Science and technology of the Casimir effect
Date: Sunday, January 03, 2021 @ 14:45:15 GMT
Topic: Science

From Physics Today: Science and technology of the Casimir effect (extracts)
Repulsion, torque, and dynamical effects

...Those experimental and theoretical results are more than just demonstrations. They point to future work in which the Casimir force can be used to manipulate nanoscale objects. In MEMS devices, high surface-to-volume ratios often result in unwanted stiction that could be mitigated with a repulsive Casimir interaction. What’s more, by producing attractive and repulsive forces and torque at the nanoscale, one can create, at least conceptually, a micro-tractor beam for moving quantum dots, nanowires, bacteria, viruses, and other minuscule objects...

...In the dynamic Casimir effect (DCE), photons are created by a rapid change in a system parameter, such as an electromagnetic boundary condition. For example, a mirror in an optical cavity moving rapidly at a frequency f generates pairs of photons with frequency f/2 from the vacuum. Moving a mirror at relativistic speeds is no mean feat, and researchers have relied on changing another system parameter such as the index of refraction instead. The effect has been seen in superconducting circuits, a Josephson metamaterial, a Bose–Einstein condensate, and photonic crystal fibers.

How is this related to the static Casimir effect? Imagine a mirror moving slowly. The quantum fluctuations can easily keep up with the mirror, and their energy, stored in the modes of a cavity, can give rise to attractive or repulsive forces. If the mirror is accelerated to relativistic speed, the virtual particles that pop into existence get separated from their partners and produce real photon pairs. The dynamic analogue is a way to essentially mine the fluctuations by stripping photons from the pairs. In the static Casimir effect, the fluctuations produce a force; in the DCE, they produce photons...

Searching for the Casimir energy

The Casimir effect emerges from fluctuations of the quantum vacuum, but its details depend directly on the nature of the materials that make up the Casimir cavity. Those details thus involve the coupling between the electromagnetic field and the walls. In the conventional Casimir effect between two perfect conductors separated by a vacuum, the positive energy density of the modes inside the cavity is less than that outside the cavity. An important question is, Can that difference in energy—the Casimir energy—be directly detected, and if so can it be exploited to reveal any novel physical phenomena?

In 1988 Michael Morris, Kip Thorne, and Ulvi Yurtsever speculated that this Casimir energy vacuum could be used to stabilize the existence of a wormhole and thus lead to the possibility of superluminal travel.12 The Casimir force also has been invoked in connection with the cosmological-constant problem—the so-called vacuum catastrophe—and dark energy in the universe. But the wide discrepancy between the estimates of the background energy density of the universe and the energy density that would result from naïve calculations of the quantum vacuum energy fluctuations remains unresolved.

Furthermore, the Casimir effect can be formulated and Casimir forces computed without reference to zero-point fluctuations. Hence, experimentalists hope to be able to measure a physical effect that can be attributed unambiguously to the existence of the Casimir energy in order to confirm the existence of what has to date simply been used as a theorist’s tool.

...the search for the Casimir energy is an active area of research.

Although daunting, such experiments may bear on other unresolved issues of fundamental physics. Indeed, if the Casimir energy exists and can alter phenomena such as the temperature at which a phase transition occurs, then an entirely new range of devices and technologies may emerge...

For a short survey of the first 60 years of research on the Casimir effect, see the article by Steve Lamoreaux, Physics Today, February 2007, page 40.

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