The Vacuum That Pushes Back

If you place two uncharged metal plates extremely close together in a complete vacuum, they will pull toward one another. This is the Casimir effect, predicted by Hendrik Casimir in 1948 and first measured with high precision by Steven Lamoreaux in 1997.

The force does not arise from gravity, static electricity, or magnetism. It is the physical manifestation of quantum vacuum fluctuations. According to quantum field theory, empty space is not truly empty; it is populated by virtual particles that constantly pop into and out of existence. These particles behave like waves of electromagnetic radiation.

When two parallel plates are positioned nanometers apart, they restrict the space between them. Only virtual photons with wavelengths that fit perfectly within the gap can exist there. Outside the plates, there are no such boundary restrictions, meaning a infinite spectrum of wavelengths can exist. This creates an imbalance: there are more virtual photons pushing the plates from the outside than there are pushing from the inside. The plates are squeezed together.

For decades, this quantum suction was considered an inescapable nuisance for nanotechnology. As engineers tried to build microscopic machines with moving parts, the Casimir force would pull the tiny silicon gears together, causing them to stick and lock up permanently.

But in 2009, a research team led by Jeremy Munday, then a postdoctoral researcher at Harvard University, alongside Federico Capasso and Adrian Parsegian, demonstrated that this quantum force could be reversed. Instead of pulling together, the plates could be made to push apart.

The Chemistry of Quantum Repulsion

To understand how Munday and his team reversed a quantum force, it helps to look at the math that governs the fluctuations themselves. The Casimir force between two materials is determined by their dielectric properties—essentially, how their atoms polarize and respond to fluctuating electromagnetic fields across different frequencies.

If you have two plates made of material A and material B, separated by a vacuum, the force between them is always attractive. However, in 1961, Soviet physicists Evgeny Lifshitz, Igor Dzyaloshinskii, and Lev Pitaevskii calculated that the interaction changes if you insert a third material—a fluid—between the two plates.

According to the Dzyaloshinskii-Lifshitz-Pitaevskii (DLP) theory, if the dielectric permittivity of the fluid sits precisely between the permittivities of the two plates, the balance of the virtual photon pressure shifts. When plate 1 has a higher dielectric permittivity than the fluid, and the fluid has a higher permittivity than plate 2, the net Casimir force becomes repulsive.

To turn this theoretical blueprint into a physical measurement, Munday’s team had to construct an apparatus capable of measuring piconewton-scale forces submerged in a liquid.

Inside the Liquid Junction

The researchers chose their three materials based on their optical properties across a wide frequency spectrum. For the first plate, they used a silica substrate. For the fluid, they selected bromobenzene, a dense organic liquid. For the second surface, they used a gold-coated sphere. At crucial frequencies, the dielectric permittivity of bromobenzene is lower than that of gold but higher than that of silica.

The experimental setup relied on a modified atomic force microscope (AFM). The researchers attached the gold-coated sphere, measuring roughly 40 micrometers in diameter, to the tip of a highly sensitive AFM cantilever. This sphere was suspended above the flat silica plate inside a liquid cell filled entirely with bromobenzene.

To measure the force, the team slowly moved the silica plate closer to the gold sphere. As the gap narrowed, the repulsive Casimir force pushed the sphere upward, bending the flexible cantilever.

The researchers bounced a laser beam off the back of the cantilever and tracked the deflection of the reflected light using a position-sensitive photodetector. By measuring this deflection as a function of the distance between the sphere and the plate, they could calculate the exact force of the interaction.

At distances below 80 nanometers, the cantilever bent away from the silica plate. The force was repulsive. Measured.

Nanoscale Levitation

The measured repulsive force was tiny—on the order of tens of piconewtons—but it matched the predictions of the DLP theory. Because the force originates from the fundamental quantum properties of the materials, it operates continuously without requiring any external power source.

The most immediate application of this phenomenon is the elimination of stiction in microelectromechanical systems (MEMS). In devices like silicon accelerometers, optical switches, and tiny sensors, moving parts frequently stall when they accidentally touch. By coating these components with materials that produce a repulsive Casimir force when submerged in a suitable fluid, engineers can create a passive, physical buffer.

Because the repulsive force increases rapidly as the gap between the components shrinks, the parts are prevented from ever making physical contact. The fluid gap acts as a self-correcting, frictionless quantum bearing. The components levitate on a cushion of virtual photons.

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