by Bob Berman
Herschel published three papers in one of the Royal Society’s journals, reporting his results. He also quickly discovered through further experiments that this same invisible heat-light also emanated from terrestrial sources such as gas lamps and candles. At the end of his second paper, Herschel suggested that light and heat are part of the same phenomenon, tracing two apparently disparate experiences to the same source. “We are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one,” he wrote.
Here Herschel was using a scientific principle already known in his day called Occam’s razor. It was posited by an English Franciscan friar, William of Ockham (1287–1347), who argued that of all the possible explanations for a given result, the one with the fewest assumptions is most likely to be correct. In other words, when it comes to scientific hypotheses, the simpler the better. If your car fails to start in the morning, it’s possible that the problem was caused by a meteorite falling on it overnight and damaging the electronic ignition circuit board. But it’s simpler to surmise that the battery is dead or you’re out of gas. If these simple assumptions turn out to be wrong, then you might gradually move on to investigate increasingly intricate and unlikely scenarios.
So Herschel could have imagined that his prism bent, or refracted, two entirely different solar phenomena and placed them side by side on his table. But it was simpler—and, as it turned out, correct—to assume that light contains one component we see with our eyes and another that we feel as heat on our skin, heat that can be measured with a thermometer. He also suggested that light’s various colors might produce dissimilar effects on chemicals and their reactions, foreshadowing the birth of photography half a century later.
Herschel called the new, invisible form of light he had stumbled upon calorific rays and announced that these rays were reflected, refracted, absorbed, and transmitted just as visible light is. (We will remember this similarity later, because other forms of unseen energy do not follow this pattern and do not behave as visible light does.) Despite deriving from the Latin word for “heat” (calor) and thus being a logical label, the term calorific rays was later abandoned, and Herschel’s invisible rays were renamed infrared radiation to indicate their location in the spectrum. This, of course, was logical, too, because the prefix infra means “below” the red.
Herschel himself suggested that if we could detect these rays in the distant universe, they might open the door to new discoveries that would remain unavailable if we limit ourselves to the visible alone. This proved true as well. Today more than half of all new telescopes are designed to detect infrared rays, including the giant James Webb Space Telescope, scheduled for launch late in 2018. This is one of the reasons modern observatories are located atop tall mountains, where our atmosphere will absorb and block the least amount of cosmic infrared radiation. When the European Space Agency launched its orbiting infrared telescope, whose 3.5-meter mirror makes it much larger than the Hubble Space Telescope (though smaller than the Webb), they named it the Herschel Space Observatory. They also took pains to explain that it was named to honor both William and his sister Caroline. It detects radiation from objects in space that are too cold to emit visible radiation.
And so the infrared energy discovered by William and Caroline lit the way, so to speak, for an ongoing flood of discoveries. It also very much paved the way for the discovery of an entirely different form of invisible light—one that was downright spooky.
CHAPTER 4
Hot Rays
Radiation that lies just beyond the red end of the spectrum may be invisible, but it has a major effect on us. First things first: we must stop ourselves from making the mistake everyone does—regarding infrared radiation and heat as the same thing. We cannot be blamed for this, since everyone calls a deep-red bathroom floodlight a heat lamp. And on a chilly April afternoon, we might claim to “feel the sun’s heat” on our skin. Neither is true. In both these cases, what’s being emitted is infrared radiation.
Infrared radiation is not heat. Rather, infrared radiation creates heat. They are two entirely different animals. You are now probably the only one on your block who knows this.
How does infrared radiation create heat? Think of infrared radiation as invisible waves of light, a millimeter from crest to crest, generated the same way other forms of light are created, by the motion of electrons. Heat, on the other hand, is simply the motion of atoms and molecules. Little solid particles moving or jiggling. When a light ray’s waves have just the right spacing so that they pass a particular spot at the correct frequency, it will give an entire atom a little push. Beam enough of those waves at anything, and all its atoms will be set to vibrate. Voilà: heat.
Visible rays of sunlight can jostle atoms, too, as Herschel discovered. But if the waves are too short and too frequent, they’ll scarcely budge atoms, which is why green and especially blue and violet light don’t heat things very much. Red does better. And when it comes to maximum efficiency, those unseen infrared waves are optimal for making whole atoms jiggle.
Say you’re sitting in a parked car. Visible sunlight comes through the glass. Some of it manages to slightly jiggle some of the atoms in the dashboard, upholstery, and every other sunlit part of the auto’s interior. Then each jiggling atom creates and releases some of its newfound kinetic energy as a bit of infrared light, so now the car’s interior is increasingly filled with infrared light. And here’s the surprise: while glass is transparent to visible rays (duh!) it is opaque to infrared rays. That’s because the regular resonance, or vibration rate, of glass atoms is naturally in sync with infrared rays, so a kind of chaotic barrier gets created, preventing the infrared rays from leaving the car. Glass lets visible rays in but keeps infrared rays from exiting. Glass creates an infrared trap. Result: the car gets hotter and hotter. This is why you never leave a living thing in a closed car on a sunny day.
And it’s why greenhouses, sometimes called hothouses, are made of glass. We want to let light in but block heat from escaping.
When you feel the “warmth of the sun” you are actually only feeling the increased vibrations of your skin’s atoms. The accelerated atomic motion was caused by the sun’s infrared radiation, but you can’t actually feel it. The sun does a good job of warming your skin, the ground, anything it hits, because slightly more than half the sun’s total emissions are pure infrared radiation. There’s lots of it hitting our planet. When it strikes the ground it heats it so efficiently that the surface of the earth in turn heats the air just above it. Hot air rises, so up go bubbles of warm air. When this starts happening, typically around 9:00 or 10:00 a.m. every sunny day between late April and late August, the air rises far enough—cooling steadily all the while at the rate of five degrees Fahrenheit per thousand feet—to eventually reach a height where it cannot hold its vapor, since cool air can’t contain as much moisture as warm air. Bingo: a cloud suddenly forms. The water has changed from its gaseous state, which is transparent, to its liquid state, meaning that the vapor abruptly condensed into untold billions of tiny droplets. There are around five grams of water droplets in each cubic meter of a typical cloud. No wonder you can “feel” that you’re in a cloud whenever you walk through fog. It feels viscerally damp.
A cloud is a visible manifestation of the power of infrared rays. The sun’s IR radiation heats the ground, which makes a large air bubble rise until it cools too much to hold its vapor in a gaseous state. (Bob Berman)
The cloud’s height reveals the altitude at which the rising air has cooled to its dew point, where the vapor-to-liquid transition occurs. In humid summer air, that’s typically four thousand feet. In the dry air of Montana it’s more like nine thousand feet. Infrared radiation is what kicks the daily weather into motion.
Another way infrared radiation differs from heat is that the latter spreads rather slowly, while infrared radiation travels at the speed of light, essentially instantaneously. Remember that heat
is the motion of atoms. So let’s say we place a frying pan on the stove and turn on the burner. The flame makes the pan’s metal atoms jiggle faster and faster as the bottom heats up. But the atoms in the pan’s handle are still cool. You can hold it safely for quite a while, even as the butter starts melting and the eggs start frying. The handle will eventually heat up, though, because heat always travels in a single direction, from hot areas to cool areas, which means that the kinetic energy of fast-moving atoms influences other, more stationary atoms in their vicinity and makes them start to jiggle faster, too. The jiggling domino effect gradually moves through the metal until at last the handle gets too hot to touch. It takes a while.
But now consider infrared radiation, which, again, moves at the speed of light. Say you’re gathering with some friends around a bonfire. Even at a distance you can feel the fire’s heat on your face, which actually means you’re feeling the flames’ infrared rays making your skin’s atoms move faster. But here’s the point. If some big person steps in front of you, blocking the light, you instantly feel the change on your face. The guy in front of you has cast an infrared-blocking shadow; you are now inside it. You can feel the infrared’s absence immediately.
Where there’s heat, there’s infrared radiation, which makes it a boon to police, the military, and weather reporters. Infrared light makes atoms jiggle, and the energy of jiggling atoms causes new infrared photons to appear. In every situation except on the hottest summer days, bodies are warmer than their surroundings, so your jiggling atoms emit infrared rays and betray your presence to infrared sensors. Such devices can also detect various crops (think marijuana), because each type of plant has an exact characteristic temperature. On a more benign note, infrared-sensing satellites can determine the heights of clouds, because the top of a cloud is cooler than the bottom.
Cloud layers hover at specific altitudes because the temperature at those elevations causes atmospheric moisture to change from a vapor to liquid droplets. The unseen energy behind all this drama is infrared. (Michael Mah)
Such orbiting instruments can also pinpoint ocean movements, especially dramatic ones such as the frigid Humboldt Current, which runs northward up the western coast of South America.
Because infrared waves bend around obstacles more easily than visible light does, they’re used in some cordless phones and audio headsets. They’ve been controlling garage doors for decades, sending invisible rays to the door opener’s IR detector, which signals the motor to start moving. Medical infrared imaging is a very useful diagnostic tool, too, because tumors, for example, are usually warmer than their surrounding tissues. Infrared cameras are effective fire-detecting tools because they can spot heat alterations in buildings and can be used to test electronic fire-prevention systems.
But the best thing about infrared waves, what sets them apart from some of the other invisible rays we’ll explore, is that they’re harmless. Infrared rays are not carcinogenic, and their ability to heat skin does not pose a mortal risk. After all, it’s okay to have your body atoms jiggling a bit faster than usual. Indeed, when you get the flu and run a fever, all your atoms are moving around three miles per hour faster than normal. You could tell the doctor that your atoms are so fast right now that they’re making you feel terrible, and he might give you some Tylenol and say, “Here: this will slow them down.” The point is that a little extra atom speed isn’t going to hurt you very much. (There are exceptions, or course. People and pets have died of sunstroke.)
But compared to more closely spaced, faster-vibrating light rays such as gamma rays and X-rays, which, as we’ll see, can be seriously harmful, long waves such as infrared rays are far more benign. Not only are they safe, they’re also essential to your coziness, since infrared radiation creates heat. And we have infrared radiation to thank for other daily comforts and conveniences as well.
And okay—it may not be technically correct, but go ahead and keep referring to that bathroom infrared floodlight as a heat lamp. At least now you know the real story.
CHAPTER 5
Ultraviolet Brings the Blues
Our next invisible light “flavor” is ultraviolet. This is an attention grabber, because it’s the variety of light that causes the most deaths every year. Its discovery was troublesome, too, and carries us back more than two centuries, to a mere year after William Herschel announced his calorific rays to the cheers of scientists around the globe.
It’s hard to imagine a story more different from Herschel’s than that of Johann Ritter. For introducing the world to infrared radiation, Herschel was justly lionized, almost worshipped, and lived happily into his mid-eighties. By contrast, Ritter’s 1801 discovery of ultraviolet light was ignored—and he died penniless at the age of thirty-three. It hardly seems fair.
Ritter was born in Samitz, in what is now Poland, in 1776—five years before Herschel discovered Uranus. The son of a Protestant pastor, he attended Latin school and, at age fourteen, was sent by his father to a nearby city to learn to be a pharmacist. As he was learning and practicing the trade, he developed a keen love of science and performed countless experiments. His life changed, however, when he was nineteen: his father died and left him a modest inheritance, enabling him to enroll in the University of Jena in April of 1796. There he met Alexander von Humboldt, one of the most famous scientists of his day, and embarked on studies of electricity and its effects on the body.
By the time he reached the age of twenty-one, Ritter’s research and published papers on electrochemistry and electrophysiology had attracted enough notice to make him an odds-on favorite for the ranks of the science immortals. Indeed, after he created the world’s first dry-cell battery, at the age of twenty-four, then married the young, pretty girlfriend with whom he’d been living for years, his trajectory seemed upward bound.
But it was not to be. Ritter had a tendency to get into disputes, and he quarreled with university officials over the issue of whether he’d be appointed as a lecturer. In time, Europe’s larger scientific community also grew suspicious of Ritter’s conclusions and even many of his stated facts because he routinely couched them in philosophical or occultist rants.
The main problem was his championing of the tenets of the Naturphilosophie, then in vogue in some German intellectual centers. Ritter based his work on the idea that the universe is a “oneness” in which all scientific disciplines are interconnected and that the cosmos possesses a “world-soul,” meaning a kind of innate, built-in, God-like intelligence. He also believed that polarity—meaning the law of opposites—ruled nature. Just as magnets have a north and south pole, Ritter was convinced that absolutely every aspect of science boiled down to, and could be explained in terms of, pairs—the elements of which, in his view, often stood in opposition to each other. He found evidence for his belief everywhere he looked. One of his experiments refined the work of earlier scientists who’d used electricity to divide water into two elements, hydrogen and oxygen. He also noted that air had been found to be essentially composed of two gases, oxygen and nitrogen. Always pairs. He became convinced that the earth had opposing electrical poles in addition to opposing magnetic poles.
It was his obsession with dualities that spurred Ritter to his greatest discovery. He had of course learned of Herschel’s bombshell 1800 discovery of invisible calorific rays, or heat rays, just beyond the red end of the spectrum. Ritter quickly hypothesized that cooling rays might dwell on the opposite side of the spectrum, the violet end.
He started out by doing exactly what Herschel had done, but he soon found that temperatures did not drop on the violet end, so he tried something else. Since he couldn’t find any physical effect produced by the rays, he looked for a chemical reaction. It had already been proved that paper soaked with silver chloride would blacken when exposed to sunlight; this discovery was one of the earliest stepping-stones toward the field of photography. Ritter wondered whether all sunlight’s colors would create this reaction with equal speed. He exposed silver chloride–soa
ked paper to various parts of the prismatic spectrum, cast onto the paper by sunshine striking cut glass. Red light had only a negligible effect in darkening, or reducing, the compound to silver, while green light did it much faster and violet did it fastest of all. Ritter then placed the chemically soaked paper in the dark spot beyond the violet end of the spectrum, and voilà—the paper darkened even more rapidly than it had in violet light. Obviously some invisible rays that lay beyond the violet end of the spectrum had a dramatic, repeatable chemical effect.
Ritter had done it—he had discovered an entirely new form of invisible light. But alas, he predictably interpreted this effect as proof of a polarity between “deoxidizing rays” near the violet end of the spectrum and “oxidizing rays” near the red end. Here again was his obsession with dualities. When the world heard about it, people soon abandoned Ritter’s term “oxidizing rays,” and this new form of invisible light came to be called chemical rays, a label that stuck throughout most of the 1800s. It took a full lifetime for Herschel’s calorific and Ritter’s chemical rays to instead be labeled infrared and ultraviolet.
You’d think such a momentous finding would have elevated Ritter to the status of Herschel. It didn’t. First Ritter continued his habit of embedding his findings in the language of polarities and soul, the Naturphilosophie tenets of “oneness between nature and people.” And soon he got worse, routinely peppering his papers with references to such occult practices as dowsing (using a divining rod to find water underground). He imagined he’d found the general principles governing the interdependencies of inorganic nature and human phenomena and named this new branch of study siderism. He even published a periodical with that title. With virtually no subscribers, its first issue was its last.