily sensible that the detection capacity of our eyes peaks in the same place.

  Infrared and ultraviolet are invisible to us, but that doesn’t mean they’re insensible. We experience them through our skin, not our eyes. We sense the Sun’s infrared light in real time as heat on our skin, but we sense its ultraviolet light only after our skin has been darkened and perhaps damaged by excessive exposure, otherwise known as sunburn.

  Earth itself radiates infrared, as does everything whose molecules are in motion, be it animate or inanimate. In other words, anything and everything with a temperature above absolute zero. Dusty galactic clouds, where stars form deep within, emit infrared. Your kitten, your canary, and your houseplants, dead or alive, all emit infrared. Some species of snakes have small pits on their heads that pick up infrared rays from tasty warm-blooded prey, readily revealed at night against the rapidly cooling surroundings. And alas for the hotel industry and tourists the world over, the antennae of bed bugs have infrared sensors that alert the bugs to a nearby source of warm blood. As for ultraviolet, flying insects—including gnats, moths, mosquitoes, and butterflies—as w
ell as birds, bats, rats, and cats see it quite well.

  Just because an object emits infrared doesn’t mean it can readily be seen by an infrared detector. You still have to single out your target from any competing sources of infrared light, either surrounding your target or surrounding you. Anything warmer than its surroundings shows up brighter. But if the target is about the same temperature, you’ll lose it in the infrared “noise.” Skywatchers improve their capacity to detect their chosen infrared target by deeply cooling their apparatus with liquid nitrogen (77 kelvins) or, for the coolest cases, liquid helium (4 kelvins). These tamp down the thermal noise of the detector itself, permitting the celestial object to shine more distinctly in the data. As you might suspect, the needs of the military aviator are the exact opposite. If targeted by a heat-seeking missile, the plane or helicopter will typically deploy infrared countermeasures such as swirling hot flares, which contribute infrared noise to what the warhead “sees” and thus render the engine’s hot exhaust indistinguishable from the countermeasures themselves.

  Infrared and ultraviolet merely hint at all the light energy we humans cannot see. Further along on the long-wavelength, low-energy end of the electromagnetic spectrum are radio waves (experimentally demonstrated in the 1880s)9 and microwaves (named as the small-end subset of radio waves in 1964–65, hence the diminutive prefix “micro”); further along in the other direction, on the short-wavelength, high-energy end are X-rays, discovered in 1895, and gamma rays, in 1900. Though we’ve assigned labels to the various bands, the electromagnetic spectrum is a continuum. Civilization is layered along this continuum. Hundreds of AM, FM, and XM stations are beaming radio waves through your body right now, the phone part of your smartphone is communicating in microwaves with a cell phone tower, and the map features of your smartphone are talking to GPS satellites overhead via microwaves too. You’re probably receiving visible light from a nearby lamp and, if its bulb is incandescent, infrared light as well. Meanwhile, across the universe, an ancient, persistent, pervasive sea of microwave radiation forms the cosmic microwave background, a legacy of the Big Bang.

  Most celestial goings-on emit light in multiple wavelengths simultaneously. For example, the explosion of a massive star—a supernova—is a cosmically commonplace (though locally rare) and seriously high-energy event that, in addition to visible light, blasts out prodigious quantities of X-rays. Sometimes the explosion is accompanied by a burst of gamma rays or a flash of ultraviolet. When it takes place in our own galaxy, it may emit so much light in visible wavelengths that it remains visible for several weeks without the aid of a telescope, as was true of the supernova spectaculars hosted by the Milky Way in 1572 and 1604. Long after the explosive gases cool, the shock waves dissipate, and the visible light fades, a supernova remnant radiates infrared and radio waves.

  The flip side of visibility is detection. When it comes to the pursuit of prey or, conversely, the avoidance of enemies, detection is key to both conquest and survival. Whether you’re the victim or the aggressor, never is it more advantageous not to see something than to see it. Either way, but especially if you’re the likely victim, you would prefer not only to see the aggressor but also to remain unseen yourself.

  Camouflage (a word of French origin, whose earlier meanings ranged from smoke and suffocating underground explosions to costume disguises and criminal sneakiness),10 the art of remaining unseen, is not uncommon among creatures big and small. Think of the kaleidoscopic changes of the cuttlefish or octopus, the twiglike insect known as the walking stick, or, before the melting induced by climate change, the polar bear’s snowy fur against the whiteness of the Arctic snowpack. Camouflage can be about either keeping yourself from getting eaten or closing in on your own dinner.

  There’s also the distinction, proposed early in the twentieth century by an American artist named Abbott Thayer, between two very different forms of visual camouflage: blending versus dazzling. Nature has chosen the option of blending for both the widespread walking stick and the threatened polar bear. Critters that live in woodsy habitats might blend in by being green on green or speckly brown on speckly brown, while others might dazzle and confuse observers with vivid stripes, prominent spots, or other garish markings that have the effect of breaking up the outlines of their bodies and making them more difficult to track while in motion. In all cases, the goal is to vanish.

  Invaders and warfighters love camouflage and stealth—the nearest they can get to invisibility—and they’ve been attempting it for millennia. In the fifth century BC, the military theorist Sun Tzu advised:

  All warfare is based on deception. . . . Hence, when able to attack, we must seem unable; when using our forces, we must seem inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near.11

  Ten centuries later, Flavius Vegetius Renatus, a prominent Roman court official and author of a military handbook, described traditional camouflage for the scouting craft that accompanied large warships for the purpose of making surprise attacks, intercepting enemy convoys, and monitoring the approaching enemy:

  So that the scouting vessels will not be betrayed by brightness, the sails are dyed Venetian blue, similar to the colour of the sea, and the tackle is coloured with the wax that ships are generally coated with. Also, the sailors and marines wear Venetian blue coloured clothing so that not only at night, but also in the daytime, they more easily remain unseen while scouting.12

  Though the sea continually changes color, at a distance the ships’ blue coloration could—under optimal conditions—merge with that of the water. Only at close range would the difference between their Venetian blue and the varied blues, browns, greens, and grays of the sea be readily perceptible. But once the difference had registered, there wouldn’t be enough time to organize an attack against the scouts. Distance buys time and advantage, which was precisely Galileo’s point when he sought support from the doge of Venice in 1609. A few other proposals for maritime camouflage were more imaginative than a coat of Venetian blue. One early twentieth-century alternative, never implemented, involved swathing ships in billowing white covers meant to simulate clouds.13

  Cladding troops and vehicles with branches and leaves to simulate forest foliage is another time-honored type of camouflage, whether in the guerilla warfare of twentieth-century Vietnam or in medieval Scotland (recall the dire prophecy in Shakespeare’s Macbeth: “Fear not, till Birnam wood / Do come to Dunsinane”). But not until World War I, when artists began to paint unrolled canvas to look like roads and observation posts to look like tree trunks, did the word “camouflage” officially enter the English language. Soon the practice of painting whole warships in a dazzle pattern (also called disruptive patterning or, much snazzier, razzle dazzle) was adopted on both sides of the Atlantic. The deciding factor seems to have been the sinking of almost a thousand British ships by Germany’s U-boats during the first nine months of 1917, causing a British painter of seascapes who was serving as a naval officer to propose that “since it was impossible to paint a ship so that she could not be seen by a submarine, the extreme opposite was the answer—in other words, to paint her in such a way . . . as to break up her form and thus confuse a submarine officer as to the course on which she was heading.”14

  Creating confusion seemed a better solution than trying to attain invisibility. Suddenly artists became facilitators of military goals, as warriors picked up some of the disintegrationist, scientistic visual strategies of the vanguard movements of Cubism, Futurism, and Vorticism. Picasso and Braque, the fathers of Cubism, were delighted to see what they regarded as their aesthetic invention being applied to ships and weaponry; walking down a boulevard in Paris one evening and seeing a convoy of zigzag-painted heavy guns heading for the front, Picasso reportedly exclaimed, “We invented that!” Franklin D. Roosevelt himself, assistant secretary of the US Navy during World War I, is said to have shouted, after being shown a bedazzled test ship, “How the hell do you expe
ct me to estimate the course of a God-damn thing all painted up like that?” In the end, though, standardized dazzle camouflage apparently did not live up to its promise. Attacks proceeded at similar rates on ships with and without such a coating. Nevertheless, despite plenty of evidence to the contrary, magical thinking about the efficacy of disruptive camouflage persisted through World War II and beyond.15

  Several options for disappearing from the visible part of the spectrum have a long wartime pedigree. The simplest is to exploit the darkness of night. Another is to blind the enemy. Set a huge bonfire, and enemy forces who look at it will be unable to see anything other than flames and therefore be unable to target you with any precision. In recent decades, both lasers and smokescreens have been used to blind the enemy: throw a white-phosphorus grenade at your target, and you’ll get an instant smokescreen that will also scorch whoever is in the vicinity while it masks your own maneuvers and your own infrared radiation. Blinding also happens to us in the universe, when the light of a host star swamps the much fainter reflected light of its exoplanets. This was a big problem until a couple of decades ago, when space scientists began using a special occulting disk in their telescope’s optics to block out the offending starlight, thus achieving the opposite of what a telescope was invented to accomplish.

  Another, very different approach to disappearance is transparency—embodied in the clear glass window. Flies, moths, birds, and visitors from outer space who don’t know about windows must be baffled by the interposition of something visually imperceptible but impenetrable between themselves and the scenery.