by Bob Berman
Second, retinal cells consist of molecules that are affected by incoming energies, which trigger the cells to emit electrical pulses. The energy of photons of various kinds determines how the retina interacts with matter. It’s no accident that retinal photochemistry only works when dealing with a very limited range of energies. If the spacing between waves of light is just a bit longer than that of the deepest red we can see, its energy is too weak to influence the retina’s protein molecules. This is why, on a strictly physical and chemical level, we cannot see infrared radiation.
At the other end of the spectrum, the energy of wavelengths shorter than those of violet is so powerful that it can damage those sensitive molecules. Fortunately the lens of the eye absorbs this light before it can do any retinal damage. (The lens pays a price for shielding the retina, though. Over time, the lens itself becomes damaged, often resulting in cataracts. This is why it’s a good idea to block the blue-violet-ultraviolet part of the spectrum by wearing sunglasses in bright natural light.)
Third, and perhaps least important, some forms of invisible light don’t bounce off the objects around us. Some of these wavelengths bend or diffract around objects instead of reflecting off them. Others, such as X-rays, penetrate objects rather than bouncing off them. Detecting these rays as a way of “seeing” our environment would be impossible and pointless.
Now that we know what they are and why we can’t see them, let’s begin our tour of the invisible rays that flood our lives.
CHAPTER 3
The Green Planet and the Red Heat
Ours is an age of discovery. A time of mind-blowing scientific revelations, ranging from magnetars (bizarre supermagnetic suns) to extremophiles (life forms that thrive in seemingly lethal temperatures). We’re used to being wowed by science.
So it’s hard for us to imagine the shock that swept the planet in March of 1781, when an unknown astronomy hobbyist abruptly pulled the rug out from under the greatest minds of the time, achieving instant global fame.
Who was this person whose name was then on everyone’s lips? Odds are you’ve scarcely heard of him. Indeed, when we ponder the greatest minds in human history—those whose discoveries were most profound or ahead of their time—we’d have to begin with an “unknown”: Aristarchus of Samos, the first person to say that the earth orbits the sun. We’d include Isaac Newton, who explained motion, and Albert Einstein, who revealed that space and time warp and shrink, meaning that the universe doesn’t have a fixed size. And among these giants we must place a man named William Herschel, born on November 15, 1738, in Hannover, Germany. He may not have been as brilliant as the aforementioned pioneers, but thanks to his tenacity—his ability to follow a scientific scent as relentlessly as a bloodhound—he made not one but two of the most astonishing scientific discoveries of his time. On top of that, he was just plain lucky.
Herschel’s early life gave little clue to his later genius. He was one of ten children, and his father was not an aristocrat but rather a musician in the military, an oboist. Following in his father’s footsteps, young William played oboe in the band of the Hanoverian Guards, where he displayed enough talent at an early age to suggest that composing might be his lifelong destiny.
He first visited England at the age of eighteen and was so impressed that he resolved to immigrate there, which he did the following year. Moving to a new place to seek one’s fortune as a musician seems like a Haight-Ashbury pipe dream in our modern times, but back then it was an almost mainstream thing to do. England during the mid- and late eighteenth century was a land of musical opportunity like no other, and Mozart, Haydn, and Handel, along with thousands of unknown musicians, gravitated to the country. As a result, the competition was fierce. Yet Herschel soon scraped together a living by copying music in that pre-Xerox era. Slowly he climbed the professional ladder, paying the rent by teaching and composing. Finally, in 1766, he was appointed organist of a fashionable chapel in Bath, the well-known spa town. Herschel was not only an accomplished oboist and organist: he was also skilled on the violin and harpsichord. He composed twenty-four symphonies and many concertos—an impressive output, even if the works are regarded today as remedies for insomnia.
Happily, his intellectual curiosity extended far beyond music. One day he read a book that changed his life: Robert Smith’s A Compleat System of Opticks, from which he learned techniques for do-it-yourself telescope construction. With his boundless energy, Herschel didn’t merely mimic the astronomers of his day by observing and sketching the moon and planets. He was primarily intrigued by the dim nebulae that peppered the heavens. Almost universally believed to be luminous fluids, they beckoned with a mystery unrivaled by the mountains and craters of the moon. What were these hazy blobs, really? They were so faint that to see them well would require a telescope larger than any then in existence, since the brightness and clarity of a telescope image was directly proportional to the diameter of its main lens, or mirror. The big problem, then as now, was that bigger telescopes were disproportionately expensive, and many were of poor quality.
If Herschel wanted a mirror big enough to see the nebulae up close, he was going to have to make it himself. Back then, specialized mirror glass didn’t yet exist, and Herschel began casting molten metal in his basement and grinding his own mirrors from blank disks he created by mixing copper, tin, and antimony. Although his first mirrors cracked on cooling and became nothing more than expensive paperweights, he ultimately succeeded, and indeed his mirrors grew ever larger, some nearly two feet in diameter, a huge light-gathering improvement from the mere six- to eight-inch mirrors that were most common at the time. More than that, their shapes—paraboloids fashioned to a precise curve—were of such high quality that all the distant starlight, planet light, and galaxy light arrived at a perfect common focus. Herschel’s homemade telescopes outperformed even those at the famous Royal Observatory, in Greenwich. He also made his own eyepieces.
Herschel was joined in this work by his brother Alexander and his sister Caroline, who remained his faithful assistant for the rest of his long life and eventually became a respected scientist in her own right.
Slowly the intellectual society of England began to hear whispers of this unusual family with their enormous, unparalleled telescopes. Using them, Herschel embarked on perhaps the most energy-intensive project of his life—a telescopic survey of the entire sky. When that was completed, he built an even bigger telescope and completed an even more thorough and detailed survey of the heavens. Then, on March 13, 1781, during his third and most comprehensive sky survey, he saw something that would change his life and astound the world.
Herschel observed a green “star” that wasn’t just a point of light but was shaped like a disk. Initially he surmised that it must be a comet. But it couldn’t be a comet, for it never developed a tail or the highly elliptical orbit comets possess. By observing the object’s slow nightly motion, he soon realized it was a new planet that had an eighty-four-year orbit around the sun. He named it Georgium Sidus after the English king George III, in a successful attempt to attract attention and curry favor. However, others soon insisted that it be named in accordance with tradition and possess the name of a god in Roman mythology. Thus the first planet anyone ever discovered came to be called Uranus.
The world was amazed. No one had imagined that the universe contained any planets other than the five familiar bright “wandering stars,” which had been known since prehistory and were mentioned in the Bible, the Vedas (the Hindu holy books, written in Sanskrit), and ancient Egyptian papyri. No prophet, no holy book, no great thinker, no prestigious council, no school of philosophy—no one had entertained the notion that there could be other worlds that were too faint to be readily seen. Moreover, the telescope had been around for 170 years by that point. Countless astronomers had carefully inspected the heavens. The cosmos contained one sun, one moon, and five bright planets. That was the nature of reality, and no one had the slightest reason to doubt it until Herschel blew
that “reality” to smithereens. The shock of the discovery was akin to a modern scientist revealing that it is our largest toenail, rather than the brain, that controls our thoughts. Or that the moon is hollow and inhabited by a race of monkeys.
As it turns out, Uranus is dimly visible to the unaided eye—I’ve seen it on several occasions without the slightest optical aid. That green world is positively brilliant through any telescope. Why hadn’t anyone found it earlier? This question caused deep consternation among intellectuals. It became the headline of the century around the world.
Overnight William Herschel went from an amateur telescope maker and mediocre classical composer to the world’s most celebrated scientist. The Royal Society awarded him the Copley Medal, the era’s equivalent of the Nobel Prize. King George III, badly needing some prestige after having just lost possession of the American colonies and tickled pink that Herschel had first tried to name the new planet George, awarded him an annual pension of two hundred pounds sterling.
Herschel could then spend all his time on astronomy, and he did so tirelessly for the next forty years. He dedicated his new career to building ever-larger telescopes and trying to solve the thorny nebula problem—deciphering the true nature of those interstellar “clouds.” He almost immediately found that his best instruments rendered most of those glowing blotches as separate innumerable stars, which made him wrongly conclude that all nebulae were of that nature—star clusters. But since some nebulae stubbornly remained blurry no matter how large the telescope or great the magnification, he knew they must be very distant as well as huge. He therefore concluded that the whole cosmos was composed of such gargantuan clusters—cities of suns—which would later be called galaxies.
In 1788 the Herschels moved to Slough, where William spent the rest of his life. On every clear night (in England such nights were few and far between; he hired a watchman to wake him if the weather cleared), Herschel observed the heavens, dictating from the eyepiece while Caroline took notes. To supplement his income, Herschel made telescopes—the finest of his day—for others.
When all was said and done, Herschel created three catalogs listing 2,500 nebulae and star clusters. He identified 848 double stars and had seventy scientific papers published. He correctly calculated our solar system’s motion and direction through space. He discovered two moons of Saturn and coined the word asteroid. He was the first to say that our Milky Way galaxy has the shape of a pancake. Not bad for an oboist.
But Herschel appears here not for his first-ever discovery of a new planet but for something he found in 1800, near the end of his illustrious career. Actually, his unveiling of the green planet was so astounding that this later revelation is scarcely mentioned in biographical sketches. Encyclopaedia Britannica’s 1,700-word article about him devotes exactly ten of those words to his having discovered the first-ever invisible light.
Though he was the first to discover them scientifically, Herschel was not the first to intuit the existence of unseen rays. But he, like everyone else, was unaware of previous hypotheses because the work of his predecessor had evaporated decades earlier, leaving no trace. Émilie du Châtelet, who died in 1749, was a French author, physicist, and mathematician. Her greatest accomplishment remains her translation and explication of Isaac Newton’s main work, his Philosophiae Naturalis Principia Mathematica, a translation still in widespread use today in the French-speaking world. Du Châtelet had been raised in Paris in the 1710s, in a thirty-room town house overlooking the Tuileries Gardens. As a child she loved eavesdropping when educated guests—especially astronomers—came to visit, and she developed a talent for science and mathematics that was rare among the women of her time, as few women had the opportunity to study these subjects. (The writer Voltaire later said that she was “a great man whose only fault was being a woman,” which underscores the virtual impossibility of gaining respect or recognition as a female scientist in that era.)
Émilie was in her late twenties when she met Voltaire, then age thirty-nine, and they fell in love. Soon they were living together in a large house in eastern France, where together they created a center for research—she in science, he in philosophy. Distinguished visitors regularly came from other parts of Europe.
One summer night, Émilie had an insight about the nature of light that would reverberate more than a century later in the wake of discoveries involving photography and infrared radiation. She wrote her ideas in accurate scientific form and, with a bit of influence from Voltaire, became the only woman in eighteenth-century France to have a scientific treatise published. That 1737 paper, Dissertation on the Nature and Propagation of Fire, ultimately published in 1744, predicted that there was an unseen form of light that she hypothesized must be the source of a flame’s heat.
After she and Voltaire broke up, she fell in love with a French poet, and when their relationship ended—badly—she discovered that she was pregnant with his child. In that era, a pregnancy in one’s forties was extremely risky, and Émilie had a premonition that she would die in childbirth. Frantically she rushed to complete her magnum opus—the translation of and commentary on Newton’s Principia. Then, tragically, her premonition came true, and she did die soon after childbirth, at age forty-two. Just as sad, a commotion erupted when word of her unwed status spread a scandal that quickly cast her life, work, and accomplishments into oblivion. She remained unknown well into the twentieth century and didn’t really emerge until a 2006 revival of interest resurrected her accomplishments. In any case, her prediction about an invisible component of light was of course utterly unknown to Herschel a lifetime after her published paper had appeared and just as promptly vanished from sight.
Fully nineteen years after his discovery of Uranus brought him global renown, Herschel was still engaged in nonstop experimentation and observation, always recorded by his sister Caroline (continuing the pattern of women making unsung contributions to Herschel’s discoveries). Neither he nor anyone else realized it, but an avalanche of publicity and acclaim was about to come crashing down on him all over again.
In 1800, Herschel was fully aware that when visible light strikes any surface, some of the light’s energy is absorbed, and the surface is warmed. He knew that dark objects apparently absorb more energy than light objects, since they get hot faster, whereas white paper, for example, reflects most of the light that hits it and scarcely warms at all.
Herschel was then using two of his telescopes to observe the sun. He employed dark glass filters to screen out most of the light so it wouldn’t blind him. Yet he could always still feel some of the sun’s warmth coming through the filters. He noticed that some filters seemed to allow more of the light to pass through, while others transmitted more of the heat. He wrote that, when observing the sun, he “felt a sensation of heat” even when a particular filter transmitted “but little light; while others gave… much light with scarce any sensation of heat.”
Always curious about the underlying nature of things, Herschel decided to see for himself the degree to which various colors of glass transmitted heat. Were some colors associated with more heat than others? It was the simplest kind of question, but it was one that nobody had previously explored.
With Caroline looking on, pen in hand, Herschel set up an apparatus that let sunlight pass through a narrow opening. The thin beam of light then struck a piece of cut glass. This prism spread a rainbow containing all the colors of the visible spectrum across his table. He then positioned three thermometers on the table. He placed two in the shadows, far outside the spectrum. Their purpose was to act as “controls,” taking the temperature of the unlit sections of the table. Then he placed the third thermometer within the bands of colored light on the table, studying each color in the spectrum one by one.
Herschel took repeated readings in the violet, green, and red regions of the spectrum. In each he observed a temperature rise, which he dictated and Caroline recorded. His findings: after he left the thermometer for eight minutes in each of the three
colored-light zones, the average temperature increase was two degrees Fahrenheit in violet, 3.2 degrees Fahrenheit in green, and 6.9 degrees Fahrenheit in red. The red was much hotter than the other colors!
Obviously either sunlight’s red beams had a greater heating effect than its green and violet beams or perhaps there was somehow more red light reaching the table than green or violet light, although this latter explanation didn’t seem likely because the red looked no brighter than the other hues.
Then something historic happened. It was the kind of fluke we frequently come across when we study the great eureka moments of scientific discovery. Herschel took a break from his experiment and left the room. As the sun moved slowly across the sky, the spectrum crept across the table until it was no longer hitting any of Herschel’s carefully placed thermometers. When he returned, he glanced at the thermometer he’d left sitting in red light, by then lying in the shadow just outside the red end of the spectrum, and was surprised to see that the temperature was much higher than it was when the instrument had been bathed directly in the sunlight’s red band. What was going on? He repeated the readings. The conclusion slowly dawned on him. Invisible “heat rays” coming from the sun were being refracted by the prism to a position just beyond the rainbow’s red boundary.
Herschel kept taking measurements at various positions. If he placed the thermometer more than a few inches past the red end of the visible spectrum, it registered no temperature increase at all and matched his two control instruments. He also looked beyond the violet end of the spectrum but found no temperature change there.
William Herschel, who made not one but two astonishing discoveries, including that of the first-ever form of invisible light. (Wikimedia Commons)