Noisy Meteors

We reported that several people were hit by falling meteors during the recent leonids shower on November 18. Now we?ve learned that some skywatchers not only saw the leonids, they heard them too.

?I am sure I could hear several of the meteors,? says Karen Newcombe from San Francisco. ?Several times when a Leonid with a persistent debris train flew directly overhead, I heard a faint fizzing noise.? She reports that there was delay between the sight and the sound and wonders, ?How is that possible when the meteor was so many miles above my head??

This question has been asked before, by some of the world?s greatest scientists. In 1719 astronomer Edmund Halley collected accounts of a widely-observed fireball over England. Many witnesses, he wrote, ?[heard] it hiss as it went along, as if it had been very near at hand.? Yet his own research proved the meteor was at least ?60 English miles? high. Sound takes about five minutes to travel such a distance, while light can do it in a fraction of a millisecond. Halley could think of no way for sky watchers to hear and see the meteor at the same time and decided that the reports were ?pure fantasy.?

But many people heard the leonids on November 18. The sounds were sonic booms or the loud crack of an explosion that came after the meteor had passed out of view. These were exotic, delicate noises, heard while the meteor was in full sight. Scientists call them ?electrophonic meteor sounds.?

Colin Keay, a physicist at the University of Newcastle in Australia, thinks he has figured out what causes electrophonic meteors. According to Keay, glowing meteor trails give off not only visible light, but also very low frequency (VLF) radio signals. Such radio waves, which oscillate at audio frequencies between a few kHz and 30 kHz, travel to the ground at the speed of light. However, human ears can?t directly sense radio signals. That means that something on the ground must be converting the radio waves into sound waves, acting as a transducer. Keay has found that transducers are surprisingly common. Simple materials like aluminum foil, thin wires, pine needles, even dry or frizzy hair, can intercept and respond to a VLF field.

?Strong, low-frequency currents can literally shake ordinary objects,? says Dennis Gallagher, a space physicist at the NASA Marshall Space Flight Center. ?When things shake, they launch vibrations into the air, which is what we hear.?Higher-frequency radio waves, like TV transmissions or FM radio broadcasts, oscillate much too fast (hundreds of millions of times per second) to shake these conductors, which is why we don?t pick up radio shows on our teeth fillings. And even if we did, we couldn?t hear the resulting MHz-frequency sound waves, which are far above the frequency range of a human ear.But we can hear VLF waves.

Keay discovered that even a pair of glasses could be made to vibrate slightly, which could explain the experience of Erich in Troy, New York, who says, ?When I was out [viewing the Leonids on Nov. 18th], I had my head back on the ground and heard a sizzling sound. My head was close to grass and leaves and I wear wire frame glasses as well. The sound was definitely simultaneous with the observation of a rather large streak.?

Dennis Gallagher says, ?I think what makes this exciting is that we?re talking about a phenomenon that has been experienced by people for perhaps thousands of years. Even in modern times folks who reported hearing such sounds were ridiculed. It was only about 25 years ago that Keay was able to do the research and legitimize the experiences of all those generations of people.

?It shows there are still wonders in nature yet to be recognized and understood. We should take this experience with meteors as reason to open our minds to what may yet be learned.?

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