The Quantum Casimir Torque that Spins Gold in the Dark

Put two uncharged metal plates extremely close together in a complete vacuum, and they will pull toward each other. This is the Casimir effect, predicted by Hendrik Casimir in 1948 and measured precisely by Steven Lamoreaux in 1997. The phenomenon is not driven by gravity or static electricity, but by quantum fluctuations. Because the narrow gap between the plates restricts the wavelengths of virtual particles that can pop into existence, more particles press on the plates from the outside than from the inside. The vacuum pushes them together.

For decades, this force was understood as a straight, linear pull. But theory suggested that if you broke the symmetry of the plates, the vacuum would not just pull them together—it would twist them.

In 2019, a research team led by Federico Capasso at Harvard University measured this twisting force, known as the Casimir torque, for the first time. They did not use mechanical gears or external magnetic fields. They allowed the empty space between two materials to rotate a microscopic gold flake.

Measured.

The Physics of the Twist

To understand how empty space can twist solid matter, look at the optical properties of the materials involved. The Casimir force depends on how a material responds to electromagnetic fields. Most materials are isotropic, meaning they respond to light the same way in all directions.

If you use anisotropic materials—where the index of refraction varies depending on the direction of the incoming light—the quantum vacuum fluctuations passing through them are polarized. When two anisotropic plates are held close together but misaligned, the electromagnetic modes of the vacuum between them are squeezed.

The system seeks its lowest-energy state. To reach it, the vacuum exerts a torque to align the optical axes of the two plates.

Calculating this torque is straightforward on paper, but detecting it in a laboratory is notoriously difficult. The Casimir force is vanishingly small, and the torque is smaller still. At the nanometer scale, electrostatic forces from stray charges or surface roughness easily swamp the quantum signature. If the plates touch, electrostatic attraction welds them together instantly.

The Measurement Apparatus

To isolate the torque, Capasso’s team designed an experiment that swapped the vacuum for a liquid.

According to their study published in Nature, the researchers mounted a microscopic, birefringent crystal of barium titanate to a silica substrate. Above this crystal, they suspended a thin, circular gold flake, tens of micrometers in diameter.

The entire apparatus was submerged in ethanol.

The liquid served two purposes. First, it buoyant-stabilized the gold flake, reducing gravity's influence. Second, by choosing a liquid with refractive properties that matched the gold and the barium titanate in specific ways, the researchers kept the attractive Casimir force extremely weak while allowing the Casimir torque to dominate. This prevented the gold flake from collapsing down onto the crystal.

Because gold is isotropic, it does not naturally experience a Casimir torque when paired with an anisotropic crystal. To break the symmetry, the researchers exploited the fact that the gold flake was extremely thin—less than 100 nanometers. They used a polarized light beam to induce a temporary anisotropy in the gold's electron plasma, or they relied on the intrinsic anisotropy of the underlying barium titanate crystal to induce a direction-dependent interaction with the gold.

By using a liquid crystal or a birefringent solid substrate, the mismatch in electromagnetic wave propagation speeds in different directions created an angularly dependent energy density in the gap. The gold flake began to rotate, seeking the angle of minimum energy.

The team monitored the rotation of the gold flake using a specialized polarization-sensitive optical microscope. They recorded the angular orientation of the gold flake as it aligned itself with the optical axis of the underlying barium titanate crystal. By measuring the rotation speed against the known viscous drag of the surrounding ethanol, they calculated the torque.

The measured torque was minuscule—on the order of $10^$ Newton-meters. Yet, the experimental data matched the theoretical predictions of quantum electrodynamics.

Engineering the Void

This measurement is more than a confirmation of a seventy-year-old quantum prediction. In the field of microelectromechanical systems (MEMS), the Casimir force is usually an enemy. It causes tiny silicon gears and switches to stick together permanently, a failure mode known as stiction.

By demonstrating that the Casimir force can be engineered to repel, pull, or rotate objects based on material orientation, the experiment opens up new design paths for nanoscale machinery. Instead of fighting quantum vacuum fluctuations, engineers can use them to build frictionless bearings, contactless nanomechanical gears, and torque-transmitting drives that do not touch.

Space itself, when properly structured by the matter around it, becomes the machine.

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