The Force of Empty Space
According to quantum mechanics, the vacuum is not empty, but teeming with virtual particles that constantly wink in and out of existence. One strange consequence of this sea of activity is the Casimir effect: Two flat metal
The Force of Empty Space
According to quantum mechanics, the vacuum is not empty, but teeming with virtual particles that constantly wink in and out of existence. One strange consequence of this sea of activity is the Casimir effect: Two flat metal surfaces automatically attract one another if they get close enough. The Casimir force is so weak that it has rarely been detected at all, but now a team reports in the 23 November PRL that they have made the most precise measurement ever of the phenomenon. They claim that their technique, using an atomic force microscope, has the capacity to test the strangest aspects of the Casimir effect, ones that have never before been tested.
The simplest explanation of the Casimir effect is that the two metal plates attract because their reflective surfaces exclude virtual photons of wavelengths longer than the separation distance. This reduces the energy density between the plates compared with that outside, and–like external air pressure tending to collapse a slightly evacuated vessel–the Casimir force pulls the plates toward one another. But the most puzzling aspect of the theory is that the force depends on geometry: If the plates are replaced by hemispherical shells, the force is repulsive. Spherical surfaces somehow “enhance” the number of virtual photons. There is no simple or intuitive way to tell which way the force will go before carrying out the complicated calculations.
Since the discovery of the theory by Casimir 50 years ago, there have been only two previous documented detections of the effect. One was in 1958 and had 100% uncertainty, and the second was last year [Phys. Rev. Lett. 78, 5 (1997)], when the theory was verified to within 5%. Umar Mohideen and Anushree Roy, of the University of California at Riverside, claim their new results verify the theory to within 1%.
Mohideen and Roy exploited the exquisite sensitivity of the atomic force microscope (AFM), which can sense forces as small as newtons. In an AFM, the force causes a slight deflection of a microscopic cantilever, which is detected with a laser system. The team affixe