Physicists Hear High-Tech Whistle While They Work

"It all starts with one slippery liquid helium-4. Ultra-cold helium-4 becomes a superfluid, meaning it flows without friction. The scientists squashed it through an array of apertures 1,000 times smaller than the width of a human hair. The liquid whooshed faster and faster, until it reached a critical velocity. At that point, in a strange phenomenon, a microscopic quantum whirlpool dashed across each aperture, carrying away some of the helium-4's flow energy. This abruptly slowed the flow. The fluid repeatedly sped up and slowed down, creating vibrations that produced a whistling sound going from high to low.

A recording of the sound, called a quantum whistle, is available online at: http://www.nasa.gov/vision/earth/technologies/whistle.html."

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