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Research in a Vacuum: DARPA Tries to Tap Elusive Casimir Effect for Breakthrough Technology

DARPA mainly hopes that research on this quantum quirk can produce futuristic microdevices
Casimir effect



© Jeremy Munday, California Institute of Technology

Named for a Dutch physicist, the Casimir effect governs interactions of matter with the energy that is present in a vacuum. Success in harnessing this force could someday help researchers develop low-friction ballistics and even levitating objects that defy gravity. For now, the U.S. Defense Department's Defense Advanced Research Projects Agency (DARPA) has launched a two-year, $10-million project encouraging scientists to work on ways to manipulate this quirk of quantum electrodynamics.

Vacuums generally are thought to be voids, but Hendrik Casimir believed these pockets of nothing do indeed contain fluctuations of electromagnetic waves. He suggested, in work done in the 1940s with fellow Dutch physicist Dirk Polder, that two metal plates held apart in a vacuum could trap the waves, creating vacuum energy that, depending on the situation, could attract or repel the plates. As the boundaries of a region of vacuum move, the variation in vacuum energy (also called zero-point energy) leads to the Casimir effect. Recent research done at Harvard University, Vrije University Amsterdam and elsewhere has proved Casimir correct—and given some experimental underpinning to DARPA's request for research proposals.

Investigators from five institutions—Harvard, Yale University, the University of California, Riverside, and two national labs, Argonne and Los Alamos—received funding. DARPA will assess the groups' progress in early 2011 to see if any practical applications might emerge from the research. "If the program delivers, there's a good chance for a follow-on program to apply" the research, says Thomas Kenny, the DARPA physicist in charge of the initiative.

Program documents on the DARPA Web site state the goal of the Casimir Effect Enhancement program "is to develop new methods to control and manipulate attractive and repulsive forces at surfaces based on engineering of the Casimir force. One could leverage this ability to control phenomena such as adhesion in nanodevices, drag on vehicles, and many other interactions of interest to the [Defense Department]."

Nanoscale design is the most likely place to start and is also the arena where levitation could emerge. Materials scientists working to build tiny machines called microelectromechanical systems (MEMS) struggle with surface interactions, called van der Waals forces, that can make nanomaterials sticky to the point of permanent adhesion, a phenomenon known as "stiction". To defeat stiction, many MEMS devices are coated with Teflon or similar low-friction substances or are studded with tiny springs that keep the surfaces apart. Materials that did not require such fixes could make nanotechnology more reliable. Such materials could skirt another problem posed by adhesion: Because surface stickiness at the nanoscale is much greater than it is for larger objects, MEMS designers resort to making their devices relatively stiff. That reduces adhesion (stiff structures do not readily bend against each other), but it reduces flexibility and increases power demands.

Under certain conditions, manipulating the Casimir effect could create repellant forces between nanoscale surfaces. Hong Tang and his colleagues at Yale School of Engineering & Applied Science sold DARPA on their proposal to assess Casimir forces between miniscule silicon crystals, like those that make up computer chips. "Then we're going to engineer the structure of the surface of the silicon device to get some unusual Casimir forces to produce repulsion," he says. In theory, he adds, that could mean building a device capable of levitation.

Such claims emit a strong scent of fantasy, but researchers say incremental successes could open the door to significant breakthroughs in key areas of nanotechnology, and perhaps larger structures. "What I can contribute is to understand the role of the Casimir force in real working devices, such as microwave switches, MEMS oscillators and gyroscopes, that normally are made of silicon crystals, not perfect metals," Tang says.

The request for proposals closed in September. The project received "a lot of interest," Kenny says. "I was surprised at the creativity of the proposals, and at the practicality," he adds, although he declined to reveal how many teams submitted proposals. "It wasn't pure theory. There were real designs that looked buildable, and the physics looked well understood."

Still, the Casimir project was a "hard sell" for DARPA administrators, Kenny acknowledges. "It's very fundamental, very risky, and even speculative on the physics side," he says. "Convincing the agency management that the timing was right was difficult, especially given the number of programs that must compete for money within the agency."

DARPA managers certainly would be satisfied if the Casimir project produced anything tangible, because earlier attempts had failed. Between 1996 and 2003, for example, NASA had a program to explore what it calls Breakthrough Propulsion Physics to build spacecraft capable of traveling at speeds faster than light (299,790 kilometers per second). One way to do that is by harnessing the Casimir force in a vacuum and using the energy to power a propulsion system. The program closed with this epitaph on its Web site: "No breakthroughs appear imminent."

One of many problems with breakthrough propulsion based on the Casimir force is that whereas zero-point energy may be theoretically infinite, it is not necessarily limitless in practice—or even minutely accessible. "It's not so much that these look like really good energy schemes so much as they are clever ways of broaching some really hard questions and testing them," says Marc Millis, the NASA physicist who oversaw the propulsion program.

The DARPA program faces several formidable obstacles, as well, cautions Jeremy Munday, a physicist at California Institute of Technology who studies the Casimir effect. For starters, simply measuring the Casimir force is difficult enough. These experiments take many years to complete, adds Munday, who recently published a paper in Nature (Scientific American is part of the Nature Publishing Group) describing his own research. What's more, he says, although several groups have measured the Casimir force, only a few have been able to modify it significantly. Still, Munday adds, the exploratory nature of the program means its goals and expectations are "quite reasonable."

Tang is pragmatic about his efforts, given the unlikelihood that Casimir force will ever provide much energy to harness. "The force is really small," he says. "After all, a vacuum is a vacuum."

Yet sometimes the best science can hope for is baby steps. "To come up with anything that can lead to a viable energy conversion or a viable force producing effect, we're not anywhere close," Millis says. "But then, of course, you don't make progress unless you try.

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