Science & technologyMay 24th 2008 edition

The Casimir effect
Much ado about nothing

The Casimir effect, a curious consequence of quantum theory, may yet have practical applications

CAN something come of nothing? Philosophers debated that question for millennia before physics came up with the answer—and that answer is yes. For quantum theory has shown that a vacuum (ie, nothing) only appears to be empty space. Actually, it is full of virtual particles of matter and their anti-matter equivalents, which, in obedience to Werner Heisenberg's uncertainty principle, flit in and out of existence so fast that they cannot usually be seen.


Virtual percussion

However, in 1948, a Dutch physicist called Hendrik Casimir realised that in certain circumstances these particles would create an effect detectable in the macroscopic world that people inhabit. He imagined two metal plates so close together that the distance between them was comparable with the wavelengths of the virtual particles. (Another consequence of quantum theory is that all particles are simultaneously waves.) In these circumstances, he realised, the plates would be pushed together. That is because only particles with a wavelength smaller than the gap between the plates could appear in that gap, whereas particles of any wavelength could appear on the other sides of the plates. There would thus be more particles pushing in than pushing out, and the plates would clash together like a pair of tiny cymbals.

A neat idea. And in 1996, it was shown experimentally to be true. But so what? The answer is that now things in the computing industry have become so small that the Casimir force is starting to affect them. Computer engineers talk of “stiction”—the phenomenon of microscopic components sticking together—and spend a lot of time trying to figure out ways to avoid it. The Casimir effect is not the only cause of stiction, but it is an important one.

Engineers are now beginning to understand how Casimir forces depend on a device's design—an essential starting point if Casimir-related stiction is to be eliminated. A forthcoming paper in Physical Review Letters by Ho Bun Chan of the University of Florida, Gainesville, shows how the force changes when one of the flat metal plates imagined by Casimir is replaced with a corrugated silicon one. Dr Chan found that the effect's size depends on the spacing of the trenches. This suggests that if you designed microelectronic components carefully, you could reduce stiction.

Stiction is not, however, always a problem, and work is also going on to turn the Casimir effect from a liability to an asset, by using it in what are known as microelectromechanical systems (MEMS). In 2001 Dr Chan and his collaborator Federico Capasso, who were then working at Bell Labs, published a paper showing one way to exploit the effect in MEMS. They made a tiny, gold-plated seesaw and slowly lowered a gold-plated ball towards it. At a distance of about 300 nanometres (a nanometre is a billionth of a metre), the seesaw began to tilt towards the ball. The effect was particularly strong at distances less than 150 nanometres. What Dr Chan and Dr Capasso had shown was that the Casimir force could be used to make a simple lever that might act as a tiny but accurate stress sensor in nanoscale machines.

What really excites researchers, though, is the idea that the Casimir effect may sometimes be repulsive. That would knock the problem of stiction firmly on the head and, according to Dr Capasso, would open up the possibility of making things like frictionless ball-bearings.

According to calculations made several decades ago by Evgeny Lifshitz, a Russian physicist, Casimir repulsion should be possible—but you have to replace the vacuum with a fluid. The maths suggest that in order for this to happen, both liquid and plates must be made of carefully chosen substances. This is because the repulsion is caused by electromagnetic charges induced in the plates and the liquid by the virtual particles, and the forces produced by these charges depend on the materials you use.

As with Casimir's original suggestion, the gap between the theory and its practical demonstration has been a long one. But Dr Capasso, who is now at Harvard, and his student Jeremy Munday, have started investigating Lifshitz's ideas.

They recently performed an experiment in which they looked at what happens when the plates are made of gold and the fluid is ethanol. Although they did not succeed in reversing the attractive Casimir effect completely, they did reduce it by 80%—a reduction in line with Lifshitz's predictions. The right combination of materials, if it can be found, should therefore produce the desired reversal. That would be very good news for MEMS indeed.

This article appeared in the Science & technology section of the print edition under the headline "Much ado about nothing"

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