An ingenious experiment in which tiny parcels of light, or photons, are produced out of empty space has confirmed a long-standing theory that a vacuum contains quantum fluctuations of energy.
In a landmark result published in the journal Nature, an international team of researchers has demonstrated for the first time a strange phenomenon known as the dynamical Casimir effect, or DCE for short.
The DCE involves stimulating the vacuum to shed some of the myriad “virtual” particles that fleet in and out of existence, making them real and detectable. Moreover, the real photons produced by the DCE in their experiment collectively retain a peculiar quantum signature that ordinary light lacks.
The static Casimir effect was first predicted decades ago by physicist Hendrik Casimir: it refers to a force that becomes apparent at small distances when two surfaces – for example, mirrors or metal plates – are placed so close together that some of the virtual particles are squeezed out. The result is a miniscule pressure pushing the two surfaces together, which had been measured by other researchers.
The new study, however, led by Chris Wilson of Chalmers University of Technology in Sweden, along with Professor Tim Duty of the UNSW School of Physics in Sydney, shows that a related dynamic effect can occur when such a mirror moves very fast through the vacuum. The DCE was predicted over 40 years ago, but had not yet been observed experimentally due to the difficulty of creating the required experimental conditions.
"The DCE was conceived as a kind of thought experiment, sort of like Schrödinger’s Cat," notes Professor Duty. “According to quantum theory, if one could accelerate a mirror very quickly to near the speed of light, the mirror would radiate light as some of the mirror’s motional energy is imparted to virtual photons lurking in the vacuum, converting them into real photons.
“But it is practically impossible to accelerate a massive mirror to such high velocities. The required accelerations would be greater than the kind of shocks found in supernova or nuclear weapons explosions.”
Instead, Professor Duty and collaborators set out to demonstrate the DCE using microwaves, like those used for mobile phone and wireless communication signals. And instead of a massive mirror, they used a tiny microcircuit called a Superconducting Quantum Interference Device, or SQUID. The SQUID acts as a tunable mirror for virtual microwave photons, fooling them into behaving as if they encountered a moving mirror when in fact nothing is physically moving. Furthermore, they had to cool the experiment to a small fraction of a degree above absolute zero in order to get rid of unwanted thermal microwaves that would mask the DCE.
“The fact that the quantum vacuum is not empty, as demonstrated in our experiment, is related to lots of other interesting effects such as Hawking radiation of black holes and the Lamb shift in atomic physics”, says Professor Duty.
Professor Duty is an ARC Future Fellow and is involved in the ARC Centre of Excellence for Engineered Quantum Systems. He is building up a new laboratory at UNSW where he plans to pursue further research into the dynamical Casimir Effect and related problems.