Force from empty space drives a machine

Quantum theory is notoriously weird. Here’s one case in point: The theory holds that empty space isn’t empty. Instead, it predicts random energy fluctuations that cause evanescent, or virtual, particles to continually pop in and out of the vacuum.

A chip with a see-saw plate suspended parallel to its surface (left) is pushed up (right) toward a ball. Quantum fluctuations in empty space produce a force that tilts the plate. Adapted from Chan et al./Science

Exploring that oddity, the late Dutch physicist Hendrik B.G. Casimir predicted in 1948 that interactions between virtual particles would be detectable as forces between neutral, but electrically conductive, objects. Since then, experimenters have verified the strength of this Casimir force to within 1 percent of predictions.

Now, scientists in New Jersey have used that force to operate a tiny machine on a chip. “This is the first device in which you get a mechanical action as a result of vacuum fluctuations,” says Federico Capasso of Lucent Technologies’ Bell Labs in Murray Hill. He, Ho Bun Chan, and their colleagues describe the device in the Feb. 9 Sciencexpress, an online supplement to Science.

Umar Mohideen of the University of California, Riverside calls the new work “a major advancement for the future of MEMS technology.” MEMS stands for microelectromechanical systems, which combine tiny mechanical components, such as gears and levers, with microelectronic circuitry (SN: 7/22/00, p. 56).

Today, MEMS-based products include auto airbag accelerometers, blood pressure sensors, and digital movie projectors. “This paper is the first step in the design and fabrication of novel MEMS devices based on the Casimir force,” Mohideen asserts.

The Lucent team adopted techniques from the microelectronics industry to fabricate a thin, broad silicon plate that’s coated with gold and sits like a see-saw attached to horizontal supports on opposite sides. The supports resist the plate’s tilting and so normally hold it parallel to the silicon slab beneath it.

To evoke the Casimir force, the researchers suspended a tiny, gold-coated ball by a wire just above the plate and off-center. As they pushed the chip up in a vacuum, the plate tilted to reach within 76 nanometers, a thousandth of hair’s breadth, of the ball. The extent to which the plate tilted as it was raised provided a clear sign that an attractive Casimir force had become operative, the scientists say.

Capasso speculates that makers of MEMS and even tinier nanoelectromechanical systems may find ways to harness the Casimir force in 5 to 15 years. One practical device could be a high-precision position sensor. Lucent, however, has no plan to commercialize its design anytime soon, Capasso adds.

Beyond the technique’s future possibilities, the experiment also indicates that the Casimir effect may become problematic for designers of tiny machines, says Paul J. McWhorter of MEMX, a MEMS-technology start-up company in Albuquerque.

MEMS devices often fail because parts stick together. At separations of less than about 100 nm, the Casimir force becomes strong enough to cause such “stiction,” Capasso notes.

The Casimir force is most often depicted as pulling together two perfectly parallel, closely spaced metal plates. Because of the plates’ electromagnetic properties, virtual photons of certain wavelengths can’t appear in the gap. No such constraint exists for the virtual photons outside the gap, so there’s a photon-based pressure difference that pushes the plates together. When the plates are 10 nm apart, for example, this pressure equals atmospheric pressure.

The plate-ball geometry used by the Lucent team eliminates the difficulty of keeping plates perfectly parallel. Scientists have predicted that other geometries can cause yet-undetected, repulsive Casimir forces, says G. Jordan Maclay of Quantum Fields LLC in Richland Center, Wis. Working within NASA’s Breakthrough Propulsion Physics program, Maclay and his colleagues are constructing MEMS devices to test this prediction.

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