If you’ve seen the flash of yellow-orange flames when a pot boils over on a gas stove, you’ve gotten a glimpse of the ghost of an atom, specifically sodium. The color is part of the atom’s spectrum, which shows which types or frequencies of light are absorbed by that particular atom.
In the late 17th century, Isaac Newton used the Latin word for ghost, spectrum, to describe the bands of colors he saw when light shone through a prism. In 1814 Joseph von Fraunhofer noticed he could see bright lines instead of the bands of colors when looking at certain flames through a prism. He went on to develop an instrument to measure these spectral lines, called a spectroscope.
Fraunhofer noticed a series of missing colors, dark lines, when looking at the sun’s light through the spectroscope, and went on to characterize the light from several stars as well. Fifty years later Jesuit polymath Angelo Secchi invented a series of spectroscopic instruments specifically for examining the patterns of colors in the light from stars and the sun and used them to build a catalog of more than 4000 stars. Secchi classified the stars by recurring patterns in the light, which were a clue to the star’s composition.
Around the same time Secchi was building his catalog of stellar spectra, Gustav Kirchhoff and Robert Bunsen (the inventor of the ubiquitous Bunsen burner) were involved in a more down-to-earth scheme. Kirchhoff and Bunsen teamed up to create a spectroscope that used Bunsen’s new hotter, gas burner to ignite samples. They noted that that when they combusted a pure element it produced a characteristic set of lines, a spectral fingerprint, that could be used to identify it.
In October of 1860, Kirchhoff and Bunsen announced they had used their spectroscope to discover a new chemical element, which they named cesium, for the blue color of its principal line. Chemists quickly began to use Bunsen’s spectroscope to find new elements. A few months later Kirchhoff and Bunsen found two bright ruby red lines in an extract of a silicate mineral lepidolite, the spectral traces of another new element, rubidium.
Thallium’s ghostly green emanations were first observed by William Crookes, indium, ironically named for its violet lines by its color blind discoverer Ferdinand Reich. Paul-Émile Lecoq de Boisbaudran spectroscopically painstakingly identified element 66 in a sample extracted from his marble hearth, and instead of naming it for the colors of the lines, called it dysprosium, from the Greek for “hard to get” — because it was.
Hunting for new elements spectroscopically meant you didn’t actually need to have any of it in your lab or even on your planet, as long as you could observe the light from a burning sample. In 1868 several chemists and astronomers independently observed a faint line in the spectrum of the sun, and assigned it to a new element, helium, which as far as they knew did not exist on earth. It would take nearly 30 years for two Swedish chemists to confirm that it was present on earth — by matching the spectrum with that of a gas found in a uranium ore. (All the helium found on earth comes from radioactive decay.)
These ghostly lines produced by elements helped fuel yet another critical discovery that would have far reaching consequences for chemists’ understanding of the periodic table: quantum mechanics. Niels Bohr’s quantum mechanical model of the atom opened the door to explaining the line spectra of chemical elements. Though more accurate and sophisticated quantum mechanical models of the atom now exist, Bohr’s model showed the relationship between the lines and an atom’s electron by insisting that the electrons’ energies were quantized, that is, they could only have certain energies.
So why do atoms have ghosts? When an atom is heated to high temperatures, as in a flame or a star, the energy it absorbs excites its electrons. You can think of the electrons in an atom as being on an energy ladder. (this isn’t quite correct as far as the quantum mechanics goes, but it is a reasonable approximation and easier to visualize.) They can only have energies that match the rungs of the ladder, and each type of atom has a unique arrangement of the rungs.
When an atom absorbs energy, its electrons move to higher rungs. Excited electrons are unstable. They quickly return to their original arrangement, giving off some their excess energy in the form of light as they fall back to their original rung. The color (the wavelength) of the light emitted depends on the difference in energy between the rungs. The colors of light emitted are the ghosts of the energy rungs. Since each element has a unique pattern of rungs, it will have a unique spectrum of emitted light and so revealing their presence to the sharp eyes of spectroscopists.
The spectra that Secchi so carefully observed (and hand drew!) were not just a way to identify a particular star, but clues to its chemical composition and even more critically to its evolution. Chemists and astrophysicists still use the light emitted and absorbed by atoms and molecules to identify their presence. We hunt for the structure of the universe in its ghosts.
If you want a way to see the ghosts of atoms for yourself, try this inexpensive DIY folding spectroscope you can attach to your phone. Use it to check out the light from a neon sign or from a street light!
For a wonderful description of the elements, including stories of how they were first discovered, read John Emsley’s Nature’s Building Blocks.
This post is a version of an essay written for a collection commissioned for the UN’s International Year of Light in 2015, and is cross posted at Quantum Theology.
[A version of this appeared at CatholicPhilly.com in July 2010.]
On Friday, I walked onto the driveway to call the cat in for the night, peering under her favorite bushes, warmed by the brick wall of the house. I happened to look up. There, on a velvety deep cobalt sky, perfectly framed between two towering trees across the street, was a brilliant Venus and a slim crescent of the moon. The sight took my breath away, so much so that after a moment, I went back inside and pulled my husband out of bed to come see.
As I sat on the driveway, watching the moon inexorably slip away, I remembered an evening sitting on a bench outside the Jesuit retreat house on Eastern Point in Massachusetts. The bench faced east, not west, so instead of watching the fiery exuberant swirls of red and gold over Gloucester bay, my view was one of an impending darkness extending further than I could see. The blue of the clear sky slowly deepened, the stars came into focus.
The sense of being intentionally and inexorably turned away from the sun to look into the depths of the universe was strong. It was as I could feel earth spinning in space, the planet’s face — and ours — turned once a day toward the vastness of space. We look out into the universe each night whether we wish it or not.
It reminded me of my youngest child. When he was a toddler and wanted to be certain of my full attention he would take my face in both his hands and turn it toward his. Does God turn us to face the immense once each day so that we will have to look into the depths of eternity? Does he want to be assured of my full attention? What does He want me to see? to hear?
Suddenly the sunset felt like a distractingly noisy party, the rise of the night, like slipping out the door into a quiet and still street, where you can hear the cicadas and the breeze in the trees. I thought of a line from Robert Alter’s starkly powerful translation of the psalms: By day the Lord ordains His kindness and by night His song is with me — prayer to the God of my life. [Ps 42:9] In his commentary Alter notes that while this verset is often read as our response to God for His goodness, our song to Him in the night, the Hebrew implies that God also sings to us in the night. What do I hear in the quiet streets of night, where God sings to me?
In the stars whose light is so strong we can see it at inconceivable distances, I hear of my powerlessness and God’s strength. No light that I can kindle will ever compare. There is a taste of eternity in these photons first sent streaking across the universe years ago that touch me only now. As it was in the beginning, is now and ever now shall be. I feel God’s timeless hands on my face. God seeking me out.
I hear, too, an invitation to dig deeper into the tangible universe that presents itself, to think about everything from the Venusian atmosphere to the optic nerve that insinuates itself into my brain to the evolution of the stars I can barely glimpse on this overlit urban street. I hear God hoping I will find these mysteries drawing me deeper into the ultimate Mystery.
The science doesn’t impede or distract. There is more than one way to understand the universe. As Pope Francis put it last week in an audience with the young astronomers from the Vatican Observatory Summer School
“To know the universe, at least in part; to know what we know and what we don’t know, and how we can go about learning more; this is the task of the scientist. There is another way of seeing things, that of metaphysics, which acknowledges the First Cause of everything, hidden from tools of measurement. Then there is still another way of seeing things, through the eyes of faith, which accepts God’s self-disclosure. Harmonizing these different levels of knowledge leads us to understanding, and understanding – we hope – will make us open to wisdom.”
In these warm summer days (or cold winter nights depending on your latitude!) look up at the stars, out into the vastness of the universe. What is God singing of to you?
When I ran across the paper that gave the title to this post, it seemed as if it would make a wonderful title to a poem, perhaps one by Marilyn Nelson or Billy Collins, about what is hidden in plain sight - the airglow, and some of the early women in science.
The moon and stars are not the only lights in the night sky, the very envelope of air that surrounds us glows, day or night. One of the sources of the earth’s airglow is a 3 mile deep layer of sodium atoms about 50 miles above the earth’s surface.
There are roughly 8000 sodium atoms in a milliliter of air up there. In fact, there’s not much air at all, the pressure is about a millionth of what it is as sea level. Even so, only 4 in a billion of the atoms in that layer of the atmosphere are sodium. When these sodium atoms get energetically excited, they can relax back to their original state by releasing some energy in the form of light.
Quantum mechanics predicts the light won’t be white light (that is light comprising many different colors or frequencies) but sharp lines, single frequencies. The marked lines on the top in the photo are reference lines from a flame test of sodium (Na) and lithium (Li) solutions. (If you’ve ever splashed salty water onto a gas burner, you’ve seen these spectral lines for sodium, the bright orange-yellow flash.) The line at 5892 Angstroms is the yellow sodium D-line (actually two lines that are so close together they aren’t resolved in this photo), the green line at 5577 is from excited oxygen atoms. Long before anyone reached these altitudes, these ghostly lines could tell us something of the elemental make up of the atmosphere.
Where did these traces of sodium come from? Before I can tell you that story, I have to tell you another one.
My interest in the sodium atoms can be traced back to a photo from a 1938 conference posted to Twitter last Christmas by @curiouswavefn, trying to identify the one woman attendee. Just like the more recent story of @mycandacejean's search for the identity of an unknown woman in a conference photograph (see the New York Times) Twitter was able to identify the woman. She turned out to be (not Marie Curie or Mrs. Einstein as some tweeters suggested) but Dr. Carol Anger Rieke, a Harvard trained astrophysicist.
Not long after that photograph was taken, Rieke would move to the University of Chicago, where she would do foundational work in my field of computational chemistry (with Robert Mullikan, who would win the Nobel Prize in chemistry in 1966). But while there she continued to contribute to the astrophysics literature with James Franck (another Nobelist, who had just moved to Chicago from Germany) with this short communication: A Note on the Explanation of the D-lines in the Spectrum of the Night Sky.
Rieke and Franck write that the source of the sodium atoms in these recently (1929) identified lines was thought to be salt kicked up into the atmosphere by sea spray. We now know that these high-altitude metals are have an extraterrestrial origin, deposited there when meteors ablate, burn off. Small amounts of sodium are found in ordinary chondrites, something less than one percent. As metals go, sodium evaporates pretty easily, and so is released high in the atmosphere.
In tracking down the source of the atmospheric sodium, I ran across another paper, which uses methodology I developed as part of my doctoral work, that can be fundamentally traced back to Rieke’s work with Mulliken. It felt a bit like coming full circle for both me and for Carol Rieke.
More about Rieke and two other remarkable women of the atomic era who were not Marie Curie — or either of the two Mrs. Einsteins — can be found in an essay I wrote, Atomic Women, which appeared this month in Nature Chemistry.
In the beginning was the Word,
and the Word was with God,
and the Word was God.
All things came to be through him,
and without him nothing came to be.
What came to be through him was life, and this life was the light of the human race;
the light shines in the darkness, and the darkness has not overcome it. John 1:1,3-5
“And by light you mean photons, right?” asks the student in the first row. “Yes, I do.” At least in this context. There is always a bit of irony in these last classes of the semester. I’m lecturing about light — about lasers and spectroscopy, photons and selection rules — as the winter darkness grows deeper. Or maybe it’s not such an unreasonable topic for these solstice-ing days.
As I packed up to return to my office, the lines from the Prologue to St. John’s Gospel ran through my head, “the light shines in the darkness and the darkness has not overcome it.” Chemists see light as active. It doesn’t just illuminate, driving away the darkness, it can fundamentally change what it touches. One molecule becomes another, electrons shift allegiances, marching ‘round materials like armies of stars. Yet more wonderfully, once the light has soaked in, it can shine forth again, in new ways and new directions: fluorescence and phosphorescence appear at different wavelengths from the exciting light. And from these depths, the light speaks to us, telling us what was and perhaps, what will be. What is the shape of this molecule? By what pathways can it change?
So, too, has the Light shone in the darkness and we are fundamentally changed: But to those who did accept him he gave power to become children of God. Moreover, the Light is the Word, speaking of what has been, calling us to what we can perhaps be: beacons of light. We have been kindled, we hear in St. Matthew’s Gospel, not to be hidden under a bowl, or within the walls of our parish churches, but to shine forth, banishing the darkness around us, fundamentally changed and empowered ourselves to change what is around us.
Reflecting on these first lines from John in his “City of God,” St. Augustine tells of St. Simplician, a late fourth century bishop of Milan, who told Augustine of a pagan scholar who thought the opening lines to John’s Gospel “should be written in letters of gold and hung up in all the churches in the most conspicuous place.” This is where our faith begins. In the darkness, yearning for light, for life and for God to come among us, calling our names.
As Advent moves more deeply into the darkness, I imagine John’s words, written in letters of gold, shimmering on the walls of dark churches everywhere, dancing in the dimness of our souls. And the Word became flesh and made his dwelling among us, and we saw his glory … full of grace and truth.
I look for the Light dwelling among us, praying that it might change me; that I, too, might be aflame with the Word, filled with grace. A beacon of light to the world. To those who did accept him he gave power to become children of God...
And the Word became flesh
and made his dwelling among us,
and we saw his glory,
the glory as of the Father’s only Son,
full of grace and truth.
[A version of this reflection appeared at CatholicPhilly.com 7 December 2016.]
Not quite two months ago I spent a late morning and early afternoon watching the moon slide across the sun, turning midday Philadelphia into twilight and back again. I stashed the eclipse filters for the occasional look at the sun, and dove into the semester. But each time I head out for a late evening walk and see the moon hanging over the neighborhood school's field, I think about it coming between the earth and the sun.
I tend to think of the moon and sun as large objects ponderously processing through space, from my perspective taking ten or a dozen hours to creak 'round the sky. Their movements mark out days, months and years, not so much minutes and seconds. So I was struck on the animations of the August 2017 eclipse by how fast the moon's shadow moved across the ground, even when you account for the acceleration (in this video slightly more than a factor of 13). With family in California, I've flown coast to coast more time than I can count. It takes me 5 to 6 hours to fly from here to there, soaring through the sky at three-quarters the speed of sound. The umbra — the shadow — took only 90 minutes to make the same trip, traveling at more than 1200 mph.
As I walked yesterday afternoon, watching the sun vanish behind the horizon as my spot on the earth rotated to face away from the sun, it occurred to me that the moon's shadow isn't the only thing moving fast. When standing "still" on earth I am, of course, in motion relative to other points in the universe. Points on the surface of the earth (at my latitude, 40oN) are moving at 750 mph. Fast indeed, but not so fast I cannot imagine it.
In this moment in history, where I can climb on a plane and be on the other side of the world in half a day, or video chat with my kids who are thousands of miles away or I can go to a lab downstairs and with a quantum mechanical trick, nudge atoms around, arranging them to suit me, I might be tempted to think of myself as commanding great powers. At least until I think about how fast the earth is moving around the sun. 67,000 mph hour. The solar system? Orbiting the galactic center at a half million miles per hour. I am moving through space at speed I cannot truly fathom: a thousand feet flash by in a millisecond, a hundred thousand in a second. Eighty thousand miles in a minute.
Lines from Psalm 29 came to mind:
The Lord's voice resounding on the waters,
The Lord on the immensity of waters;
The voice of the Lord, full of power,
The voice of the Lord, full of splendor.
The Lord on the immensity of waters, the Lord on the immensity of space. Adore the Lord in his holy court.
The psalm ends with an assurance that God, whose strength we cannot fathom, who with a word can strip the forests bare, and spin a universe into being, will grant us peace. I can think of nothing else we need more now than this. Peace and God's unimaginable strength to sustain and protect us on this tiny world hurtling through space.
I have had Jon Larsen's In Search of Stardust on my stack of books to read because last spring the upper division research methods course I taught did an experiment to measure the heat capacities of meteorites, using the method developed by the Vatican Observatory's Guy Consolmagno, SJ and Bob Macke, SJ and colleagues. The students were curious about the astrochemistry context (where do the samples come from, how can you distinguish regular rocks from these stony aliens) and I've been collecting resources for this coming spring when a new batch of students will make these measurements.
I tend to think of meteor strikes as spectacular and rare events, fireballs roaring through the sky that finally come crashing to earth. Still they aren't as rare was you might think — tens of thousands of meteorites weighing as much or more than a euro coin hit the earth's surface each year, most of them landing in the water. It gives me a visceral sense of how big the earth is! But what takes my breath away are the hundreds of trillions of micrometeorites that come to rest on earth each year, adding as much as 100,000 metric tons to the earth's mass. Invisible, unremarked. Perhaps as many as one a day hits the roof of my house, there are surely some of these ancient bits of dust in the water I drink, still others stuck to my hands after weeding the garden.
Larsen, a Norwegian jazz musician and visual artist, discovered that you could find and identify these micrometeorites by looking at the dust on urban roofs. Previously it had been thought it wouldn't be possible to find or identify these microscopic objects from outer space amidst the general detritus of a city. But Larsen's careful eye is rewarded, as these cosmic intruders have a characteristic morphology. Their unique appearance means you can sort them out under a microscope, much like Pasteur manually sorted the crystals of tartaric acid. And they are astonishingly beautiful.
Larsen offers a brief and readable glimpse into the science of micrometeorites, but I also enjoyed simply browsing the images, reading them as I might clouds. There is a golden glass meteorite with deep blue inclusions (p 51) that looks like some alien aquatic creature's shell, while the burnished cryptocrystalline specimen on page 45 looks like a bronzed wasp's nest — until one remembers it is only a few tenths of a millimeter across.
As much as I learned about the dust from outer space, Larsen's compendium of the terrestrial imposters (it's important to know what you aren't looking for!) gave me an entirely different view of road dust, containing polished spheres of glass from the reflective markings on the roads and crystals, microgemstones.
In one of the most beautiful passages in Isaiah (Is 54:11-12) we hear, "I lay your pavements in carnelians..." Who knew it was literally true. The world is a beautiful place, if only we know where and how to look.
In Search of Stardust: Amazing Micrometeorites and Their Terrestrial Imposters by Jon Larsen (Voyageur Press, 2017)
Guy J. Consolmagno, Martha W. Schaefer, Bradley E. Schaefer, Daniel T. Britt, Robert J. Macke, Michael C. Nolan, Ellen S. Howell, "The measurement of meteorite heat capacity at low temperatures using liquid nitrogen vaporization" Planetary and Space Science, 87 (2013) 146-156
"Aunt Chel," called my youngest niece as she bounded through the front door of my dad's house, "it looks funny outside."
I got up and went to check. I agreed, something was off. The sky was dimmer than it should be and an odd color, not the desert blue I expected late on a Sunday afternoon, but tinged green. Thunderstorm incoming? No, not a cloud in the sky. And I'm in the desert. Right. Fire? This is more of a worry, there is only one road out from my dad's small farm. We don't smell smoke, but still, I'm uneasy. And then there are the trees....something is just not right.
We go back inside to check if there is anything on the Cal Fire site about nearby fires. My dad and sister-in-law have worried looks on their faces as I describe the sky, will we need to evacuate? As I'm opening up my laptop , my stepmother mentions in passing that she'd heard something about an eclipse coming next month. Next month? "Or perhaps today?" I wonder aloud. I hadn't heard anything, but I live on the other side of the continent, and I'd been on retreat for the last week, staying in a hermitage in a spot even more remote than my dad's farm, and before that, spinning around in the end of semester chaos.
I type "eclipse" into the search box. We are indeed in the middle of an annular eclipse of the sun, the moon's shadow will sweep over California, but not reach the East Coast. 80% of the sun's disc will be obscured by the sun at the peak. This is not an insignificant loss of light, enough for my 9 year old niece to have noticed immediately when she went outside.
I breathe a sigh of relief, and take my niece and nephew out to show them how to observe the eclipse by making pinhole cameras with sheets of paper, and by looking at the crescent shadows on the ground (the leaves on the trees serve as ad hoc pinholes, or you can make your own grid with your fingers).
Fast forward five years. I know there is an eclipse tomorrow. The reports on the radio, TV spots, news reports are hard to ignore. I am prepared. I have glasses to watch with, and a pair of binoculars with the appropriate filters on them.I have a good sense of what the sky will look like; outside Philadelphia, where I live the sun will be just under 80% obscured.
But I wonder if being so prepared will change the experience. Will it be as viscerally disturbing, or just a fun science-in-the-neighborhood day, much like the Wallops' rocket launches we gather at the school field to watch? What do I miss when I am not sitting uneasily on the edge of uncertainty?
The mathematics and science that let us predict eclipses, not only their time and track, but also the phenomena we ought to observe, take my breath away, but I confess I don't long for a universe that I can completely predict. It reminds me of a line from one of Alice Walker's poems (Before you knew you owned it): “Live frugally on surprise.” Surprise is part of the delight of doing science, the interesting questions for me come when molecules surprise me, in their structures or or in their behavior.
Similarly, my heart and soul are not captured by an utterly predictable God, a clockwork deity. I long to be surprised by mercy, ambushed by God, caught in a whirl of life and love beyond my comprehension, just as I was caught by surprise by that eclipse.