Casimir Physics

Quantum theory has profoundly altered our conception of empty space by forcing us to consider vacuum as the realm of quantum field fluctuations. Even a perfect vacuum at zero temperature has fluctuating fields known as 'vacuum fluctuations', with a mean energy corresponding to half the energy of a photon. This leads to an extremely high vacuum energy density which we do not observe in our universe, but poses a fundamental problem for any attempt to unify quantum theory with gravity. Surprisingly, the same quantum phenomenon results in potentially very useful forces, e.g. for micro/nano-electromechanical systems (MEMS/NEMS) at microscopic distances, the archetype of which is the Casimir force. Predicted by Hendrik Casimir in 1948, it is an attractive force between two ideal metallic plates that increases rapidly when the latter approach below sub-micron distances. Fundamentally, the Casimir effect comes into play when probing non-Newtonian gravity at sub-micron distances. The unsolved connection between the Casimir energy and the cosmological constant, the Casimir momentum and quantum friction, is a topic at the very foundations of our understanding of physics. Another fascinating phenomenon occurs when the two plates are vibrating in vacuum, where photon pair emission by the moving plates has been predicted, converting mechanical motion into photons. This so-called dynamical Casimir effect (DCE) has recently been observed in transmission lines by emulating the motion of the mirrors via periodic changes of the optical length of the electromagnetic field.

Casimir forces between macroscopic surfaces have the same physical origin as atom–surface interactions (Casimir–Polder forces) and van der Waals (vdW) forces between two atoms or molecules. In general, Casimir forces result from selective confinement of long-range fluctuations of any field, regardless of its quantum or classical nature, which are encountered in a variety of systems, such as in mixed liquids where it is named the critical Casimir effect, or in the vdW interactions between protein molecules or cellular membranes. vdW forces are crucial in bioassembling including virus-like type objects. Casimir forces also have important technological potential for applications in MEMS/NEMS (switches, accelerometers, quantum levitation systems, etc): they have surface areas large enough but gaps sufficiently small for the Casimir force to draw components together and affect their actuation dynamics. For interacting bodies with geometrically nanostructured surfaces or bulk anisotropic bodies, the Casimir force also shows a lateral component and long-range torques which could both be used for contact-less force transmission in MEMS/NEMS.

Casimir physics embraces today fundamental topics, ranging from the role of quantum vacuum in fundamental physics and cosmology, the static and dynamic properties of the quantum and classical Casimir effect, atom–surface interactions in the physics of ultracold and thermal (Rydberg) atoms and molecules, to non-equilibrium phenomena and vdW forces in biological and soft-matter systems. The Casimir field has seen very rapid development in the last few years, leading to the formation in Europe of the recent ESF RNP CASIMIR (2008–2013) of 60 European groups with different expertise and close interactions/collaborations with leading groups in the Americas and Asia, and the DARPA program on Casimir Physics in the USA. Moreover, an increasing number of precise measurements of Casimir and Casimir-like effects have been performed using modern experimental techniques, while complementing theoretical approaches have tackled fundamental physics questions and provided insight into opportunities for novel applications for MEMS/NEMS architectures, optoelectronic systems, superadhesion (Z-Man/Gecko project), and atom chips. Casimir physics and fluctuation-induced forces find applications in various domains of physics and related disciplines. Indeed, the precise laws governing the long-range interactions between atoms, molecules, clusters, nano- and bio-assemblies or surfaces amongst each other, immersed in vacuum, in air or in a liquid, are important not only for physics but also for biological and chemical processes. Understanding the Casimir force in a whole variety of flexibly shaped boundaries will open novel techniques in the engineering of nano-mechanical systems.

The end of the ESF RNP CASIMIR and the DARPA program on Casimir Physics, as well as the start of a new era of the dedicated international Conference Series 'Casimir Physics', with the most recent in 2014 at École de Physique des Houches (France), have inspired this special issue showcasing cutting edge research in the field of Casimir physics. Although as Guest Editors we would have wished to have a wide range of research topics covering the whole interdisciplinary framework of Casimir Physics, the focus of the journal was limited to papers related only to condensed matter physics, which unfortunately left out several main actors in the field. Still, we hope that this special issue will serve as a reference source for those wanting to learn some of the most recent advances in the field.