Capturing a Phantom Force

Someone has finally managed to get something from nothing. In the current issue of Physical Review Letters, a physicist describes the first successful measurement of the Casimir force, a pressure exerted by seemingly empty space.

Classical physics predicts that empty space is devoid not only of matter but of energy as well. But quantum mechanics holds that even at absolute zero, the vacuum is seething with activity: "virtual" photons that, like unobservable Cheshire cats, wink in and out of existence. In 1948, Dutch scientist Hendrik Casimir predicted that by creating a tiny cavity, small enough to exclude some of these virtual photons, physicists could coax a weak pressure from the crowd of photons outside the cavity. Although a similar phenomenon--the Casimir-Polder force--was measured 3 years ago, the more subtle Casimir force eluded measurement for almost 50 years.

Until now, that is. Physicist Steven Lamoreaux of Los Alamos National Laboratory coated a quartz plate and a spherical lens with a thin gold film and attached both to a torsion pendulum, which measures force by twisting. The plate and lens, separated by about three-quarters of a micrometer, allow only virtual photons of certain wavelengths to exist between them--just as a glass bottle's size allows only certain notes to sound when you blow on it. But because virtual photons of many more wavelengths exist outside the plates, there is proportionally more photon energy per cubic micrometer on the outside. The imbalance results in a pressure that forces the plates together.

After carefully measuring the twisting force that the plate and lens applied to the pendulum, Lamoreaux came up with a value for the force--less than 1 billionth of a newton--that agreed to within 5% with Casimir's original prediction. "It's extremely nice to see it done," says Yale physicist Malcolm Boshier. "This is one of those experiments which is going to wind up in all of the textbooks." Once again, the subatomic world has proved to be every bit as weird as quantum mechanics predicts.