Repulsive quantum effect finally measured
A quantum effect that causes objects to repel one another – first predicted almost 50 years ago – has at last been seen in the lab. According to Harvard physicist Federico Capasso, a member of the group who measured the effect, it could be used to lubricate future nanomachines. The team detected the weak repulsive force when they brought together a thin sheet of silica and a small gold-plated bead, about half the diameter of a human hair. The force is an example of the Casimir effect, generated by all-pervasive quantum fluctuations.
The simplest way to imagine the Casimir force in action is to place two parallel metal plates in a vacuum. Thanks to the odd quantum phenomenon, these become attracted to one another. It happens because even a vacuum is actually fizzing with a quantum field of particles, constantly popping in and out of existence. They can even fleetingly interact with and push on the plates. However, the small space between the two plates restricts the kind of particles that can appear, so the pressure from behind the plates overwhelms that from between them. The result is an attractive force that gums up nanoscale machines. (To learn more about the Casimir force see Under pressure from quantum foam.) Capasso says that the Casimir force needn’t be an enemy. “Micromechanics at some point will have to contend with these forces – or make use of them.”
In 1961, Russian theorists calculated that in certain circumstances, the Casimir effect could cause objects to repel one another – a scenario Capasso’s team have finally created experimentally. The team achieved this by adding a fluid, bromobenzene, to the setup. The Casimir attraction between the liquid and the silica plate is stronger than that between the gold bead and the silica, so the fluid forces its way around the bead, pushing it away from the plate. The effect is akin to the buoyancy we experience in the macro world – where objects less dense than water are held up by the liquid around them. But in this case the bromobenzene is less dense than the solid bead. “You could call it quantum buoyancy,” Capasso told New Scientist. The force he measured was feeble – amounting to just a few tens of piconewtons – but that is still enough to buoy up nanoscale objects.
“The next experiment we want to do is use a TV camera to track the motion of one of these spheres, then we should be able to see easily whether you have levitation.” Harnessing the repulsive Casimir force could provide a kind of lubrication to solve the problem of nanomachines becoming gummed up by the better-known attractive version, says Capasso. In theory you could instead use a liquid denser than the components to buoy them up, but that wouldn’t be practical. “These gizmos are usually made of metal, so you would have to use mercury,” he explains. Quantum buoyancy bearings could be used to build delicate sensors, such as a floating “nanocompass” to detect small-scale magnetic fields.