Historical research guide · 1030 · 1984
Extracting electrical energy from the vacuum by cohesion of charged foliated conductors
Robert L. Forward
In one minute
Casimir predicted in 1948 that two conducting plates a whisker apart are pushed together by the zero-point fluctuations of the vacuum. Robert Forward, writing from Hughes Research Laboratories and the Air Force Rocket Propulsion Laboratory, asks the engineer’s question: can that pull be made to do useful work? His answer is a device he calls a vacuum-fluctuation battery. Stack ultrathin aluminium leaves a few micrometres apart, put the same positive charge on every leaf so that they repel each other, then trim the voltage until the electrostatic push is a shade weaker than the Casimir pull. The leaves drift together, the Casimir force does work against the electric field between them, and an active power supply takes that work out as electricity. Raise the voltage again and the stack re-expands, so the battery recharges. Forward’s own analogy is a hydroelectric dam: the foliated conductor is water held high, the vacuum field is the catalyst, and the servo-controlled leaves are the turbines.
Connection
This is the founding engineering proposal of chapter 6 — the first time the Casimir force was written down as a power-cycle component rather than a curiosity, by a working propulsion physicist with a contract number on the paper. Chapter 2 gains from it the precise boundary Forward himself drew: the vacuum field is almost certainly conservative, so what he designs is a battery that stores and returns energy, and the open question is what happens at plate separations nobody had yet measured.
From the source
The Casimir attraction is treated as a fundamental force of the vacuum rather than a material effect: it exists between any pair of conductors, it is independent of the metal the plates are made of, it is purely attractive, and it falls off as an inverse power of the separation — properties the paper explicitly compares with gravity. Forward quotes the standard reading of it as a zero-point pressure of electromagnetic waves, and notes that Lifshitz later extended the calculation to dielectrics.Section I, Casimir force, page 1700
The architecture is stated as an analogy that keeps the thermodynamics honest. A conductor prepared in a foliated state sits in a high vacuum-fluctuation potential-energy state because of its large surface energy; the Casimir force then converts that potential energy into kinetic energy as the leaves cohere into a solid block in a low state. Forward’s comparison is a hydroelectric dam, where gravity is the catalyst and the water’s starting height is the stored quantity — and where the dam is also used to store energy by pumping the water back up.Section on the vacuum-fluctuation battery, page 1701, the hydroelectric-dam passage
The device itself is specified. A large number of leaves of ultrathin aluminium foil are stacked a few micrometres apart; each leaf is wired to an active bidirectional power supply and its shape and position are watched by sensors; the supply puts a small positive charge on every leaf so that electrostatic repulsion holds the stack open against the Casimir pull. That suspension is unstable, so each leaf is stabilised electronically by feedback from the position sensors, with the option of insulating frames of a material such as aluminium oxide giving partial mechanical support.Section on the vacuum-fluctuation battery, page 1701
The cycle is run one leaf pair at a time, or all at once. Lower the voltage between the end leaf and its neighbour until the electrostatic repulsion is slightly less than the Casimir force at that distance; the two leaves are drawn together and do work against the repulsive field; by keeping the electric field always slightly below the Casimir force the bidirectional supply extracts electrical energy as the pair moves from wide separation to minimum separation. Repeat leaf by leaf until the foliated conductor is condensed into a solid block, or bring all the leaves together at once like compressing an accordion.Section on the vacuum-fluctuation battery, page 1701 to 1702
It is a battery, and Forward says so in the paper’s own terms. If the collapse is halted before the aluminium films cohere, the device is recharged by making the applied electrostatic force slightly larger than the Casimir force, pushing the leaves apart at the cost of energy supplied from the bidirectional power supply. He also gives a second geometry that would be easier to fabricate and more stable — a wide flat spiral of foil built along the lines of a Slinky toy, where each turn acts against its neighbours so that only one conductor has to be contacted, and the spacing stays uniform through a large compaction.Page 1702 and Figure 1
Two open questions are named, and both are experimental. First, no rigorous proof is known that the vacuum-fluctuation field is conservative like the gravity field; Forward judges it highly probable that it is, on the ground that otherwise one could design Casimir machines from which an infinite amount of energy could be extracted. Second, there were then no good measurements on metal plates in the one-to-twenty-nanometre range where theory predicts the crossover from the retarded law to the much stronger nonretarded van der Waals law — and he argues that molecular-beam-epitaxy monolayer deposition had just made plates flat enough to run that experiment below eight nanometres, where, if the very strong predicted forces appear, they may be a new source of energy.Section II, Experimental verification of the Casimir force, pages 1700 to 1701, and the conservative-field paragraph
The way in
https://doi.org/10.1103/physrevb.30.1700How to cite it
Robert L. Forward (1984) Extracting electrical energy from the vacuum by cohesion of charged foliated conductors doi:10.1103/physrevb.30.1700