A discovery that may someday help measure how clouds and
earthquakes change Earth’s rotation has come from an experiment
that made friction-free helium whistle.
By manipulating ultra-cold liquid helium-3 in a hollow,
doughnut-shaped container, NASA-funded scientists at the
University of California at Berkeley produced a whistling sound
that got louder or quieter depending on the orientation relative
to the North Pole and Earth’s rotation. In principle, small
changes in Earth’s daily rotation rate will also vary the
loudness of the whistle. Although Earth rotates every 24 hours,
clouds and the motion of Earth’s crust can make any given day
slightly longer or shorter. These new findings might provide an
unusual new way to measure such changes.
“This research was an exciting breakthrough for us,” said
Dr. Richard Packard, a U.C. Berkeley professor. “The successful
demonstration of this effect may enable scientists to measure
extremely slight increases or decreases in the rotation of
objects, including Earth.” Packard led the research team, along
with Dr. SÈamus Davis, also a U.C. Berkeley professor.
“Current Earth rotation measurement techniques are not
sensitive enough to detect rotational changes caused by
earthquakes, even those as large as magnitude 8,” said Dr.
Richard Gross, a geoscientist at JPL. “If we had more sensitive
techniques, like those being developed by Dr. Packard, then we
could measure the effects on Earth’s rotation. That would help us
better understand Earth’s structure.”
The team cooled the doughnut-shaped vessel filled with
liquid helium-3 temperature nearly 1 million times colder than
room temperature. At this ultra-cold temperature the liquid
becomes a superfluid. A superfluid is a state of matter that has
no friction, so the liquid can flow continuously inside the
vessel. The liquid in the doughnut acts like a single, super-
giant atom that does not follow everyday behavior, but is
dictated by the strange rules of quantum physics.
This latest discovery builds on the team’s previous
research. In 1997, they discovered the quantum whistle when they
pushed helium through a single perforated membrane between two
superfluid-filled chambers. This experiment demonstrated a
phenomenon called the Josephson effect. As they tried to push the
fluid through the holes, each 1/500th as thick as a human hair,
it jiggled to and fro. The vibration frequency increased as they
pushed harder on the fluid. They used the world’s most sensitive
microphone and ordinary headphones to hear the vibrations–an
oscillating, whistling sound.
In this latest research, they put two thin membranes, each
with an array of more than 4,000 tiny holes, at opposite sides of
the doughnut to divide the fluid. When the researchers tried to
push the fluid through the holes with electrostatic pressure, it
did not flow in the direction they were pushing. Instead, it
flowed in a strange, oscillating pattern, which produced a
whistle. In flowing through the doughnut-shaped vessel, the
whistle got louder or softer, depending on the vessel’s
orientation with respect to Earth’s rotation axis.
The promising new research might also lead to extremely
precise gyroscopes to help navigate future NASA spacecraft. This
experiment used a tiny amount of helium-3, but by using a much
larger amount, an ultra-sensitive gyroscope might be created.
“Earth is probably too noisy to realize the full potential
of this technology,” Packard said. “The best environment would be
on a free-floating satellite, which could have zero vibration.”
The Berkeley team calls the most recent effect they observed
“quantum interference of a superfluid.” They found that by
linking two superfluid quantum systems using a doughnut shape,
even a tiny effect of Earth’s rotation influences them both
through laws of quantum mechanics, and the two systems
“interfere” with each other.
“In essence, we demonstrated that two weak links behave as
one weak link whose properties are influenced by Earth’s
rotation,” Packard said. “The successful demonstration of this
effect has been a goal of low-temperature physicists for more
than 35 years.”
This research program was conducted under a grant from
NASA’s Biological and Physical Research Program. Packard co-
authored the paper, which will appear in the July 5 issue of
Nature, with NASA fellow Ray Simmonds and Drs. Emile Hoskinson
and Alexei Marchenkov. More information about the quantum fluids
research program at U.C. Berkeley is available at
http://physics.berkeley.edu/research/packard and
http://ist-socrates.berkeley.edu/~davisgrp .
The whistling helium sound can be heard online at
http://www.jpl.nasa.gov/heliumwhistle .
More information on the Biological and Physical Research
Program and Fundamental Physics Program is available at
http://spaceresearch.nasa.gov and http://funphysics.jpl.nasa.gov .
JPL manages the Fundamental Physics in Microgravity Research
Program for NASA’s Office of Biological and Physical Research,
Washington, D.C. JPL is a division of the California Institute of
Technology in Pasadena.