Testing New Physics With Nothing

To detect new forces, particles, and dimensions in a sub-micron-sized force experiment, physicists must inevitably confront the Casimir force, an exotic quantum phenomenon in which empty space can push together a pair of metal plates. Empty space, or the "vacuum," is actually teeming with fleeting particles and electromagnetic fields.

But in between a pair of narrowly spaced plates, the vacuum does not pack energy as densely as it does outside the plates. Just as an underground tunnel blocks AM radio signals with wavelengths that are bigger than the opening of the tunnel, the metal plates keep out electromagnetic fluctuations with wavelengths greater than the distance between the plates.

In a 1600s demonstration by Otto von Guericke, the invisible atmosphere pushed together a pair of evacuated hemispheres so strongly that even horses could not pull them apart. Similarly, in the Casimir effect, the more energy-dense vacuum outside the plates pushes together the metal plates, because they enclose a less energy-dense vacuum.

However, the Casimir effect occurs much more subtly than the 1600s demonstration. Nonethless, this vacuum pressure, which has been confirmed experimentally (Update 300), can become large enough at short separations to conceal the effects of new physics.

To overcome this problem, theorists at Purdue University and Wabash College (contact Dennis Krause, kraused@wabash.edu) propose to exploit a key fact: the metal material itself influences the strength of the Casimir force, primarily through electronic interactions between the metal and the vacuum.

On the other hand, the plates' interaction with any new forces, particles, or dimensions would likely depend on the metal's nuclear as well as electronic properties.

Therefore, the theorists suggest making differential measurements of the Casimir force. Together with experimentalists at IUPUI and Lucent, they would compare the Casimir force for plates made with different metal isotopes of the same element. Isotopes of an element have fairly identical electronic properties, but different nuclear and gravitational properties.

If there is a difference between the measured Casimir forces, the researchers can attribute it to new physics after other effects (such as sample preparation) are taken into account. (Isotopes do affect the plates' electronic properties slightly, but the researchers compute the resulting change in Casimir force to be tiny compared to other effects, about 10,000 times smaller than the magnitude of the Casimir force itself.)

Their technique has another advantage: by directly measuring Casimir force differences, rather than the force itself, they reduce the dependence upon theoretical assumptions. (Krause and Fischbach, Physical Review Letters, 4 November 2002; also Fischbach, Krause, Decca, Lopez, Physics Letters A, upcoming; also see Purdue News release).