As if to downplay the risk of becoming too grown-up here in my thirties, I unexpectedly ended up spending the last few days playing with glow-in-the-dark powder. As I did, I found a range of old memories and impressions washing over me, and I also learned one or two new things.
There is a special wonder and strangeness about luminescence. Things that glow are usually linked in the imagination with uncanny hidden powers, whether mythological or scientific. In the former case I think of magical swords in the Tolkien universe that glow when orcs are near, or hidden cities of elves; the glow of ghosts and angel’s halos, and the disturbing augurs long ago linked with comets, aurorae, and St. Elmo’s fire (and still, in some parts of the world).
In the latter case, I picture the Curies in their little laboratory, filling vials with luminous radium, knowing they were face to face with some tremendous new power but tragically unaware of what it could ultimately do to them. I think of Roentgen noticing the unexpected glow of a fluorescent sheet on the other side of the room whenever his cathode ray equipment was switched on, and wondering what the nature of these strange new “X-rays” could be.
I remember first getting hold of those cheap chemical glow sticks and glow necklaces when I was a kid, my total amazement when I first saw them being passed out by a vendor at the state fair some still summer night many years ago. With their pure sharp colors so unlike anything in nature, they pierced the night, and seemed to me not like plastic do-dads filled with cheap reagents, but magic-steeped fragments of another world. Kids who got them were visitors to that world, and to be immediately envied. When I finally got hold of one it was rapture; when the reactants ran out after a few hours and the like dimmed away I felt a unique disappointment. But I soon consoled myself with the discovery that the glow could be saved for weeks by just putting the glow sticks in the freezer. Warmed back up in my hands after its long sleep, the glow would return for me to admire, before I carefully put it back in its suspended animation.
I soon learned about radioactivity and the long-time uses of radium salts in watch dials–the elusive, evanescent element lingering in tiny traces in uranium ore, itself an incredibly exotic substance to me. Knowing what radium had done to the Curies and the girls who painted those pretty luminous dials almost increased my awe, for it showed this was not just a matter of glowing salt but something deeper, something to be feared, with the power to burn and destroy any impudent mortals who dared come too close. The atom was an Aladdin’s lamp concealing a terrifying genie, whom we could only hope would not come raging out at us.
This was, of course, not much of a deterrent. I finally got hold of an honest-to-goodness glowingly radioactive thing in high school. It was a tritium powered keychain–no radium at all. Instead of the deadly energetic bazooka explosions of alpha radiation given off by radium, the tritium gave off beta particles, near-weightless free-flying electrons–and ones so weak they couldn’t even get through a sheet of paper. But the keychain’s soft blue glow perfectly jibed with my amazement at the idea of a light that could go on and on for decades without the slightest recharging. In this incredibly small but incontestable way, I could carry a tiny nuclear-powered energy source around in my pocket.
The peak of my nuclear-themed interest in luminescence came when I figured out how to create my own makeshift spinthariscope. In the early freewheeling years where radioactivity was a breathtakingly novel phenomenon, a kind of emblem for the wonders of the new scientific-technological age, spinthariscopes were a popular party toy: a tiny speck of radium was mounted on a needle, held at the right distance in front of a luminous screen, and then curious party-goers would gaze through a lens at the screen.
It’s hard to imagine parties quite that nerdy ever being the norm. But what they saw through that lens was, if you were in the right frame of mind to appreciate it, utterly breathtaking: a blizzard of tiny flashes of light, each flash marking the place where an alpha particle shot from a disintegrating radium had crashed into the screen at 5% the speed of light, creating a tiny storm of atomic excitement so intense you could see it with the naked eye. Here, luminosity was not just the general marker of radioactivity’s presence, but marked the specific decay of a single atom.
I had to see this. I make-shifted my own spinthariscope by busting open an old smoke detector to get at the americium (smoke detectors use a tiny piece of radioactive material to makes the nearby air conductive; if smoke gets in, the air the conductivity drops and the alarm goes off). Then I broke open a tiny old black-and-white TV and took a chunk of the screen, whose inside surface had its special power of glowing when struck by radiation of all sorts. Holding the pieces together in a tiny closet with my bare hands, the fabled blizzard of light came suddenly into view on the screen before my own eyes. I was watching individual atoms explode–on television at that. For sheer scientific gee-whiz moments, I don’t know if I’ve ever topped it.
For some reason, I didn’t give much thought to luminescence for years after that–maybe I thought that last experience was impossible to top. I worked in laboratories with all kinds of fluorescent dyes and markers, but it all seemed unremarkable, too systematic, like a mere part of the workaday grind. It’s funny how some circumstances inspire wonder right away and some simply don’t, even after years of chances.
But a few days ago I found some old glow-in-the dark powder in a drawer and felt myself inexplicably intrigued by it. A few minutes in the sun or under a blacklight, and it would glow a sickly yellow-green for several minutes in the dark. It was like a sponge for light, I thought. What a strange idea–that a substance could somehow grab hold of light, hold it in an inert form, and then gradually set it free again!
As usual when I’m hit by these kinds of whirlwinds of curiosity about some sciency thing or other, my instinct was to go online and try to buy that thing cheap, which I promptly did. I quickly found that the old stuff I had was an antiquated kind of glow-in-the-dark powder–zinc sulfide with copper added–and that there was now a substance that could glow ten times as brightly. Apparently a lot had happened in the world of luminescence during my post-spinthariscope years of neglect; there was a bit of catching up to do.
It turns out the story goes back pretty far. The first known discoverer of phosphorescence was an Italian cobbler and alchemist, Vincenzo Casciarolo. In 1602, Casciarolo was exploring around a volcanic deposit on Mt. Paderno, near Bologna, when he discovered pieces of a whitish stone which he decided to collect and bring home to his lab. It’s not clear just what particularly interested him about this stone, but with the search for the philosopher’s stone very much a going concern in those days, he likely hoped this sample would hold the ticket to alchemical superstardom–and riches beyond belief.
Though Casciarolo failed to turn lead into gold, he found something almost as interesting–after calcining (roasting) the newfound rocks, they gained the ability to, as accounts of the time described it, “attract the golden light of the sun”. Placed in a sunny spot for a few minutes, the rocks would glow with a golden hue in darkness for a number of hours. Casciarolo’s discovery drew the wonderment and admiration of other alchemists, made him widely known, and marked the beginning of the science of luminescence.
It was to prove a slow beginning indeed. The white stone Casciarolo found is now recognized as barite–barium sulfate–and the “Bologna stone” he produced by roasting it was barium sulfide, with a trace of copper impurity that imparts the awesome power of phosphorescence. However, for nearly 400 years, almost no new substances were found to have this weird property. Zinc sulfide, activated with impurities of copper or silver, was known through most of the 20th century to be about the best phosphorescent material going in terms of length of glow, but it still wasn’t very bright, and would fade in just a few minutes–all in all, not much better than poor Casciarolo had been able to achieve.
Finally, in 1996, there came one of those great yet relatively unsung breakthroughs in materials science, rather like the invention of neodymium magnets or even the advent of high-temperature superconductors. A group of material scientists in Tokyo had been experimenting with adding (doping) different kinds of rare-earth elements into a phosphor called europium-doped strontium aluminate. This phosphor had been known about for decades, but had an even shorter glow time than zinc sulfide. It was only of academic interest. But the researchers found that after adding a small additional amount of the rare-earth dysprosium, this boring old phosphor underwent a remarkable transformation–when fully charged, it could now store light for a day or longer, all while glowing with ten times the brightness of older phosphors. It became, to use a very non-technical term, a super-duper-phosphor.
That brings us back to the present. Like so many obscure yet groundbreaking innovations, the advent of this new phosphor quickly and quietly revolutionized the production of luminescent products–toys of course, but also watches, artwork, exit signs, and other kinds of emergency lighting. When I started looking online, it wasn’t at all hard to find strontium aluminate phosphor–in fact it has become common and cheap, replacing zinc sulfide almost everywhere. I ordered a small amount of the coarsest-grained version–supposed to be the brightest and longest-glowing–and it arrived promptly.
When I took out the small pouch, it looked completely unimpressive, just a small bag of off-white colored sand. I set it underneath a bright fluorescent light and left it alone, vaguely unimpressed. After ten minutes I came back, turned off the light, and was given a rare treat. The strontium aluminate was blazingly bright, literally throwing shadows across the room, like a strong night-light. The glow was a remarkable light blue, almost exactly like the tritium light I’d first been amazed by all those years ago. I had seen glowing stuff before, but never so intensely. Even half an hour later, it was still dazzling, still throwing shadows across the room. Here again after so long was one of those rare moments where wonder alights and you feel presence of a kind of magic–and then I was back in time, remembering my strange childhood fascination with glowing stuff.
Here’s how luminescence works. First, there are two main kinds: fluorescence and phosphorescence. Lots of common things, for example hi-liter ink and the fabric brighteners in T-shirts, will fluoresce; they glow when a light of another color shines on them. But fluorescent substances don’t store any light and instantly go dark again when the other light is taken away.
Phosphorescence, on the other hand, is when the absorbed light gets stored up instead of immediately released. This trick depends on quantum mechanics in action. With fluorescence, the luminescent substances can absorb light, boosting their electrons to a high-energy state. But the high-energy and resting (“ground”) states have the same angular momentum, so it is easy for the electrons to fall back down almost at once. In phosphorescence, on the other hand, the electrons then become “trapped” in a high energy state that is much more stable, because it has a different angular momentum or “spin” than the ground state.
Angular momentum, along with energy and electric charge, is one of those fundamental entities that is conserved in our universe–it can’t be created out of nothing or destroyed. This is why the re-emission of light in a phosphor is so slow. But quantum mechanics allows some wiggle room: over time, even “forbidden” events like a spontaneous change in angular momentum can take place, given enough time, and if they are small enough. This is why the light sponged up by a phosphor is not trapped there forever.
The process of making the forbidden jump depends on heat energy, in the form of tiny vibrations in the material, so phosphorescence is strongly dependent on temperature. Indeed, like the chemical glow sticks of my old memories, I found that strontium aluminate phosphor is very sensitive to temperature. Just sticking it in the freezer will kill the glow, but it starts right back up when warmed in the hand (an interesting way of visualizing heat flow).
This is a roughly correct explanation of how phosphorescence itself works, but note that it tells us nothing about why, of all things, strontium aluminate with europium and dysprosium added makes for such a spectacularly bright and long-lived way of storing light energy.
In fact, there is as of this writing no universally accepted fundamental explanation of this phenomenon.
Like so many scientific wonders we now enjoy, the strontium aluminate super-phosphor was not designed or predicted in any way from theory, but was the result of luck and inspired guesswork. In their paper announcing its discovery, its own inventors did propose a mechanism for how it stores light so much efficiently than anything else out there–but they came up with it only after they knew about the remarkable phosphorescent properties.
In the years since then, that first mechanism has been mostly discredited, and one new theory after another has been put forward to replace it, none with complete success. In a very real way, the ongoing attempt to understand the mechanism behind this new phosphor technology is like a microcosm of how scientific theories develop–not by the awesome power of deductive insight, but in a bootstrapping, post-hoc fashion, one experimental defeat after another.
This lack of an overarching theory may be pretty typical of whole fields in science, but it still means that progress has been completely hit-or-miss–and mostly miss. For although the super-phosphor has been known for about 20 years now, there is still nothing on the horizon that is much better. The search for brighter or longer-persistence glow materials after strontium aluminate has turned up little.
From another perspective, one can be tempted to take this as a sobering lesson about the limitations of science, and the way that progress in many sciences does not always follow the dependably exhilarating exponential progress that we have gotten so accustomed to from the semiconductor age and Moore’s law (and even Moore’s Law is not looking very healthy these days).
Again the comparison to high-temperature superconductivity and high-strength magnets is apropos: after a heyday in the 1980s, no superconductors have been found that work above liquid-nitrogen temperatures (except at freakishly high pressures). And while the discovery of neodymium magnets, also in the ’80s, created its own quiet revolution in electronics and other technologies, there are serious physics obstacles to creating magnets any stronger. As with the super-glowing aluminates, in both cases we know how these substances work only in a post-hoc, hand-wavy kind of way.
To me the lesson of all this is that we live with the uneasy paradox–we stand upon an unprecedented pinnacle of scientific understanding, yet, to an alarming degree, that pinnacle rests not on reliably cranking out advances like a factory, but rather on a vanishingly small number of game-changing breakthroughs that we cannot repeat or even always fully explain.
Often, overwhelmed by the fire-hose of marvels of our technological age–more of which seem to be about marketing and fashion than true novelty–we fail to appreciate “simple” discoveries that aren’t really so simple after all but that have quietly remade our lives. We should be more appreciative of these for the fantastic one-offs that they very possibly are. To sit in a dark room, childishly transfixed by the riddle of how a bit of dust can store up a day’s worth of light, seems to me as good an homage as any.