Unraveling the mysteries of quantum vacuums
The mysteries of the quantum vacuum forge the hook upon which many popular speculations are hung. The quantum vacuum is linked with unresolved theoretical inconsistencies; for example the quantum field theory (QFT) estimates vacuum energy density roughly 10123 larger than the general relativistic estimate. Accordingly, new techniques to probe the quantum vacuum are eagerly awaited.
Two recent publications report experimental results on one such probe - the Dynamical Casimir Effect (DCE). In the DCE, a mirror accelerating in empty space loses energy by amplifying virtual photons into pairs of real photons.
I spoke with AAAS member G. Jordan Maclay (Quantum Fields LLC and Professor Emeritus of the University of Illinois at Chicago) about the DCE and the recent experiments. He pointed out that there are (at least) two ways of understanding DCE. The direct QFT model shows that rapid changes in the mirror's velocity result in nonadiabatic changes in quantum vacuum modes. Nonadiabatic effects amplify virtual photons into a pair of correlated real photons whose frequencies add up to the vibrational frequency of the mirror. The photon correlations and energies provide a signature of the DCE.
Maclay noted another way to understand the DCE. The electrical field at the mirror's surface is held equal to zero through the motion of screening electrons along the surface. If the mirror is moved periodically, an observer will see an oscillating surface charge accompanied by electromagnetic radiation.
The first DCE photons were observed by C.M. Wilson et al. DCE photons were generated by rapidly changing the electrical length of a superconducting waveguide by applying a ~10GHz magnetic field, thereby effectively oscillating a mirror at 0.05c. Observed photons exhibited the right energies and correlations to be DCE photon pairs. Maclay commented that, aside from a few issues arising from the non-ideal nature of the apparatus, this is a 'convincing, if not quite air-tight, demonstration of the DCE.'
The second publication by P. Lahteenmaki et al offers a different approach. They embedded an array of superconducting SQUIDs into a microwave cavity. A ~10 GHz magnetic field effectively produces a mirror velocity of 0.5c. The result was again photons with the correct energy and correlations to be DEC photon pairs.
These early results on the DCE appear consistent with QFT predictions. Not a surprising result, perhaps, as QFT deals very well with energy differences. However, any experimental results may provide a guide to deeper understanding of the controversial and seemingly paradoxical quantum vacuum and its interactions with the 'real' world.