The phenomenon relies on the long-established fact that empty space is not at all empty, but fizzing with particles that pop in and out of existence (see “Out of the ether: the changing face of the vacuum”). This is down to the laws of quantum mechanics, which say that even a vaccum cannot have exactly zero energy but must exhibit small fluctuations of energy. These fluctuations show themselves as pairs of short-lived particles.
The presence of these “virtual” particles, usually photons, has long been proved in experiments demonstrating the standard Casimir effect, in which two parallel mirrors set close together will feel a pull towards each other. This happens because the small space between the mirrors limits the number of virtual photons that can appear in this region. Since there are more photons outside this space, the radiation pressure on the mirrors from the outside is larger than the pressure between them, which pushes the mirrors together.
Now Chris Wilson at Chalmers University of Technology in Gothenburg, Sweden, and his colleagues have gone a step further, pulling photons out of the void in a process called the dynamical Casimir effect. “It was a difficult technical experiment,” says Wilson. “We were very happy when it worked.”