How do you measure the Casimir effect

Researchers from Harvard University in Cambridge and the National Institutes of Health (NIH) from Maryland have shown that a gold ball and a quartz plate repel in bromobenzene solution. Almost frictionless nano devices are thus becoming possible.

Artistic impression of the experiment

Cambridge (USA) - The quantum mechanical Casimir effect was theoretically predicted in 1948 and confirmed by experiment in 1956. It says that two plates that are only a few nanometers apart must attract each other if they are only separated by air or vacuum. The effect is not visible in everyday life, but it plays a role in many experiments in the nano and micrometer range. It is particularly undesirable for the tiniest of circuits or devices. Because the Casimir forces increase the smaller the distance between the individual objects, they can, in extreme cases, block nanomachines.

Jeremy Munday, Federico Capasso (Harvard University) and Adrian Parsegian (NIH, Maryland) have now found a combination of materials in which the Casimir effect causes repulsion. This is possible because Casimir forces are based on electromagnetic charge fluctuations, so the electrical properties of the bodies involved play a decisive role. If a suitable liquid is brought into the space between the bodies involved, this can turn the attractive force into a repulsive force.

Together, gold, bromobenzene and quartz are one of the few combinations for which the Casimir effect ultimately leads to a repulsion of the two solids. Superfluid helium between air and a solid is another example of an attractive Casimir effect. In this case, the helium flows up the container walls.

Since the Casimir effect only generates very small forces, it is relatively difficult to measure. Munday and his colleagues used a converted atomic power microscope for this. It is usually equipped with a very sharp tip that moves line by line over the sample to be measured. The deflection of the tip is measured optically. The researchers replaced the usual tip with a 40 micrometer tiny plastic ball coated with gold. They immersed this sphere in a cell with bromobenzene that covers a quartz plate. When the ball approached and removed from the quartz plate, a repulsive effect was found at short distances. The physicists ruled out hydrodynamic effects and static electricity as the cause.

According to the researchers, the proof that the Casimir effect can also have a repulsive effect enables new applications of the almost frictionless "quantum levitation". Steve Lamoreaux from Yale University comments in the current issue of "Nature" that one could probably find combinations of materials in which one object floats at a fixed distance from another, so that devices with negligible friction are conceivable. However, this would only be possible for devices ranging in size from nano- to micrometers.