Historical research guide · 0875 · 1993
Extracting energy and heat from the vacuum
Daniel C. Cole · Harold E. Puthoff
In one minute
In 1984 Robert Forward described a small machine that draws electrical energy out of the vacuum: a charged, foliated conductor whose leaves are pulled together by the Casimir force against their own electrostatic repulsion, leaving the charge stored at a higher potential than it started with. Harold Puthoff later proposed a more plentiful route using a charged plasma, and raised the possibility of getting heat as well as work. Daniel Cole and Puthoff take up the obvious objection: the vacuum is a state of thermal equilibrium at absolute zero, so how can anything be drawn out of it without breaking thermodynamics? Their answer, worked in classical language, is that two different kinds of operation are being run together. Slow, reversible squeezes at absolute zero move no heat, and that much is right. But letting two plates fall together and then stopping them is irreversible, and irreversible operations do make heat. Track the entropy through both and the books balance. The second law holds and the proposals stand.
Connection
Chapter 6 depends on a question that has to be answered before any hardware matters: is drawing energy from the zero-point field forbidden by thermodynamics, or merely difficult? This is the paper that answers it in the affirmative in a mainstream physics journal, and it is careful to keep the two things chapter 2 insists on keeping apart — what the laws permit, and what an engineer can build.
From the source
The paper’s own verdict on the question it was written to settle: recent proposals for extracting energy and heat from electromagnetic zero-point radiation by means of the Casimir force are analysed, and the conclusion is that, yes, in principle, these proposals are correct. The authors are equally explicit about the boundary of that verdict — technological considerations for actual application and use are not examined.Abstract, page 1562
The apparent contradiction is a confusion of two different thermodynamic operations, and naming them dissolves it. Quasistatic displacements of van der Waals or Casimir systems are thermodynamically reversible, so no heat flows at absolute zero — that is the earlier result. Heat generation instead requires a thermodynamically irreversible operation, and an irreversible operation can produce heat even when the initial temperature is absolute zero.Page 1562, column 2, the paragraph beginning with the apparent contradiction
The mechanism is put as a thought experiment with an ordinary shape. Take a large supply of uncharged parallel-plate capacitors. The fluctuating but correlated induced charges on each pair, which arise from the zero-point plus thermal fields, pull the plates together. Let each pair collide, collect the usable heat, discard the plates and start the next pair. The fuel is the supply of capacitors and the spent capacitors are the exhaust, exactly as burnt fuel is the exhaust of an engine.Page 1562, column 2, the heat-generation thought experiment
Energy extraction and heat extraction are separated with care, and the separation is what makes the accounting work. Energy extraction means having the systems perform positive work — releasing two attracting bodies and letting them displace a movable stop or probe. Heat is a different quantity: at absolute zero it appears only in the irreversible case, where kinetic energy that did no work on the stop must be converted into electromagnetic radiation, which becomes heat once it is randomised.Page 1563, the discussion of energy extraction versus heat extraction
The second law is checked rather than assumed. The irreversible free contraction is compared with a reversible quasistatic contraction that ends in the same configuration, and the reversible path is then brought to the same final state by contact with a series of infinitesimally warmer reservoirs. Summing the heat over temperature along that path gives the entropy change of the irreversible process, and since the region outside the container does not change, the net entropy change of the universe comes out positive, in agreement with the second law.Pages 1563 to 1564, Figures 1 and 2 and the entropy argument
A concrete system is carried through to show how the curves would actually be computed: two fluctuating dipole harmonic oscillators in a conducting box, close enough together that the unretarded van der Waals interaction dominates, with the thermodynamic state fixed by just the separation and the temperature. Following the reversible adiabatic path from a start at absolute zero gives no change of temperature, so that path becomes a vertical line at absolute zero — the third law appearing in the geometry of the diagram.Page 1564, equations 1 through 4 and the accompanying discussion
The way in
https://doi.org/10.1103/PhysRevE.48.1562How to cite it
Daniel C. Cole, Harold E. Puthoff (1993) Extracting energy and heat from the vacuum doi:10.1103/PhysRevE.48.1562