Feb 22, 2007
New look for elusive force
Physicists in the US have shown that the elusive, quantum-mechanical Casimir-Polder force increases as a surface gets warmer. The result, obtained by Eric Cornell and colleagues at the JILA laboratory in Boulder, Colorado, could affect the design of devices that rely on small-scale interactions, such as atom chips, nanomachines and nanoelectromechanical systems (NEMS).
Quantum fluctuations mean that the vacuum is not empty, as is assumed in classical physics. These fluctuations are a consequence of the uncertainty principle, and they give the vacuum a structure that manifests itself in a variety of different ways, such as the Casimir effect.
The fluctuations become stronger closer to a surface and nearby, isolated neutral atoms experience them as a subtle pull. This tiny pull is like an invisible rubber band between bulk objects and atoms, which can be a source of friction in tiny devices. Tiny though it is, the Casimir effect actually causes parts in NEMS and microelectromechanical systems to stick together.
Previous work in this field by the JILA team included the most precise measurement ever of the Casimir-Polder force, which was hundreds of times weaker than the best previous record. The researchers were also able to measure the force at distances as large as 5 μm. Now, Cornell and co-workers have taken this work a step further by making the first measurement of the temperature dependence of Casimir-Polder. The results also confirm a 1955 prediction by Evgeny Lifschitz that temperature affects the Casimir force.
To measure the force, the JILA physicists placed a Bose-Einstein condensate (BEC) of 250 000 rubidium atoms in a magnetic trap a few microns away from a glass plate. A BEC is an ultracold cloud of gas atoms that are all in the same quantum state. The researchers then observed the "wiggling" of the BEC as it was brought closer to the surface of the plate.
Based on the changes in the oscillation frequency of the BEC, the JILA team calculated the Casimir-Polder force between the BEC and the plate. Measurements were made as a laser beam heated up the glass plate from room temperature to around 605 K, while the surroundings were kept near room temperature (around 300 K). Cornell and colleagues found that the strength of the Casimir-Polder force at 605 K was nearly three times greater than it had been at room temperature. The researchers were also able to separate the forces coming from the glass-plate surface, from those coming from the environment.
The experiments demonstrate a new practical use of a BEC, a form of matter that was created at JILA more than 10 years ago. Cornell, who shared the 2001 Nobel Prize for Physics for this work, says that the purity and sensitivity of a BEC makes it uniquely useful as a tool for measuring very slight forces, such as Casimir-Polder.
The researchers published their work in Phys. Rev. Lett..