The Casimir Effect Heats Up

For the first time, a group led by Nobel laureate Eric Cornell at the National Institute of Standards and Technology and the University of Colorado in Boulder has confirmed a 1955 prediction, by physicist Evgeny Lifschitz, that temperature affects the Casimir force, the attraction between two objects when they come to within 5 millionths of a meter (approximately 1/5,000 of an inch) of each other or less. These efforts heighten the understanding of the force and enable future experiments to better account for its effects.

Tiny as it is, the Casimir effect causes parts in nano- and microelectromechanical systems (NEMS and MEMS) to stick together. It confounds tabletop experimental efforts to detect exotic new forces beyond those predicted by Newtonian gravity and the Standard Model of particle physics.

In their work, the researchers investigated the Casimir-Polder force, the attraction between a neutral atom and a nearby surface. The Colorado group sent ultracold rubidium atoms to within a few microns of a glass surface. Doubling the temperature of the glass to 600 degrees Kelvin while keeping the surroundings near room temperature caused the glass to increase its attractive force threefold, confirming theoretical predictions recently made by the group's theorist co-authors in Trento, Italy.

What was happening here? The Casimir force arises from effects of the vacuum (empty space). According to quantum mechanics, the vacuum contains fleeting electromagnetic waves, in turn consisting of electric and magnetic fields. The electric fields can slightly rearrange the charge in atoms. Such "polarized" atoms can then feel a force from an electric field. The vacuum's electric fields are altered by the presence of the glass, creating a region of maximum electric field that attracts the atoms. In addition, heat inside the glass also drives the fleeting electromagnetic waves, some of which leak onto the surface as "evanescent waves." These evanescent waves have a maximum electric field on the surface and further attract the atoms.

Electromagnetic waves from heat in the rest of the environment would usually cancel out the thermal attraction from the glass surface. However, dialing up the temperature on the glass tilts the playing field in favor of glass's thermal force and heightens the attraction between the wall and the atoms.

Obrecht et al., Physical Review Letters, 9 February 2007
Also see the NIST press release