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Psst! Sounds like a meteor: in the debate about whether or not meteors make noise, skeptics have had the upper hand until now
Natural History, July-August, 2002 by Alan Burdick
In the wee hours of November 18, 2001, while meteor gazers sat outdoors enjoying a dazzling Leonid shower, a puzzled few sat indoors typing e-mails to NASA.
"Do meteors make noise?" two perplexed viewers in North Carolina wanted to know. They had heard "a crackling to hissing sound" several times that evening, just as the meteors flared overhead. Likewise, an observer in Mississippi heard "a hissing sound" right when a meteor streaked across the sky. A third person heard sizzling; a fourth, "a kind of swish." That shouldn't happen, one viewer pointed out. Sound travels far more slowly than light; you might hear a sonic boom several moments after a meteor appears, but simultaneously hearing and seeing the "swish" of a meteor is as impossible as seeing distant lightning and hearing the accompanying thunder at the same time. Yet this viewer, too, had heard "a faint fizzing" noise from several Leonid meteors that night. "I hope I'm not going crazy!" she added.
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Not to worry, says Dejan Vinkovic, coordinator of the Global Electrophonic Fireball Survey. Vinkovic, a graduate student in physics at the University of Kentucky in Lexington, began the survey two years ago in an effort to gather a database of earwitness reports of these whispering meteors ("electrophonic fireballs" in the literature). He knows full well that reputable scientists have dismissed the phenomenon for centuries; in 1719, astronomer Edmund Halley discounted such anecdotes as "the Effects of Fancy." But not only has Vinkovic heard the sounds himself, he was recently part of an international team of scientists that, for the first time in history, successfully captured them on tape. "Nobody had actually recorded these sounds under controlled conditions," he says. "We proved that it can be done."
Reports of noisy meteors date back to at least the year 817, when a Chinese observer documented a meteor with a sound "like a flock of cranes in flight." In 1676, Italian astronomer Geminiano Montanari observed one that sounded like "the rattling of a great Cart running over Stones." Montanari was also the first to doubt his ears: his calculations put the meteor thirty-eight miles up in the sky--too far away, he knew, for its sound to reach him instantly. A spectacular Leonid storm in 1833 generated further anecdotes of meteors that swished, whooshed, or, in one case, "resembled the noise of a child's popgun." Failing any suitable explanation, however, the reports were again dismissed as figments of imagination.
There the matter rested for a century and a half. How could sound waves travel at the speed of light? A few inconclusive experiments were conducted. Tenuous theories arose and sank. Finally in 1980, Colin Keay, of the University of Newcastle in Australia, offered a strange yet geophysically rational explanation. As meteors fall through Earth's magnetic field, he proposed, they generate radio signals audible to the human ear.
Ordinarily that would be impossible, Keay knew. Radio waves are electromagnetic, not acoustic. Acoustic waves are vibrations of molecules: when a wing flaps, it compresses the surrounding air in a series of waves that reach the inner ear and are perceived as sound. Electromagnetic waves, which include radio signals and visible light, don't need a medium to propagate and don't make an impression on the human ear. Even radio waves of 20 Hz to 20,000 Hz--quite low frequencies, corresponding to the range of acoustic frequencies that humans can register as sound--are, by themselves, inaudible.
To bridge this gap, humans invented the transducer, a device that efficiently translates electromagnetic waves into physical, air-moving waves and hence sound. Amazingly, in laboratory experiments Keay found that even ordinary objects can act as transducers. Slips of paper, aluminum foil, even eyeglasses: when Keay exposed them to rapidly shifting electromagnetic fields (that is, radio waves of very low frequency), the objects oscillated ever so slightly, creating weak--and faintly audible--acoustic waves.
Keay postulated that the same thing happens outdoors under a meteor shower: falling meteors generate very low-frequency radio signals that travel at the speed of light to the ground, where they cause pine needles, blades of grass, and other small objects to tremble slightly and whisper to any stargazer within earshot. Mundane objects become celestial heralds, instantly announcing the arrival of shooting stars. A meteor plummets, and the lawn chair or the pine tree speaks.
Keay's theory helped explain the rarity of noisy meteors. Only very rarely does a meteor generate sufficient electromagnetic energy to make pine needles move. Even then, the sounds are faint; one meteor watcher may hear noises, while a friend standing ten feet away may hear nothing. Keay suggested that his theory could also account for controversial age-old reports that the aurora borealis, which is essentially an electrical disruption of Earth's magnetic field, sometimes speaks to its viewers.
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