Empty space isn't actually empty. Even if you somehow managed to suck every single atom out of it, the Universe is filled with various fields that dictate the behavior of particles and forces. These fields even create pairs of "virtual particles" that pop into existence briefly before annihilating each other.

This counterintuitive view of the nature of the Universe is an outgrowth of quantum field theory, but it was difficult to figure out any obvious consequences. That changed in 1948, when Dutch physicist Hendrik Casimir figured out a specific situation where the contents of empty space matter. Now called the Casimir effect, it creates a tiny force when two conductive metal plates are placed in close proximity.

In a new paper published in today's edition of Science, researchers show that the Casimir effect can also be repulsive and use the balance between attractive and repulsive forces to cause a tiny flake of metal to levitate above a surface.

Quantum suckage

Traditionally, the Casimir effect has been demonstrated using two small metal plates in a vacuum. If the plates are uncharged, then we typically think of there being no electromagnetic field between them. But in the quantum world, virtual electromagnetic connections pop into and out of existence as random fluctuations. If the plates are brought close enough together—held only nanometers apart—there isn't enough physical space for the full set of fields to form. In essence, some of the fields are excluded from the space between the two plates.

As a result, the area outside the gap between the plates contains more electromagnetic fields than the space between them. This creates a pressure that forces the plates together. By tweaking the materials or shape and orientation of the plates, it's also possible to convert this to a short-range repulsive force.

The new work, done by a collaboration between scientists at the University of California, Berkeley and Hong Kong University, involved figuring out how to do both attraction and repulsion at the same time. The attraction was the same as it has always been: a gold plate, with a tiny (25µm across) flake of gold, floating in ethanol. On its own, this would generate an attractive force that would quickly suck the flake onto the larger gold plate.

To change that, the researchers covered the plate with teflon. The Casimir interaction between teflon and gold is repulsive and functions primarily at short wavelengths. Thus, when the gold flake gets close enough to the teflon-coated surface, it feels a repulsive force, keeping it from being sucked onto the surface. As it gets farther away, however, the short-range repulsion starts to fade out. Meanwhile, the attraction to the underlying gold never went away, becoming dominant once the repulsion fades out.

Hovering in alcohol

The result is an equilibrium that causes the gold flake to float about 45 nanometers above the teflon-coated gold. The distance is flexible, too. By changing the thickness of the teflon coating, the researchers were able to vary the separation from about 20nm to 45nm. That separation, however, was only an average. Over time, the gold flake would bounce up and down within about a 10nm range over the course of 20 minutes. But it was stable in the sense that there was no indication the gold flake was going to do anything beyond shift up and down slightly.

Before you get excited about riding a quantum hoverboard, it's important to emphasize that absolutely none of this scales up. You can't make bigger plates and get a larger repulsive force. In fact, as you make the gold plate bigger, the force generated by gravity operating on its increased mass will overwhelm the Casimir force, sending it floating down onto the surface. This is one of those things that you have to accept is just pretty damn cool, rather than a gateway to obvious practical applications.

That doesn't stop the authors from suggesting some less-than-obvious uses, such as "contact-free nanomechanical systems and controlled self-assembly." The latter is actually where this could turn out to be useful. As we've started exploring the value of various nanoparticles, the Casimir effect can actually be a problem, causing these tiny particles to aggregate rather than dispersing through a suspension. It is possible that we could design a tiny coating that preserves the nanoparticles' features while allowing them to repel each other if they get too close.

Science, 2019. DOI: 10.1126/science.aax0916 (About DOIs).