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In 1948, physicist Hendrik Casimir was working at Philips Research Laboratories in Eindhoven, the Netherlands, studying the properties of colloids. At the time, it was thought that the properties of these materials were determined by van der Waals forces, the long-range attractive forces that exist between neutral atoms and molecules. In 1873, van der Waals introduced the concept of intermolecular force, but did not explain it theoretically. Then, in 1930, Fritz London pioneered the use of nonrelativistic quantum mechanics to explain intermolecular forces.

Casimir realized, however, that the theory used to explain van der Waals forces could not correctly explain his experimental measurements on colloids. Casimir then collaborated with Dirk Polder to find a simple expression for the intermolecular force that included relativistic effects. Their result is a generalization of the London-van der Waals force, including delays due to the finite speed of light.

Casimir was interested in the simplicity of the results and tried to find simpler explanations. After talking to Niels Bohr, he suggested that this might have something to do with vacuum energy. Casimir found that calculations based on vacuum energy were further simplified when molecules were replaced by perfectly conductive plates. The idea is that when two electrically uncharged plates are placed in a vacuum a few nanometers apart, an attractive force is created.

In a quantum vacuum, electromagnetic fluctuations appear and disappear as intermittent electromagnetic modes, spanning an infinite range of wavelengths in free space. Between the two plates, larger wavelengths do not exist, and the optical cavity formed by the gap between the two plates limits the number of modes that can exist in the cavity. The difference between the outer waves and the inner waves then creates an effective net force that pushes them inward.

Since this force decays rapidly with distance, it can only be measured when the distance between objects is very small. At the submicron scale, this force becomes so powerful that it becomes the dominant force between uncharged conductors. When the spacing is about a hundred times the typical size of atoms, the Casimir effect produces pressures equivalent to about one atmosphere.

Casimir's original goal was to calculate van der Waals forces between polarizable molecules. In 1956 Evgeny Lipschitz discovered a general theory for calculating van der Waals forces between non-ideal conducting plates and was able to prove that Casimir forces were only a special case. In 1975, Schwinger discovered another way to calculate Casimir forces without reference to vacuum energy. Then, in 1997, Steve Lamurox quantified the force to within 5% of theoretical predictions.

Generally, high-energy physicists regard the Casimir force as a force originating from vacuum energy. But in condensed matter, the more popular view is that it has the same physical origin as van der Waals forces and does not depend on vacuum energy. The vacuum energy approach focuses on macroscopic origins, while van der Waals 'approach focuses on microscopic origins. In the professional literature, these two methods are often considered complementary, but which of the two methods is more basic?

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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