Place two parallel plates close to each other in vacuum and a strange thing happens, as Dutch physicist Hendrik Casimir learned. The Casimir effect that he described draws the plates together, an effect that was successfully measured first in 1958 and, with greater precision, by Steve Lamoreaux in 1996. The effect becomes important at distances less than 100 nanometers. And if it seems like little more than a curiosity, be aware that Robert Forward looked at the possibilities of engineering to put this energy to use in an intriguing 1984 paper.
That paper (“Extracting Electrical Energy from the Vacuum by Cohesion of Charged Foliated Conductors” — see reference below) looks at the attraction between two parallel plates in a vacuum as the result of vacuum fluctuations of the electromagnetic field. As the two plates close on each other, longer electromagnetic waves no longer fit between them. The result: The total energy between the plates is less than the amount pushing them together from the vacuum that surrounds them. The Casimir effect increases as distance decreases, and if you get to atomic-scale distances, it has been measured at tons per square meter.
From a propulsion perspective, the idea that empty space contains energy in the form of these fluctuations of electric and magnetic fields is quite interesting, but we are in the earliest stages of understanding how the effect works. Interesting news, however, has just come out of Harvard University, where researchers have experimented with replacing the vacuum with a fluid. They were then able to measure a repulsive form of the Casimir effect. Working with a gold-coated microsphere attached to a mechanical cantilever in a liquid, the team measured its deflection as they varied the distance from a nearby silica plate.
Image: This is an artist’s rendition of how the repulsive Casimir-Lifshitz force between suitable materials in a fluid can be used to quantum mechanically levitate a small object of density greater than the liquid. Figures are not drawn to scale. In the foreground a gold sphere, immersed in Bromobenzene, levitates above a silica plate. Background: when the plate is replaced by one of gold levitation is impossible because the Casimir-Lifshitz force is always attractive between identical materials. Image courtesy of the Capasso lab. Credit: Harvard University.
A repulsive Casimir effect gets us into some practical uses for these strange phenomena, as Federico Capasso (Harvard School of Engineering and Applied Science) points out:
“Repulsive Casimir forces are of great interest since they can be used in new ultra-sensitive force and torque sensors to levitate an object immersed in a fluid at nanometric distances above a surface. Further, these objects are free to rotate or translate relative to each other with minimal static friction because their surfaces never come into direct contact.”
By contrast, attractive Casimir forces could obviously hamper extreme miniaturization. At the nanoscale, then, developing bearings that use this ‘quantum levitation’ is helpful in reducing friction among tiny components. Nanotechnology itself is a major interstellar driver — if we can reduce payload sizes down to nanotechnological scales, the propulsion problem becomes much more tractable. We can envision sending, for example, tiny probes that can use assembler technology upon arrival to build a robotic research station in a new planetary system, needing to accelerate far less mass than that of a conventional probe.
To the extent, then, that this work takes us deeper into a workable nanotechnology, it becomes useful for interstellar purposes. But it is also notable in that it suggests possible (though highly problematic) long-range uses of Casimir forces in propulsion, as Brian Wang notes in NextBigFuture. Brian flags Jordan Maclay’s “Study of Vacuum Energy Physics for Breakthrough Propulsion,” available here, which grew out of work for the Breakthrough Propulsion Physics project, and which takes a detailed look at ways this energy might be exploited.
The paper is Munday et al., “Measured long-range repulsive Casimir–Lifshitz forces,” Nature 457 (8 January 2009), pp. 170-173 (abstract). A Harvard news release is available. The reference for the Robert Forward paper is “Extracting Electrical Energy from the Vacuum by Cohesion of Charged Foliated Conductors,” Physical Review B 30, no. 4 (August 1984), pp. 1770–73.