Reversing the Casimir force

Metamaterials can be used to fabricate high-resolution “perfect” lenses and even invisibility cloaks (see Physics Today February 2007, page 19 ). Ulf Leonhardt and Thomas Philbin, both at the University of St. Andrews in Scotland, reveal another surprising property of negatively refracting perfect lenses: They can reverse the direction of the attractive Casimir force between parallel conducting plates. As Hendrik Casimir demonstrated in 1948, in otherwise empty space the electromagnetic zero-point energy increases with the conductors’ separation, whence the attractive force. The physicists’ analysis of the force-reversal exploits a special property of the left-handed materials used to make perfect lenses: The constitutive Maxwell equations for such materials are the same as they would be in an empty space obtained by changing the sign of the coordinate perpendicular to the conducting plates. What’s the implication for the Casimir force? Suppose that the space between conducting plates is largely filled with a left-handed medium and that the plates’ separation is increased by a small distance. Because of the coordinate sign change, the zero-point energy behaves as it would in an empty space when the conductors are brought together—it decreases. The Casimir force is thus repulsive. Leonhardt and Philbin estimate that the force could be great enough to levitate a piece of 500-nm-thick aluminum foil. (U. Leonhardt, T. G. Philbin, New J. Phys. 9 , 254, 2007 .)