when -- bam! -- night turned to day.A large fireball lit up the night as it raced northward, turning from white to cobalt before exploding in a shower of sparks. Although the rock broke up miles above in the atmosphere, the computer analyst said he simultaneously heard at least seven seconds of sound as he watched the rock tumble and break up.
"Not even a second later we could hear sounds. I didnt think at the time why we could hear it," Hammer said. "It sounded pretty much like a firework, just like that static-like crackling, and then when it broke up it popped. It was like watching an airplane without the delay."
This event is a rare phenomenon called, electrophonic sound.
Several weeks ago, a small number of people fortunate enough to be outdoors on Memorial Day weekend, caught the special effect in action.
An audiovisual treat
At times when meteors fall our way, some lucky observers both see and hear the streaking space rocks at precisely the same time, even though they enter the atmosphere high overhead.
Jack Murphy, the Denver Museum of Nature and Sciences curator of geology and
"The people who did hear it didnt know anything about it," Murphy said. "Then you tell them they were lucky to hear it because it was a rare thing."
The trick to the simultaneous sight and sound are the megawatts of electromagnetic radiation given off by the fireball, or bolide.
The very-low-frequency radiation, traveling at the speed of light, can be so intense that it can make objects on the ground -- anything from eyeglasses to dental fillings, both of which Hammer has -- quiver and vibrate.
The key is what sort of objects -- even hair or pine needles will suffice -- lie in the observers immediate surroundings. Murphy said another eyewitness to the bolide who was near Hammer at the time did not report hearing anything.
"A wide variety of mundane objects -- not necessarily metallic -- may transduce the electromagnetic energy into acoustic energy, which may then be heard by the normal hearing process," said Colin Keay, a research associate in the physics department at the University of Newcastle, New South Wales, Australia. Indeed, its not the fireball that is heard, but the vibration of nearby objects.
The electrophonic debate
But hearing is not always believing. Experts have quarreled about the sounds for hundreds of years.
Astronomer Edmund Halley derided witnesses to a 1719 fireball visible over much of England who reported "hearing it hiss as it went along, as if it had been very near at hand." Halley wrote off the reports as the "effect of pure fancy," a notion that stuck until this century, Keay said.
Keay, an expert in the phenomenon, said scientists have since compiled thousands of reports of electrophonic sounds.
"It looked and sounded like a cheap firework," Hammer said. "I kind of thought it was a flare at first."
Keay said his investigation into the phenomenon led him to discover that turbulence in the plasma trail left by the bolide is the likely culprit.
"The turbulence tangles the geomagnetic field into what I sometimes call magnetic spaghetti, which is trapped until recombination of the plasma releases it and makes the field lines vibrate within the Earth-ionosphere cavity, generating the electromagnetic waves," Keay said, in an e-mail exchange with SPACE.com.
Although the physics may be impressive, the actual sounds arent necessarily so.
"It looked and sounded like a cheap firework," Hammer said. "I kind of thought it was a flare at first."
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