Originally published 6 November 1995
Stout little X, with its feet planted firmly on the ground and its arms uplifted in surprise, is our emissary to the unknown.
It was the philosopher René Descartes, in his book on geometry in 1637, who first used x to stand for the undetermined variable in his equations. Since then we have employed X as an alias when the true identity of a thing is unknown: the mysterious Mr. X, the creature from Planet X, secret ingredient X.
And so it was that when Wilhelm Konrad Röntgen discovered penetrating radiations of an unknown nature on Nov. 8, 1895, he called them X‑rays.
It turned out that the mysterious rays were not so mysterious as he at first imagined; they are of the very same nature as the rays that enter our eyes from the sun, only of a shorter wavelength. Röntgen had stumbled into a previously unexplored part of the electromagnetic spectrum.
We live in a sea of electromagnetic waves, from the very long to the very short: AM radio, FM radio, television, radar, microwaves, radiant heating, visible light, ultraviolet, X‑rays, gamma rays, cosmic rays. The wavelength (distance from crest to crest) of a typical radio wave might be as long as a city block. A billion X- ray waves can fit across the head of a pin.
What is it that is waving? Electric and magnetic fields. What are electric and magnetic fields? Things that wave.
That didn’t help much. Let’s try again.
What are electric and magnetic fields? The things that physicists represent in their equations by the letters E and B. What are E and B? The electric and magnetic fields.
Still unsatisfied? That is because you want to see or touch these waves, the way you see and touch waves in water or vibrating piano strings. Can’t be done. Electromagnetic waves are electromagnetic waves — and that’s that. Not even physicists can tell you much more about them. We see the effects of these presumed waves. The rest we make up. There’s a bit of Alice in Wonderland in all of this. The grin without the cat. But it works.
Electromagnetic waves are undeniably real. Turn on your radio. Thaw the hamburg in your microwave. Look at the X‑ray photograph that your dentist made. Never mind that electromagnetic waves seem spookily immaterial, evanescent, hard to describe in familiar terms. We use them. Our equations precisely describe their behaviors. X goes forth in the world as respectable E and B.
The most familiar electromagnetic waves are visible light. Ten or 20,000 of these waves would fit across my thumbnail. They are focused by the lens of the eye and detected by the retina. We were born with electromagnetic wave detectors to either side of our noses.
Our bodies are less dramatically sensitive to waves a little shorter than the visible, called ultraviolet. These can change cells in our skin and give us a tan or skin cancer. We are also sensitive to waves a little longer than visible, called infrared. We feel them as heat.
The rest of the spectrum, stretching away toward the very long and the very short, was X‑territory — until the late-19th century.
We followed X into the unknown. Down the spectrum to longer and longer wavelengths, and up the spectrum to shorter and shorter wavelengths. We learned how to produce each kind of wave, turn it to our purpose. We learned how to detect those waves falling upon the Earth from space, and used what we detected to discover more about the universe. For example, special telescopes placed in orbit above the atmosphere detect Röntgen rays from deep space, confirming, among other things, the possible existence of black holes. One of the most prominent black hole candidate is called, appropriately, mysteriously, Cygnus X‑1.
We are creatures of X. We love our mysteries. When Röntgen announced his discovery, the news spread like wildfire. Public demonstrations sprang up everywhere. “Wondrous rays.” “See the bones in your hand.” “Count the coins within your purse.”
But now Röntgen’s magical rays have become commonplace, threatening even. So we turn to other sources of mystery. Black hole X‑1. Television’s X‑Files. X marks the spot.
Röntgen’s rays have now been assigned to their appropriate place in the electromagnetic spectrum. They are as well-understood as the colors of the rainbow. We should have long since given them a name less suggestive of mystery: Röntgen-rays, perhaps, or more simply, R‑rays.
But “X‑ray” marches on, unmasked. The name retains its hold on our imaginations, most likely because of the use of the rays in revealing the interior of the human body: the clots, cancers, tumors, malignancies, and the skeleton, that Halloween costume we wear on the inside all year round, the causes and symbol of our mortality.
Death, with its grinning skull and crossed bones, is the deepest of all unknowns — the ultimate and final X.
