GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - GR4009FK110IR157K

Witnesses to a meteor in Australia in 1978 claimed to have heard strange noises as it streaked overhead. Yet, given that the meteor was 30 kilometers up, if these sounds had come directly from the meteor, people on the ground could not have heard them until almost a minute after the meteor had disappeared. Physicist Colin Keay hypothesized that the light given off by a meteor's trail must be accompanied by invisible electromagnetic radiation in the form of very low frequency (VLF) radio waves. Such waves, which travel at the speed of light, would reach the observer when the meteor itself came into view. Subsequent experiments in a soundproof chamber showed that many things can act as transducers to convert VLF waves into audible vibrations. Aluminum foil, thin wires, pine needles, or dry hair all responded to a VLF field. VLF waves induce small charges in such objects, thereby causing them to vibrate in time with the waves' oscillation. This transducer effect would explain why some people heard the noises while others close by heard nothing. Those who heard sounds were simply nearer to transducers. It could also explain why attempts to record meteor sounds have failed: scientists carefully place their microphones away from possible sources of interference.