Light


LIGHT

for the JSC Astronomical Society at the Lunar and Planetary Institute,
7:30 PM, Friday, September 8, 2000
(An earlier version of this talk was given at the Retinal Research
Foundation Luncheon, River Oaks Country Club, Noon, Wednesday, November 3, 1999.)

by John H. Lienhard
Mechanical Engineering Department
University of Houston
Houston, TX 77204-4792
jhl@uh.edu


I doubt that anything can be as central to your work as astronomers as is light. At the same time, can anything be as ephemeral? So let's play with light as an idea. What is light that activates the retina and imprints the world out there upon our brains?

I guess we can agree that it's electromagnetic radiation in a range of wavelengths between 0.4 and 0.5 microns. I suppose we can think of it as a rain of photons. But if we do that, then we have to say what a photon is. And that means falling into the maelstrom of counterintuitive ideas that make up quantum mechanics. So, it appears, we'll have to devote the entire evening to a lecture on quantum physics. Are you up for that?

Okay, okay -- I'll take a different tack. I'll begin instead on a Saturday afternoon back in 1942. The great weekly event was the matinee at the Uptown theater. Forty cents bought a bag of popcorn, a box of Milk Duds, and a movie ticket.

I would sit in the dark and watch young Ronald Reagan flying his airplane against terrible odds. Each week, it seemed, he shot down the same Chinese actors, dressed in Japanese uniforms, and America stayed safe for democracy.

Death was bloodless in the Uptown theater. The enemy was mowed down from afar. Bullets, like laser beams, eliminated peril while it was still distant. They didn't actually break the skin. The bad guys were killed. The good guys were only wounded, and they healed before the next battle.

Weapons in the Uptown theater were like fictional death rays. You pointed a gun, and the enemy fell down. You dropped a bomb, and a bridge disappeared. It wasn't until 1946 that we began to learn what those bombs had really been doing.

18th century conception of a burning glass.So we aren't too surprised when we hear an enduring early myth about light. According to the story, Archimedes created a huge mirror. He focused the sun's rays on the Roman fleet as it invaded Syracuse. He set it on fire from the distance of a bow shot. That tale has gone in and out of favor with historians ever since. Historian D.L. Simms gives us his careful analysis of this story. He thinks the tale hangs on the edge of plausibility. Archimedes might just barely have known enough optics to make such a mirror. It's conceivable that he could've made it with an adjustable focal length. Archimedes might even've been able to keep a beam fixed on one spot long enough to ignite wood. But beyond all those terrible if's was the fact that the burning mirror didn't appear in the earliest accounts of the battle. The first versions tell us only that Archimedes' ingenuity had something to do with winning the battle and that fire was involved. Simms concludes that the burning mirror was a wishful interpolation. Archimedes might well've found a way to hurl fire. But this 2200-year-old death ray was almost certainly imagined by a Byzantine writer hundreds of years later and attributed to Archimedes.

Death rays relate to warfare the way perpetual motion relates to energy production. They're devices that make everything easy -- machines to lift us above our dirty problems. We've dreamt of technology like that ever since Archimedes, and we probably always will. Those dreams are a beginning, but good technology weds our dream to the world it creates. Whether we're engineers or astronomers, we have to pursue dreams that can be taken out of the Uptown theater and held up to the bright light of a Saturday afternoon.

So let's meet another Greek who really did cast light upon optics but who is, for us, a shadowy figure. His name might've been Diocles, but we don't really know. All we have is a text that he wrote over 2000 years ago. It's not even in his own tongue.

What we have is a manuscript that was copied out by one more in a long string of scribes in AD 1462. And he left only spaces where Diocles's figures should've gone. But it's enough to tell us that it was this Diocles who invented the parabolic mirror.

Historian G. J. Toomer picks through this skimpy legacy -- this ancient text, titled On Burning Mirrors. Most of what we know of Diocles comes from reference to this very work by a noted sixth-century mathematician. But now you and I finally get to read this copy of his book, penned sixteen hundred years after the fact.

Toomer does his historical detective work. He decides that Diocles flourished in Greece just after 200 BC. He was a mathematician -- a geometer. Toomer takes us through the text, recreating the figures. We read Diocles's opening:

The burning-mirror surface submitted to you is the surface bounding the figure produced by a section of a ... cone ... revolved about [its axis].

A parabolic section of a coneWhat Diocles is describing, of course, is the shape of a paraboloid, and paraboloids touch our modern world in the technology of solar towers: A solar tower holds a steam-generating boiler high above a surrounding field of mirrors -- a field shaped into one gigantic paraboloid, one great burning mirror that gathers in the energy of the sun and focuses it on the boiler. The boiler, in turn, supplies a turbine. Today, Diocles's mirror is the heart of a power system that we're still trying to develop.

Diocles began by talking about Archimedes's work a century earlier -- with the legend of burning mirrors and the Roman fleet. Then we realize that Archimedes's technology would had to've exceeded the yet-incomplete technology of solar towers. Worse yet, the only burning mirrors Archimedes had access to were far more primitive than Diocles's parabolic collector.

Before he gets down to geometrical analysis, Diocles takes a moment to talk about the practical use of his theory. He tells how one might use burning mirrors in temples for cremations -- how his optics might improve sundial design. And, finally, he writes down the same arithmetic I studied as an engineering student.

But I still wish I knew who Diocles was -- this man bent by the distant lens of history. Who was this almost-forgotten Greek whose mathematical burning glass illuminated scholars in Baghdad and Renaissance Europe -- who put us on the way to the solar collectors that will, one day, light our cities?

Now, back for a moment to Aristotle. Aristotle may not've created a Star Wars technology, but he contributed in a very different way to the science of light. Let's move to the year 1168, when Robert Grosseteste was born in England.

Grosseteste, educated in the cathedral school of medieval Oxford, was as near to being a scientist as that world had to offer. When Oxford became a University in 1215, he was its first chancellor.

Grosseteste was an Aristotelian in a Platonist world. Historian Margaret Wertheim tells us he lived a very strict monastic life. But he had Aristotle's talent for scientific observation. While the Church around him tried to understand God through processes of deduction, Grosseteste hoped to find God's perfection by seeing it in the physical world around him.

That meant seeing in the most literal way. For Grosseteste, God was light. Understanding light meant understanding God. And since light followed the rules of Euclid, the way to light, and to God, was through geometry.

It was Grosseteste who first figured out that a rainbow wasn't just reflected light; it was refracted light. Light is bent in the mists that form rainbows the same way it's bent in a pool of water.



Incoming light from the upper left being refracted downward
(For details see the Chen, Lienhard, Eichhorn reference below)

Roger Bacon was Grosseteste's student. He created less science than Grosseteste, but he was a powerful champion of his ideas. After Grosseteste died, Bacon set out to convince the papacy that science and math were proper arms of theology. To make his point, Bacon described wonders that would, one day, flow from science -- self-propelled vehicles, lamps that wouldn't burn out, flying machines, explosive powders, better medicine, longer life, high-yield agriculture. But primarily Bacon talked about optics. He predicted that we'd one day have telescopes and eyeglasses. We didn't have to wait long for eyeglasses (more on that in a moment). The rest of his dreams all came to pass much later.

Bacon's life was really about visual realism. He said God should be shown to the faithful in the most realistic possible way. Up to then, religious art was mnemonic. A rough image of a saint or event simply reminded people to think about Sebastian or Jonah. Now Bacon called for geometric figuring. By that he meant three-dimensional realism that would bring saints to life on church walls. It would be 150 years before the rules of perspective were formalized. Yet Bacon had sown the seeds of Renaissance art.

Wertheim reaches an odd conclusion about Bacon. His fusion of art and geometry, his insistence on compelling realism, has led us back to disembodied light flowing on the networks. Bacon's realism helped start a progression that's finally reached twenty-first-century computer graphics. By a strange trick of illogic, realism has taken us back to the ultimate inner reality -- the ultimate unreality -- of cyberspace.

Now more about Bacon's eyeglasses: I spent the summer of 1956 hunched over a drawing board at the Pacific Car and Foundry Company. By July, I was feeling steady nausea. It turned out I had some astigmatism. I needed my first pair of glasses. That took some getting used to.

Forty-four years later, I feel undressed without glasses. Yet those first glasses seemed so modern! Not until much later did I realize that spectacles have been around for seven hundred years. Historian John Dreyfus went looking for origins and found the first reference to glasses in a sermon preached in Florence in 1306:

It is not yet twenty years since there was found the art of making eyeglasses ... So short a time is it ... I have seen the man who ... created it and talked with him.

He didn't give the inventor's name, but he did give us a clean date of origin. The first eyglasses were made just after AD 1286.

An Arab scientist had figured out how to make a spherical lens back in 1036. It was when his writings reached Europe in 1266 that Roger Bacon asked if lenses might help old people with weak eyes. Twenty years later, this Italian inventor managed the trick.

Now the plot thickens. In 1286 people thought our eyes sent out rays that bounced off the things we saw, back into the eye. Theologians also thought sight worked a little like radar. We were told to view God's world directly, without the distortion of mirrors and glass. Surely spectacles would bend the outgoing and returning rays, and truth would be distorted.

But utility won out. By 1300 Venetian crystal workers were in the eyeglass business. Their best lenses were ground from quartz crystal. Cheaper lenses were made from glass. Those fancy lenses made such a lucrative trade that crystal workers weren't allowed to leave Venice once they'd joined the guild.

Early eyeglasses either pinched the nose or were held on a stick -- uncomfortable either way, but no matter! Handwritten book production had been skyrocketing for two hundred years. Now spectacles would drive demand even further. Two generations later we'd reach the point where Gutenberg simply had to invent the printing press. Then the widespread use of eyeglasses made it possible to start reducing the size of books. More words could be squeezed on to a page. The cost of books dropped, and sales rose.

In 1494 Sebastian Brant wrote the original Ship of Fools book. In it, Brant showed a foolish scholar, surrounded by too many books. The fellow uses a feather duster to whip through pages faster than he can read them. He looks like an owl in his oversized spectacles. And so, I suppose, do we, armed as we are with our eyeglasses, flipping through more words than we can digest, five centuries later. [For this and other Brant images see this excellent website by the UH Special Collections Library.]


Image courtesy of Special Collections, UH Library

Back, then, to the question of what vision is: People asked whether it was the result of an emanation from our eyes or light entering from the world around us. That led to the next question: "How fast does light travel -- or is it instantaneous?" Of course the speed of light is something we've figured out only in the last century, right? What could we possibly have known before the likes of Einstein? Well, once more, our forebears surprise us.

It turns out we've known the speed of light since before the birth of Johann Sebastian Bach. That knowledge came close on the heels of the invention of the first telescopes in the early 1600s. In 1675, a young astronomer named Ole Roemer, from Denmark, went to Paris to study optics and telescopes. Roemer was interested in the movement of Jupiter's nearest moon, so he tracked it as it orbited in and out of Jupiter's shadow. It entered the shadow and then re-emerged 42 hours, 28 minutes, and 35 seconds later. It moved with metronomic regularity.

All that fit the clockwork perfection we saw in God's firmament during the seventeenth century. In a hundred transits, Jupiter's moon could be relied on to emerge once more, right on schedule. Six months (a hundred laps) later, Roemer set his clock and focused his telescope on Jupiter. He waited. No moon! Minutes passed. No moon. Finally it danced out of the shadows a full fifteen minutes late.

So Roemer considered what could've happened. Earth had swung hundreds of millions of miles away from Jupiter during the long winter months. Light had to travel that vast distance. It'd obviously taken the extra time to do so. He put pencil to paper and concluded that light had to move 192,500 miles per second to lose that fifteen minutes. Not bad! Roemer was only three percent higher that the right value. And that was less than seventy years after we first had telescopes.

I came on all this reading John Tyndall's 120-year-old physics text. Tyndall tells Roemer's story. Then he quotes contemporary nineteenth-century estimates of the speed of light. One is worse than Roemer's. That means that your great-grandparents had no better knowledge of the speed of light than people had when Isaac Newton was still young. It's sobering to reflect on the knowledge of our forebears. We knew the diameter of earth within fifteen percent two thousand years ago. We've done brain surgery and cataract operations even longer than that.

And so there was life before the computer, before Einstein, before the Industrial Revolution. It's amazing how much the mind did before it had all our modern instruments to rely on.

I'd like to finish with two simultaneous experiences about four years ago. Both were about light. One was the first of the great light and fireworks shows over downtown Houston. For half an hour, lasers, fireworks, and searchlights lit up our skyscrapers in the largest such display North America had ever seen. A million people watched from parks surrounding the center of town. Hundreds of thousands went in among the buildings to be in the middle of it.

If I'd been told I was seeing outtakes from the movie Independence Day, I would've believed it. The whole city seemed to rise into the sky on tongues of flame. Afterward, when onlookers talked to reporters, their words were flat. No one could put the experience into words. People in the streets and city officials alike only stood with eyes glazed muttering clichés -- "Wow!" "Truly spectacular!" The only articulate response came from a teenager who ran off a few lines of impromptu rap about the show.


Photo by Roger Eichhorn, with his permission

The other experience was quite different. The night before the light show, I'd met contemporary modern artist James Turrell at a fund-raiser. We'd talked about his art, which also depends on manipulating light. That evening had a very different texture, but it dealt with the same issues.

Turrell is best known for his Roden Crater project. He's been turning a volcanic cone near Flagstaff, Arizona, into a special place where you can view the changing sky. Turrell does that in many ways. He creates interior spaces with roofs that open into the sky to catch its variable light. It was Turrell who designed the lighting scheme for that tunnel under Fannin street -- the one that takes you from our old Museum of Fine Arts to the new building.

This gathering, four years ago, took place in a home that included a Turrell light-sculpture. A focused light sent a rectangular beam from one upper corner of a room and struck the opposite walls where they joined at ninety degrees. As you gazed at the pool of light, your mind turned it into a large three-dimensional cube standing out from the corner of the room. It works because Turrell arranges for you and the light source to form vanishing points of a two-point perspective. You don't know that in your head. Rather, you feel it in your stomach.


Photo by John Lienhard

All Turrell's art calls upon our inner eye to see what it missed at first. He was quite clear on that point. We viewers are ultimately the real creative artists. He provides the light and we complete the picture.

So I'd had two adventures with light. One was pure pop culture. The other was cerebral -- intellectual. Both helped me to see what art is. Both were abstract. Both showed how art leads the mind, how it rises out of unwashed commonplace experience, and how it exposes what we first don't even know is there.

Earth really is a spaceship. The changing sky really does illuminate beauty that we miss until our senses see it anew. And our own mind is the engine that completes the work that any artist begins. We have to tie into that primal experience. We need to be transmuted by the subtle means by which light shows us so much more than mere reality. I'll finish with that idea -- that seeing is much more than just perceiving the outside world.

Whether we close our eyes and see with our fingers, or turn off our senses and see on a computer screen, or even if we trust what reaches our corporeal eyes, no seeing is completed until our minds have worked their own magic upon it.

And you know, better than I do, how that is every bit as true for the objective work of an astronomer as it is for an artist.


SOME SOURCES

The image of a burning glass is taken from Le Entretiens Physiques d'Artiste et d'Eudoxe, ou Physique Nouvelle en Dialogues, Qui Renferme Précisément ce qui s'est Découvert de plus Curieux & de plus Utile dans la Nature, Vol. III. 1745.

Simms, D.L. Archimedes and the Burning Mirrors. Technology and Culture, Vol. 18, No. 1, pp. 1-24.

Toomer, G.J. Diocles on Burning Mirrors. New York: Springer Verlag, 1976.

Wertheim, M. Pythagoras' Trousers. New York: Random House, Inc., 1995, Chapter 2, God as Mathematician. (for more on Grosseteste and Bacon.)

Wertheim, M. The Medieval Consolations of Cyberspace. The Sciences, November/December, 1995, pp. 24-25.

Maloney, S.S. The Extreme Realism of Roger Bacon. The Review of Metaphysics, Vol. 38, June 1985, pp. 807-837.

Chen, J. I., Lienhard, J. H., and Eichhorn, R. A Method for Measuring Transparent Droplet Diameters. Int. J. Multiphase Flow, Vol. 4. pp. 233-235.

Dreyfus, J. The Invention of Spectacles and the Advent of Printing. Into Print: Selected Writings on Printing History, Typography and Book Production, London: The British Library, 1994, pp. 298-310.

Tyndall, J. Light and Electricity: Notes of Two Courses of Lectures Before the Royal Institution of Great Britain. New York: D. Appleton and Co., 1883, pp. 19-20.

Routledge, R. Discoveries and Inventions of the Nineteenth Century. ca. 1890. (Reprinted by Bracken Books, New York, 1989.) pp. 298-301.

Much has been written about Turrell's work. See, e.g., James Turrell: Light & Space. New York: Whitney Museum of American Art, 1980. For more on the Roden Crater project see Brades, S. F. James Turrell: Air Mass. London: The South Bank Centre, 1993.

Herbert, L. M., Lienhard, J. H., McGehee, J. P., and Riley, T. James Turrell: Spirit and Light. Houston, TX: Contemporary Arts Museum, Houston, 1998.