The Day We Found a World Beyond the Senses
William Herschel is best remembered as the astronomer who discovered Uranus. But on March 27, 1800, he revealed an entire unseen side of nature.
It’s peculiar which historical discoveries we decide are crucial to know, and which ones don’t even deserve a mention. Like pretty much every kid of my generation in the United States, I learned in school that Christopher Columbus “discovered” the New World. Never mind that there were already plenty of people living here when Columbus arrived, or that he wasn’t the first European to reach the Americas, or the horrific aspects of his voyages that were omitted from the official narrative. This was considered an important discovery by the educators who wrote textbooks and designed curricula.
Now what about another sweeping discovery: the discovery of a world beyond the human senses, illuminated by types of light that we cannot see? Conceptually, it was one of the great awakenings in human history. Much of modern technology would not exist without an understanding of invisible light. But I never learned about that discovery in school, and I am willing to bet that you didn’t learn about it, either.
Maybe you’ve heard the name William Herschel as the astronomer who discovered Uranus in 1781, instantly doubling the size of the known universe and proving that the solar system contained planets beyond those known since antiquity. There was much more to Herschel, however.
He was an astute instrument-maker and an observer of rare discernment, who pushed back the frontiers of perception everywhere he looked. He made the first serious attempt to map our home galaxy, the Milky Way. He sought out the birthplaces of stars. He speculated on the nature of life on other worlds (even, he suggested, on the Sun!). And — back to our story — starting in early 1800, he carried out a series of experiments into the nature of heat and light, culminating in the astonishing detection of rays from the Sun that carry a warming effect even though they are invisible to the eye.
In modern terms, Herschel had discovered infrared rays, the first indication that visible light is just one tiny slice of a far-ranging spectrum, most of which is off-limits to the human senses.
“A sensation of heat, though I had but little light.”
Herschel’s discovery of the unseen side of reality was completely in character for a man whose CV could have been summed up simply as “curiosity.” He fled Hanover as a teen when the French invaded, and moved to England. There he worked as a music teacher, then a composer and organist, then became interested in music theory, then in optics theory, then in telescope making. By the 1770s his home-made telescopes were more powerful than the one at the Greenwich Observatory; by 1789, he had constructed the largest telescope in the world — simply because he wanted to know what was out there.
Herschel was as methodical at observing the sky as he was at shaping his finicky copper-tin alloy telescope mirrors. At least two renowned observers (including John Flamsteed, Britain’s first Astronomer Royal) had looked right at Uranus without comprehending what it was. Herschel immediately recognized that it was no star, and quickly deduced that it was no ordinary comet, either. His celestial investigations continued into the daytime as well, when he set out to develop better light filters so that he could study the surface of the Sun. At this point, serendipity struck again.
While experimenting with various types of filters that would let him look at the Sun without frying his eyes, Herschel noticed a bizarre disconnect. “[W]hen I used some of them, I felt a sensation of heat, though I had but little light; while others gave me much light, with scarce any sensation of heat,” he wrote in a paper read before the Royal Society on March 27, 1800. He recognized two huge, intriguing implications: Certain types of rays were capable of transmitting heat, and some of those rays were somehow different from the types of light rays that stimulate the eye. (I’m indebted to Jack R. White for gathering together many of the details of this history.)
Now Herschel had a ripe mystery to solve: What were these heat-transmitting rays? Did they bend through a prism, like ordinary light? If so, did they correspond to the colors of light in some way? Herschel being Herschel, he set up a prism, got his hands on the best thermometers available at the time, and went to work chasing answers. He passed sunlight through his prism, cast a rainbow, and measured the temperatures of the different colors. A clear pattern emerged as he moved from the violet to the red end of the prismatic rainbow. Green light produced more warming than blue; red produced more warming than green.
Which made Herschel wonder…what if? What if he kept going beyond the red, where there is no light at all — what would he find there?
Journey into the unseen
William Herschel’s initial journey into the unseen universe extended all of four inches across his tabletop apparatus — but they were four very significant inches. In his updated experiment, he relocated his thermometers to the seemingly dark zone past the red edge of the rainbow that beamed from his prism. There, to his amazement and delight, the instruments continued to register heat, even as his eyes registered nothing.
When Herschel took the readings from his thermometers, he determined that the warming caused by the Sun’s rays not only continued beyond the red zone, but the effect actually intensified as he ventured deeper into the darkness. He had to move his thermometers far outside the visible spectrum before the warming effect peaked and then tappered off. Clearly he was measuring a rising and falling distribution of something that existed beyond the human senses — as if he were running his hands over the arched back of an invisible cat.
To be methodical, Herschel also poked around at the other end of the spectrum, above the violet edge. There he found no warming effect at all, confirming his inference: He had discovered a new type of ray from the Sun that is undetectable to the eye, warmer than visible light, and (illogical as it sounds) redder than red.
On April 24, 1800, Herschel relayed the results of his investigations to his society colleagues.
Perhaps I am just projecting, but as I read the yellowed pages of the 224-year-old volumes of Philosophical Transactions of the Royal Society, Herschel seems to be sputtering with excitement. Normally he confined himself to sober notations of his empirical observations and shied away from extravagant speculations. In this case, though, he couldn’t contain his outpouring of ideas.
In his two presentations to the Royal Society, Herschel introduced an entire new scientific vocabulary. One March 27, he coined the now-familiar term “radiant heat.” A month later, he distinguished between the visible colors that he could observe and the invisible “colors” that he could only feel based on their warmth, calling the former the “prismatic spectrum” and the latter the “thermometrical spectrum.”
Most perceptively, Herschel realized that the two types of rays fundamentally have one and the same nature, rejecting the existence of “two different causes to explain certain effect, if they may be accounted for my one” (the good old principle of parsimony). The difference between the two types of rays, he deduced, was simply that the human eye could perceive one kind but not the other.
Herschel concluded that “radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light.”
Invisible light! What a concept, especially for a scientific explorer who had spent much of his life glued to the eyepiece of his telescopes, relying on his human vision to tell him about the nature of the universe.
The invention of the telescope two centuries earlier had proven that there are objects too faint and distant to be visible to the unaided human eye — but when their light is concentrated, they pop into view. The invention of the microscope likewise had demonstrated the existence of objects too small to be seen — but when they are magnified, they, too, pop into view. Herschel’s radiant heat was something altogether more shocking. It indicated that some unknown portion of reality is invisible to us not because it is lacking in quantity (brightness, size) but in quality.
A scanner, darkly
Through the remainder of 1800, Herschel kept returning to this fundamental puzzle. Were his rays of radiant heat truly invisible, or were they merely very, very faint? He tried increasing their intensity and found it made no difference. The heat rays had, in Herschel’s words, “such a momentum as to be unfit for vision.” In his culminating experiment, presented to the Royal Society on November 6, 1800, he produced a “spectrum of heat.” It was, in essence, a line drawing of his invisible cat.
Herschel even offered what we, from our modern perspective, would call an evolutionary explanation of why much of the universe is invisible to us: “[A]dmitting, as is highly probable, that the organs of sight are only adapted to receive impressions from particles of a certain momentum, it explains why the maximum of illumination should be in the middle of the refrangible rays; as those which have greater or less momenta are likely to become equally unfit for the impression of sight.” We see only what we need to see.
Implicit in Herschel’s discovery was another, even more radical idea. His rays of radiant heat could be felt but not seen. What if there were other rays that could neither be felt nor seen?
The answer arrived swiftly. Inspired by Herschel’s work, the German chemist Johann Wilhelm Ritter set out in search of invisible rays at the other end of the spectrum, beyond violet light. Just one year after the Herschel experiments, Ritter reported success. He discovered that the compound silver chloride, which turned black in reaction to sunlight, would continue to transform when placed in the dark zone past the violet edge of the spectrum (into ultraviolet radiation, in modern terms). Ritter attributed this effect to “chemical rays” that were not only invisible, they were completely undetectable to the human senses.
And then — the story stopped, or at least it paused for quite a while. After 1800, Herschel gave up on his explorations into what we now call the infrared and returned to his primary telescopic explorations of the cosmos. His foray into the invisible realm was practically limited by the simple thermometers available to him, which could do little more than demonstrate the reality of radiant heat. He was limited, too, by the theory of the time: In the early 19th century, most scientists still thought of light as a shower of particles, and heat as a movable fluid called “caloric.”
Ritter moved on to other topics as well, and made little progress in understanding the identity of his chemical rays before his early death in 1810. It took another 60 years before James Clerk Maxwell developed a unified theory of visible light, infrared, and ultraviolet as different forms of electromagnetic radiation. Then it took decades more for scientists like Wilhelm Röntgen and Heinrich Hertz to show the true grandeur of the invisible spectrum, which stretches a trillion times wider than the spectrum we see.
Still, Herschel’s work set all of that in motion. So why don’t we celebrate the first sighting of the invisible universe, an actual “new world” as opposed to the sighting of lands already inhabited and well known?
Unlike the westward voyages of Columbus, Herschel’s discovery of radiant heat had no immediate, obvious practical implications. It delivered no gold to the treasury; it conferred no political power; it opened no lands to colonial settlement. All it did was expand the range of human consciousness.
Such fundamental discoveries typically have enormous payoffs in the long run. In the short term, though, they tend to get drowned out by more immediate dramas of human affairs. Popular appreciation may arrive only later. Or, in some cases, it may never arrive at all.
Note: If you are interested in recreating William Herschel’s experiment for yourself, you can do it in an afternoon, with about $10 worth of materials. Full instructions here [PDF link].