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Detecting an electromagnetic vacuum force.

The electromagnetic force, which binds electrons to atomic nuclei, can have such distinctive effects in different situations that physicists have often given these effects special labels. Manifestations of the electromagnetic force range from the van der Waals forces of attraction between molecules and atoms to the postulated Casimir-Polder interaction between a neutral atom and an electrically conducting plate.

Now, researchers have for the first time obtained experimental evidence clearly demonstrating the existence of the elusive Casimir-Polder force. Edward A. Hinds and his co-workers at Yale University report their findings in the Feb. 1 PHYSICAL REVIEW LETTERS.

The Casimir-Polder interaction arises out of a quantum effect associated with fluctuations of electromagnetic fields in a vacuum. In 1948, H.B. Casimir and D. Polder proposed that such vacuum fluctuations would cause an observable attraction between an isolated, neutral atom and a flat, conductive plate.

Though extremely small, this attractive force would be the dominant influence when plate and atom are separated by distances much greater than an atomic diameter. At such distances, the time it takes for an electromagnetic field (or photon) to travel back and forth between atom and plate becomes significant. Known as retardation, this phenomenon affects how atom and plate interact with each other.

To detect the Casimir-Polder force, Hinds and his colleagues studied the deflection of sodium atoms traveling down the gap between two nearly parallel plates coated with gold. In the absence of other interactions, the sodium atoms would experience a Casimir-Polder force that pushes them sideways toward the plates as they move along the gap.

To detect such a minuscule effect, the researchers had to be particularly careful to avoid contamination of the gold film, which could give rise to stray electrical fields. Such fields would cause effects obscuring any attraction that could be attributed to the Casimir-Polder force.

The experiment "was a lot harder to do than it looks," says graduate student Charles I. Sukenik, a member of the Yale team.

The measurements reveal the presence of an atom-plate interaction that clearly fits a Casimir-Polder force much better than it does a van der Waals force. "Our results confirm the magnitude of the [Casimir-Polder] force and the distance dependence predicted by quantum electrodynamics," the researchers conclude.

"It's a really elegant experiment, beautifully carried out," comments Stephen R. Lundeen of the University of Notre Dame (Ind .).

Lundeen and his co-workers have attempted to detect the Casimir-Polder interaction in a different type of experiment. They studied transitions from one energy level to another in a helium atom in which one electron has been excited so that it tends to remain much farther from the helium nucleus than it would in its ground state. "We wanted to do a high-precision test on a microscopic scale," Lundeen says.

These experiments yielded the most precise measurements yet of energy levels to which the Casimir-Polder force makes a discernible contribution. However, the researchers found a minute but significant and puzzling discrepancy between their experimental results and theoretical calculations- based on quantum electrodynamics -- of what those energy levels should be.

"We're seeing a vast difference from the energy levels that would exist in the helium atom if there were no retardation," Lundeen says. "But we have a clean experimental result that is in rather dramatic disagreement with the best available calculations."

Whether the Casimir-Polder force plays any role in this discrepancy remains unsettled. "It'll be interesting to see how this matter gets resolved," Lundeen says.

- L Peterson
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Title Annotation:evidence of Casimir-Polder interaction found
Author:Peterson, Ivars
Publication:Science News
Date:Feb 13, 1993
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