This happened a while ago, but that doesn't make it any less wonderful.

To finish out the first half of my lab electronics course, they had the class tackle a medium-size design task. We split up into small teams, each of which had responsibility for a different module. We drew up designs at home, then came into class on Friday, wired them up, and put them together to make a complete optocoupler.

Optocouplers come in a lot of different shapes and forms. Ours was to take audio input to audio output by way of a 30kHz frequency-modulated infared signal. More informally, the centerpiece of the device is a little light bulb which gives off infared light, invisible to the eye. The first half of the circuit takes an audio signal and turns the light on and off more or less rapidly, based on that signal. The second half of the circuit starts from an infared detector and reverses the process, spitting out (ideally) a replica of the original audio signal.

The benefit to going through this whole rigamarole is that it's possible to separate the bulb from the detector, so that the signal can be transmitted, without wires, reasonable distances (in our case, across the room). Kinda cool for something we were to design and build from scratch.

Cool, yes, but not yet magical. The magic came later, after much building and testing and debugging and redesigning and rebuilding and connecting, when we were on the verge of a working device. We could hear, at the output, a battered but triumphant version of the sine-wave test input, a single clear tone somewhere near middle C, peeking out from behind mists of fuzz and buzz. And then, before we had all the bugs worked out, before everything was crystal-clear and perfect, the professor had pulled the plug from the function generator, taking away the tone and leaving only the static.

And then he pulled out an audio cable, connected one end to the optocoupler input, and bent over a boom box for a moment, making a connection and adjusting something or other. He stepped back, and we resumed the calibration process, lining up the transmitter and the detector.

Then we heard it. Underneath the static, richer than a plain tone. Piano, string bass, and a female vocalist singing "I Get A Kick Out Of You." And the lab was filled with the strains of Cole Porter, distant and distorted, but unmistakeable nonetheless. A few adjustments to the alignment and we could make out the full bass line. Twiddling the value of the VCO pot slid us around the resonant frequency, until a tiny tweak locked us into a region of beautiful clarity. There was something going on here, something wonderful, something more than just matching the response curves of transmitter and decoder.

Telephone, radio, phonograph: we were in the laboratories of their birth, hearing the results of years of experiment. Sounds captured, transformed, reconstructed, released again. Experimental science revealed as art. Bearded gentlemen in suits stood behind us, tears in the corners of their eyes, as they heard and felt what they had thought for so long.

Or, perhaps, it was 1938, and we were in a small Hudson River Valley town, playing with homebuilt equipment, made from discarded vacuum tubes and mail-order parts, listening to the great New York City radio stations. A city, three times as far away as we had ever been from home, singing to us through a box of glowing tubes and jury-rigged wiring.

Japanese pilots flying towards Pearl Harbor, picking up the all-night big-band broadcast from an island radio station like an ironic beacon. The children in Beyond Thunderdome listening to a hand-cranked language instruction record with mystical awe. The alien civilization which discovers the Voyager probe and listens to the gold-plated phongraph recording of Bach, Chuck Berry, and Javanese gamelan.

In the Middle Ages, people believed in invisible powers which shaped the world. Demons and angels flitted by all around, unseen by human eyes; witches and warlocks could cause things to happen at great distances, with no physical intermediation. Folk tales tell of the power of sounds, of princesses whose voices are stolen by witches for their own use, of the ability of a single true name spoken aloud to bring palaces tumbling down.

Today, we still believe in invisible powers which shape the world: we call them by different names, and they have different properties, but the distance is not so vast. It is impossible to see the signal sent through an optocoupler -- there is not even a wire inside which it is possible to imagine the visible signal passing. And yet, if you place your hand between the transmitter and the detector, as we did, the music stops, the signal path having been broken. I have heard the invisible, and I have seen both silence and sound on an oscilliscope screen. Is this really so different from seeing the elemental spirits summoned up by a cunning man, or from hearing the cries of mermaids?

To believe in the unseen is not to retreat from the real and rational world, nor does saying that the air is filled with electromagnetic fields make it unfit to breathe. The spirit is actually much the same, whether one speaks of spirits or of gravity waves: to replace small mysteries with deeper, greater ones. Sometimes, there's nothing like a little science to make the world a stranger, more magical place.