Light and dark have always been powerful metaphors for the truth and its counterpart, ignorance. Plato saw light as representing the ultimate truths and the reality we saw nothing more than obfuscating shadows.
But what if light were actually just a tool, like language and agriculture, which we used to help transform our species from bands of hunter-gatherers into a global civilization capable of producing such wonders as New York City?
As I’ve written before: understand the light, and you understand physics. And as Tyson says at the beginning of the episode, “The age and size of the cosmos are written in light.”
This is the story of how we came to see.
It’s a funny thing that light does: shove it through a tiny hole and onto a flat surface and the light will actually recreate the image of whatever’s on the other side of the hole, except upside down. This is an effect called camera obscura, first recorded by the Chinese scholar Mo Tze. Tze, who lived before the first Chinese emperors, was already part of a long tradition of writers and scholars and the show argues he was onto some of the ideas that paved the way for what we call science, including open inquiry and a reluctance to blindly follow ritual.
But soon after Tze, the first emperor, Qin Shi Huangdi came along with his army of terra cotta warriors and laid down a particular kind of law. There was to be no questioning of anything. Adherence to ideas outside of Qin’s worldview was punishable by death. It brought stability of a certain kind, but only in the way many suppressive regimes are. Books by Tze and Confucius were burned; Scholars who tried to preserve them were buried alive.
Science needs the light of free expression to flourish and an open exchange of ideas, Tyson tell us. The Qin, in what was to become an all too familiar pattern, stomped these out.
Consider that a thousand years ago, Iraq was the center of all the highest learning in the world.
Ibn Alhazen, an Islamic scholar in Basra, Iraq, worked in a time when people from Cordoba to Samarkand flocked to the fertile crescent to learn. The caliphs sent people out in search of books, rather than burn them. “Much of the light of ancient Greek science would have been permanently extinguished without their efforts. The reawakening to science that took place in Europe hundreds of years later was kindled by a flame that had been long tended by Islamic scholars and scientists.,” Tyson says. The numbers we use today, our words “algebra” and “algorithm,” as well as the concept of zero all came to us through these Arab scholars. That last idea “comes in handy when you want to write billions and billions,” Tyson says with a twinkle in his eye.
Alhazen first set down the rules of science as a systematic and relentless way to sift out misconceptions in our thinking. Like every great scientist following him, questioned things everyone took for granted, like “how do we see?”
Conventional wisdom at the time said that we were able to see because rays come out of our eyes, travel to things we see, and come back. But Alhazen realized this could be true, as the stars were too far away for this to happen.
Alhazen also discovered that light moves in straight lines, which helps explain the camera obscura. If you have a small opening to restrict light rays, only a few can pass through it. If you’re looking at a horse outside your makeshift camera obscura/tent, the only rays from the horse’s head that can make it through the hole will be travelling at a downward angle. Reverse goes for the feet. In fact, effect only works because it restricts light to travelling in fewer directions, which makes the image sharper.
But the camera obscura only produces visible images in broad daylight. To see images of stars, you’d need larger opening but this puts images out of focus. Enter the telescope, which collects light across the entire lens, but focuses it. When Galileo first took a telescope and pointed it up at the sky, he “pulled aside the heavy curtain of night and began to discover the cosmos.”
This actually led to some problems for Galileo. When comparing Venus and Jupiter with the naked eye, Venus, which is brighter, appeared bigger. But when he looked through a telescope, Jupiter was clearly the larger of the two. It turns out this was due to an optical illusion that was recently explained by modern neuroscience.
“Our urge to trust our senses overpower what our measuring devices tell us about the actual nature of reality,” Tyson explains.
What is light anyway?
Several scientists, whom we’ve already met, made pioneering insights into the nature of light. It was Newton, surprise, surprise, who discovered that white light is actually a mixture of all the colors of the rainbow. He named it a spectrum, from Latin for phantom. And William Herschel was the first to realize that there were kinds of light outside the visible spectrum. Everyone knew sunlight carried heat, but Herschel was astute enough to ask if some colors carry more heat? To test this, he created a spectrum out of sunlight and placed thermometers at different points along it. Being a good scientist, he knew that he needed a control for this experiment, to test what the temperature would be with no direct light hitting it, and chanced to place this control thermometer just beyond the red part of the spectrum.
He found that red is warmer than blue. Interesting, perhaps, but not groundbreaking. But when he looked at his control, he saw it was even warmer than red light. Herschel had discovered the infrared part of the spectrum, a kind of light our eyes are not sensitive to but our skin is. This is just another moment proving that more often than not, science advances because of “huh?” moments rather than “eureka!” moments.
Holes in the spectrum
Our journey to understand the nature of light next takes us to Germany, in the late 1700s. Joseph Fraunhofer was an apprentice to the royal Bavarian mirror maker, having been orphaned at 11. He was basically a slave who spent his days stirring vats of toxic chemicals and his nights performing household chores. Then one day his entire luck changed. The mirror maker’s house collapsed and the Bavarian prince Maximillian came to check out the scene. He took an interest in Joseph and offered Fraunhofer and escape, a position at the optical institute. It’s an amazing thing how human potential often just needs the right conditions to flourish. By 27, Fraunhofer became the leading designer of instrumentation in Europe. His work with glass at Benediktbeuren Abbey became so important it was considered a state secret. But he also did basic research into the nature of light spectra.
To help us understand what’s really going on with light, Tyson takes us to Benediktbeuren, specifically it’s organ room. Light, like sound, can be considered as a wave. Organ pipes have different lengths and it’s the length of pipe determines length of wave inside it. Short waves have high pitch or frequency, longer waves have lower pitch. It’s the distance between adjacent waves that gives the all important measure of wavelength.
Tyson plays a little ditty on the organ before being brushed aside by a more experienced organist, who plays O Fortuna from Carmina Burana, also known as the sample from Nas’s “Hate Me Now.” Fun fact: The texts upon which Carl Orff based his stirring choral masterpiece were found in this very abbey.
By now Tyson has made his analogy. Just as wavelength of sound determines pitch, wavelength of light determines color. But how?
When travels through air or space, all colors move at same speed. But inside a prism, each color moves at different speed. “These changes in speed pry the colors apart,” Tyson says.
Why then, do we see colors in nature? Are there prisms in everything, splitting white light into the colors?
Color is just the result of light wavelengths bouncing off the surface of an object. For example, a blue flower absorbs all the longer red wavelengths but reflects the high energy blue ones. And when it reaches your eye, it’s just a measure of how the eye perceives the light wave’s energy Tyson explains. All the feelings and moods that color can inspire are the result of a particular variation in the frequency of light waves triggering something inside of us. Powerful stuff, this light is.
Back to Fraunhofer now. What he did in his abbey work room was no less than officiate the marriage of physics and astronomy. When he put a magnifying glass in front of a spectrum, he saw that there were tiny black lines tucked into the rainbow. Fraunhofer asked, “why?” and the field of astrophysics was born.
“When Fraunhofer put a prism in front of a telescope, he brought the stars much closer to us,” Tyson says. With his discovery, Fraunhofer would define a new field of study, which would lead to a staggering conclusion. The visible cosmos is all made of the chemical elements; the planets, the stars, you and I are all made of the same star stuff. Examining light spectra is such a powerful tool in astronomy that Tyson can say with supreme confidence, “show me the spectrum of anything and I’ll tell you what it’s made of.”
Why that’s so wouldn’t be explained until the modern physics understood the quantum nature of the atom. Those black lines happen when something absorbs that light wave, the result of electrons doing a quantum dance around the energy levels of atoms.
Let’s take a look at the simplest atom, the hydrogen atom. It has just one proton, but more importantly, it has just one electron. As the electron orbits the nucleus, it jumps randomly between energy levels. “It’s as if you took an elevator from the second floor, to the fourth floor, but ceased to exist in between,” Tyson says. And quantum elevators only stop at certain floors. That’s why the elements are different, chemistry is completely determined by its electron orbits. It has nothing to do with gravity, but electrical attraction.
Electrons have to get energy to leap to a higher orbit and lose energy to move down. Every upward leap means that the atom absorbed a photon. We don’t know why downward leaps happen, but they always produce a light wave corresponding to energy difference between the orbits.
Those dark lines Fraunhofer saw “are the shadows cast by hydrogen atoms in the sun.” As the hydrogen atom absorbs and re-emits light waves, they’re fling them off in random directions, so they don’t continue on the original line of sight headed our way. There is then a relative lack of those wavelengths in the light headed our way, resulting the thin black line of the absorption spectrum. Each element has it’s own absorption spectrum, since their electrons dance to different tunes.
So if a star has a lot of iron or oxygen, say, we can see it in the black lines that make up it’s fingerprint in the spectrum.
Fraunhofer died at only 39, perhaps a result of the exposure to toxic chemicals as a young boy. He transformed Bavaria from a backwater into a technological powerhouse and as he lay dying, the state tried to extract every last bit of knowledge from him. His method for making optical glass remained a state secret for more than a hundred years.
But he wasn’t secretive about his basic science research and a lot of discoveries about the cosmos wouldn’t have happened with out the tool of light spectra.
Using spectra, scientists have revealed not only the composition of stars and galaxies, but also their motion towards or away from us.
Infrared is not the only other type of light outside the visible spectrum. Just next to violet is the high energy and skin-cancer inducing ultraviolet. Even more energetic are x-rays and gamma rays and below infrared lie the microwave and radio wave sections, which are integral to many technologies we use. And we can do astronomy with all of them!
“Confining our perception of nature to visible light is like listening to music in only octave,” Tyson explains. Not only do we get different views of the same things, but something cannot be see unless we look at a particular part of the spectrum, like gamma-ray bursts or radio pulsars. With microwaves, we can see all the way back to the birth of the universe.
To experience the glory of the electromagnetic spectrum, we close the episode with a montage of New York City as “seen” with different kinds of light. Infrared brings out the buildings, while gamma-rays—far too energetic to safely be created in the city—reveal the Manhattan skyline in negative space. Call it a rhapsody in the electromagnetic spectrum.
This has been a fun episode combining history, narrative, physics and even music. My high school band director would have approved of the inclusion of both George Gershwin’s “Rhapsody in Blue” and Orff’s “Carmina Burana.” It was able to work in a soapbox moment about the necessity of free thought and openness to ideas and while heavy on exposition, my hope is that the series can build on this, and explore many of the ways in which light has helped us explain the cosmos. Tyson focused a lot on the wave nature of light, so one can only hope he’ll dive into the quantum nature before it’s all done.
Photo credit: Fraunhofer lines by Yellowcloud via Flicker