Turn, Turn, Turn

A New Gyroscope Might Eventually Test an Einstein Theory

by Robert Sanders

An ultrasensitive, superfluid gyroscope developed by physicists here has the potential to surpass today's most sensitive devices for measuring absolute rotation or spin.

In a paper in an April issue of Nature, physics professor Richard Packard and his colleagues, graduate students Keith Schwab and Niels Bruckner, report a proof-of-principle demonstration of the new device.

Their prototype superfluid gyroscope already is quite sensitive, and they believe its sensitivity will eventually surpass that of the ring laser gyroscope, a highly sensitive device used in advanced commercial aircraft inertial guidance systems. Packard's immediate goal is to create a version with a sensitivity 10,000 times greater than the team has achieved to date.

"We have demonstrated a new kind of instrument that can detect absolute rotation at a very sensitive level," Packard said. "If these devices obey the theoretical design equations, they will surpass the ring laser gyroscope."

He predicts that, if he and his colleagues can boost sensitivity by very large factors, the superfluid gyroscope may be able to detect some of the strange effects of general relativity predicted by Albert Einstein more than half a century ago, such as gravitomagnetism.

"This may be a better mousetrap, but we don't know if there are any mice to be caught," Packard said.

The gyroscope is based on the fact that superfluids such as helium-4 have an uncanny ability to sense absolute rotation. The team developed a way to detect the quantum changes in superfluid helium-4 as the device's rotation changes. Their prototype has a sensitivity equal to one-half percent of the Earth's spin rate.

Such sensitive rotation or spin detectors are needed in fields such as geodesy, where geologists look for slight changes in the Earth's rotation to provide clues to what is happening in the planet's interior. At present such changes are determined using radioastronomy.

An extremely sensitive rotation sensor could also, in principle, detect gravitomagnetism, an analog of electromagnetism seen when massive objects move.

The superfluid gyroscope works because of a well-known property of superfluids: if you travel around any closed loop in the fluid you find that the net flow is zero. Put in mathematical terms, the integrated velocity around any closed path in the fluid must be zero.

That means if you start spinning a superfluid-filled doughnut, analogous to spinning a bicycle wheel on its axle, the fluid inside initially remains still while the doughnut spins. If you place a wall inside the doughnut to force the fluid to move, it will not flow like water-as a solid block of fluid-but rather develop complex flow patterns so that the fluid still retains the property of zero net flow around a closed loop.

The researchers went one step further and put a sub-micrometer sized pinhole in the wall, which causes a high velocity backflow through the pinhole, in a direction opposite to the rotation-a result also of the requirement for zero net flow. Packard saw that the high velocity fluid squirting through the hole essentially amplifies any change in rotation of the doughnut. Therefore, if he could detect these changes, he could measure very small alterations in spin.

The device is essentially a hollow doughnut filled with helium-4, chilled to near absolute zero. The instrument they built (pictured at right) is a one-centimeter-square silicon chip, etched with a channel for the superfluid and capped by a diaphragm, which operates like a drumhead, vibrating to drive superfluid back and forth in the channel, forcing it in each cycle to squirt through a microscopic hole.

The work is supported by grants from the Air Force Office of Scientific Research, the Office of Naval Research, and NSF.



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