Demonstration of the Casimir Force in the 0.6–6 μm Range
Steve K. Lamoreaux
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In one page
Casimir predicted in 1948 that two parallel conducting plates in empty space would pull toward each other, because the plates exclude some of the electromagnetic modes that would otherwise fill the gap between them. For nearly fifty years there had been exactly one attempt to measure it, and that one carried effectively a hundred per cent uncertainty. Steve Lamoreaux, then at the University of Washington, settled the question. He tried parallel plates first and gave up: holding two centimetre-wide plates parallel to within ten microradians is not practical. So he used a sphere against a flat — a quartz optical flat and a lens of 11.3 centimetre radius of curvature, each coated with copper and then with gold — hung the flat from a torsion pendulum inside a rough vacuum, and ran a feedback loop that held the pendulum angle fixed while he measured the voltage the loop needed. Across separations from 0.6 to 6 micrometres the measured force matched theory to about five per cent, with no adjustable parameters.
Why it matters hereChapter 2 rests on the vacuum being a real, structured medium rather than a bookkeeping convenience, and this is the measurement that put it on a laboratory bench; the detail chapter 6 needs is buried in the introduction, where Lamoreaux notes that the sign of the Casimir force depends on geometry.
What it claims
01The vacuum stress between closely spaced conducting surfaces, arising from the modification of the zero-point fluctuations of the electromagnetic field, has been conclusively demonstrated, with agreement with theory at the level of five per cent.Abstract; Conclusion
Settled physics02Before this measurement there had been only one attempt at measuring the effect — Sparnaay in 1958 — which showed an attractive force not inconsistent with the prediction but with effectively one hundred per cent uncertainty.Introduction
Settled physics03The geometry was chosen to sidestep the problem that had defeated earlier work: parallel-plate attempts failed because parallelism must be held to about ten microradians for centimetre-scale plates, whereas with one spherical surface the system is described simply by the separation at closest approach, and the proximity force theorem makes the force proportional to the sphere’s radius of curvature and independent of plate area.Introduction, Eqs. 1–2
Settled physics04The apparatus was an electromechanical system built on a torsion pendulum: a 2.54 cm diameter quartz optical flat and a 4 cm lens of 11.3 cm radius of curvature, each evaporation-coated with copper and then gold on the facing surfaces, the flat mounted on one arm of the pendulum and the sphere on a micropositioning tripod, in a vacuum of order ten to the minus four torr, with a feedback loop holding the pendulum angle fixed.Apparatus section, Figs. 1 and 2
Settled physics05The Casimir force is not merely van der Waals attraction under another name: the van der Waals force is always attractive, whereas the sign of the Casimir force depends on geometry — a thin spherical conducting shell cut in half yields two hemispheres that repel each other.Introduction
Settled physics06Lamoreaux is explicit about what the measurement does not yet reach: the data are not accurate enough to demonstrate the finite-temperature correction, and the closest approach of about 0.6 micrometres may be limited either by dirt on the surfaces or by an instability in the feedback system rather than by anything fundamental. Pushing below half a micrometre, where the force rises steeply, is the next measurement.Conclusion; Fig. 4(b)
What to watch
The way in
https://link.aps.org/doi/10.1103/PhysRevLett.78.5Published by the American Physical Society and held in copyright, so this page carries a summary; the record is indexed at NASA ADS and the paper is one of the most widely reproduced measurements in the field.
How to cite it
Steve K. Lamoreaux (1997) Demonstration of the Casimir Force in the 0.6–6 μm Range. doi:10.1103/PhysRevLett.78.5
Where it sits in the curriculum