Physicists Hear High-tech Whistle While They Work

It was music to the ears of physicists at the University
of California (UC), Berkeley, when they forced liquid helium-
4 through thousands of tiny holes and heard a whistling
sound.

Why the big fuss about an odd sound? It seems this
breakthrough might eventually lead to enhanced earthquake
studies and more accurate navigation systems, including the
Global Positioning System (GPS).

It all starts with one slippery liquid-helium-4. Ultra-cold
helium-4 becomes a super fluid, 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. This repeated speeding up and
slowing down created vibrations, which 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

“This whistle caught us by surprise,” said UC Berkeley
physics professor, Dr. Richard Packard. “It turns out a
single aperture will not make the whistle, because of random
speed fluctuations. Our experiment shows all the flows
through the holes are acting together, coherently, producing
the whistle. We suspect it’s like hearing thousands of
crickets chirping in unison on a summer night,” he said.

Packard said this new phenomenon might lead to improved
whistling super fluid navigation gyroscopes that detect
extremely small rotational motion. As their motion changes,
the whistle’s volume would change. This would be especially
useful on submarines or airplanes in regions where GPS
signals are unavailable.

The GPS navigation system relies on knowing the state of
Earth’s rotation. Because weather and other factors affect
Earth’s rotation, the GPS system needs constant updating for
accuracy. GPS gets its Earth rotation data from an array of
radio telescopes distributed around the world. A very
sensitive rotation sensor might complement the existing
telescope array, providing data quickly and inexpensively.

Super fluid gyroscopes are devices that detect very small
rotational motion. They use a specially-shaped, super fluid-
filled vessel containing two aperture arrays; when the vessel
rotates, the sound of the quantum whistle changes. This
provides a telltale clue and allows for sensitive measures of
the movements.

“This phenomenon may also permit scientists to develop very
sensitive rotation sensors to measure small surface twisting
signals created when an earthquake’s vibrations travel
through irregularities in the Earth’s crust,” Packard said.
“In fact, we can take this concept even further. If
seismologists can measure rotation signals from seismic
activity on Mars, they might learn a lot about martian
structure.”

Packard and his colleagues have a history of hearing whistles
while they work. Their experiments in 1997 and 2001, using
liquid helium-3, produced a whistle. But the temperatures
needed in those experiments were extremely low, just a few
thousandths of a degree above absolute zero, which is almost
one million times colder than average room temperature. Very
few people are trained to work with such ultra-cold
technology, which limits its potential applications. This
latest phenomenon occurs at a relatively high temperature of
2 Kelvin, which is 2,000 times warmer than the previous
helium-3 studies. This might make the technology available to
more users with off-the-shelf cryocoolers.

This research was conducted under a grant from NASA and the
National Science Foundation. The findings appeared in the
January 27 issue of Nature. More information about Packard’s
research is online at:

http://www.physics.berkeley.edu/research/packard/

JPL, a division of the California Institute of Technology,
Pasadena, Calif., manages the Quantum Technology in Life
Support and Habitation Program for NASA’s Exploration Systems
Mission Directorate.