Vacuum is what remains when there is nothing left in space. But this "nothing" is still able to act on objects placed in vacuum. The archetype of such an action of vacuum is the so-called Casimir force.

In fact vacuum consists in fluctuating electromagnetic fields propagating through space with the speed of light, and having the minimal fluctuation energy allowed by quantum theory. Since vacuum energy is a minimum, it cannot be used to extract work. But the fluctuations have observable consequences in atomic and subatomic physics. An atom interacting only with vacuum fields suffers spontaneous emission processes which can be considered as induced by these fields. When fallen in its ground state, the atom no longer emits photons but its coupling to vacuum results in a measurable Lamb shift of the absorption frequencies.

Two atoms located at different places experience an attractive van der Waals force which plays an important role in physico-chemical and biological processes. Casimir was studying this effect when he discovered in 1948 that two mirrors placed in vacuum are attracted towards each other.

There are many reasons for the growing interest in this Casimir force. Progress in experimental techniques have recently allowed for accurate measurements and have then led to tests of this crucial prediction of quantum electrodynamics. These tests have required theoretical developments for dealing satisfactory with the description of the optical response of mirrors or of geometry of the experiments. They have also made it possible to extend the search for hypothetical new weak forces into the distance range where the Casimir force dominates over other forces.

Nevertheless, in spite of all this progress and renewal interest on the subject there are still basic questions that have not been answered and extensions that have not been achieved, for example a consistent treatment of temperature effects or a full theory of geometrical effects between mirrors made of real materials.

Recently it has been recognized that vacuum induced forces played a considerable role in micro- and nano-electromechanical systems (MEMS or NEMS) with typical sizes in the µm range or below. The emerging interface between quantum Casimir forces and nanophysics is already clearly visible in this Focus Issue, and it will certainly gain more and more importance in the forthcoming years.

The topics covered by the articles in this Focus Issue of New Journal of Physics reflect the increasing activity and broadening spectrum of the studies connected to the Casimir forces. We hope that the reader will find in this collection of papers new research results or proposals as well as reviews by experts of a large set of open issues which are currently addressed by an expanding community. We also hope that the issue will interest the non-expert readers and draw their attention to this exciting and interdisciplinary research field.

The articles below represent the first contributions and further additions will appear.

Focus on Casimir Forces Contents

Casimir energy with a Robin boundary: the multiple-reflection cylinder-kernel expansion
Z Liu and S A Fulling

On the use of hydrogen switchable mirrors in Casimir force experiments
Sven de Man and Davide Iannuzzi

Thermal corrections to the Casimir effect
Iver Brevik, Simen Ellingsen and K Milton

Casimir forces and non-Newtonian gravitation
R Onofrio

Sample dependence of the Casimir force
I Pirozhenko, A Lambrecht and V B Svetovoy

Casimir effect: a novel experimental approach at large separation
P Antonini, G Bressi, G Carugno, Giuseppe Galeazzi, Giuseppe Messineo and G Ruoso

Exact zero-point interaction energy between cylinders
F D Mazzitelli, D A R Dalvit and F C Lombardo

Stability and the proximity theorem in Casimir actuated nano devices
R Esquivel-Sirvent, Luis Reyes and Jeffrey Bárcenas

Casimir effect for arbitrary materials: contributions within and beyond the light cone
W L Mochán and C Villarreal

The Casimir effect within scattering theory
A Lambrecht, Neto P A Maia and S Reynaud

Quantum electrodynamical torques in the presence of Brownian motion
Jeremy N Munday, Davide Iannuzzi and F Capasso

Rubén G Barrera, Universidad Nacional Autónoma de México, Mexico
Serge Reynaud, Université Pierre et Marie Curie, Paris, France