July 6, 2001 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. Séamus 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 to a 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.
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