Forces in between

Scientists have for the first time measured the force that attracts two closely-held plates

Published: Monday 31 March 1997

A WEAK force comes into play when two parallel plates are placed a fraction of a millimetre apart in a vacuum, pulling the plates together. But numerous attempts to measure this effect, predicted by the Dutch scientist Hendrik Casimir in 1948, have failed.

Now Steven Lamoreaux at the Los Alamos National Laboratory in New Mexico has used a torsion pendulum connected to two surfaces to measure the force and has come up with a value which matches Casimir's prediction to within five per cent.

According to quantum theory, empty space is not truly empty but contains virtual photons and other virtual particles which pop into and out of existence. In the space between two plates erected close together in a vacuum, only those virtual photons whose wavelengths fit into the gap many times can exist. So there must be fewer virtual photons between the plates than outside them, and this imbalance produces a small force that pushes them together.

Lamoreaux found that the Casimir force varied with the separation between the two surfaces. When they were 0.75 micrometres apart, the force was around one billionth of a newton. Ed Hinds of the Sussex Centre for Optical and Atomic Physics at Sussex University is enthusiastic about this result: "The experiment is a beautiful demonstration of an effect we all believed in."

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