Vacuum Fluctuations

Dear Colleagues,

Quantum electrodynamics (QED) describes the presence of a field of vacuum fluctuations in the quantized electromagnetic field. This fundamental field is present everywhere, even in the absence of matter and at absolute zero temperature. It consists of virtual photon creation and annihilation processes.  The average value of the field is 0, but is not zero. This field is responsible for the Lamb shift and other phenomena in the QED that have been experimentally verified to 14 decimal places, the most precise predictions in modern science. The virtual electron–positron field is often included as a vacuum fluctuation. It produces a much smaller shifts in the atomic energy level due to the polarization of the vacuum. It is the source of Hawking radiation.

All quantum fields have vacuum fluctuations, but the effective distance of their influence decreases with the associated mass. Hence, the electromagnetic field has the greatest influence in most environments. In elementary particle physics, the vacuum fluctuations of the gluon and Higgs field are dominant.

One of the conundrums about vacuum energy is that QED predicts that the energy density of the quantum vacuum fluctuations is formally infinite or near infinite, which conflicts with General Relativity. There are several theories that may explain this disagreement. On the other hand, in all calculations to date of physically measureable phenomena, the changes in vacuum energy, which tend to be small, have precisely corresponded to predicted and measureable quantities.

The vacuum fluctuations of the electromagnetic field explain the origin of van der Waals forces and other dispersion forces, Casimir forces, and the atomic emission linewidth. Modifications in the boundary conditions of the vacuum fluctuations have been used, for example, to control spontaneous emission, to alter dispersion forces, to generate torques and repulsive forces, to enhance chemical reactions, to transfer heat in the vacuum, to transfer mechanical energy from one oscillator to an adjacent oscillator by the modulation of Casimir forces, and to tune attractive and repulsive forces in microelectromechanical systems.  Vacuum fluctuations have also been linked to cosmological phenomena, for example, to dark energy, to the Big Bang, to inflation, and to the Cosmic Microwave Background radiation. Vacuum fluctuations have been used to induce entanglement. In some systems, vacuum fluctuations are a source of noise and may limit performance, for example, in quantum computation, or cause decoherence in electron diffraction.

Recent efforts are providing direct measurements of spectral regions of the virtual vacuum field using a variety of techniques.

The role of vacuum fluctuations can be enhanced in microcavity electrodynamics with subcycle control, where the strong coupling of matter to the vacuum fields has been achieved, for example, with graphene. New quantum states of matter, quantum cavity chemistry, cavity-controlled transport (for example, inducing a THz energy gap in the band structure of a carbon nanotube), and vacuum-modified superconductivity have been investigated.

It is often said that one cannot extract energy from the free quantum vacuum because it is the lowest state of the electromagnetic field. This statement is generally true but ignores some important nuances, for example, the stochastic transfer of energy. Theory indicates that charged particles in the vacuum will show Brownian motion. Vacuum fluctuations in the electromagnetic field induce current fluctuations in resistively shunted Josephson junctions that are measurable in terms of a physically relevant power spectrum, and vacuum fluctuations have been used to induce a persistent current in a quantum ring.  One can reduce the energy density of the free vacuum by modifying the vacuum field, for example, by means of surfaces and therefore allow energy transfer. The attractive Casimir force between two metal surfaces could, in principle, perform positive work on the surfaces, moving them together quasistatically.  One predicts theoretically that the energy density of the vacuum field between the plates would be reduced correspondingly to conserve energy. Unfortunately, this experiment, which would demonstrate the direct exchange of vacuum energy with mechanical energy, has not been conducted as far as I know.

A device has been described recently for which it is suggested that the boundary conditions of the vacuum field at a thin metal surface result in the transfer of energy from the vacuum fluctuations to electrons in the metal, injecting some into a dielectric and producing a small current. A theoretical calculation showing fluctuations in the kinetic energy of an electron near a surface supports this interpretation. It appears that under certain conditions, the exchange of quantum vacuum energy with mechanical or other forms of energy may be possible. The dynamical Casimir effect and the Unruh effect involve excitation of the vacuum field by mechanical means. Other dynamical vacuum excitation effects have been proposed. The recent demonstrations of the transfer of mechanical and thermal energy through the quantum vacuum also seem to support this possibility.

Over the past few decades, the role and significance of vacuum fluctuations have increased significantly as researchers have developed novel applications.  With their pervasive role in physical phenomena, there is an opportunity to take advantage of this universal, tunable, vacuum field to accomplish scientific and engineering objectives with vacuum engineering.

Prof. Dr. G. Jordan Maclay
Prof. Dr. Roberto Passante
Guest Editors

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