Historical research guide · 0468 · 2006
Casimir forces and non-Newtonian gravitation
Roberto Onofrio
In one minute
Roberto Onofrio, who works at Dartmouth College and the University of Padova, reviews a measurement problem that sits exactly where two frontiers meet. Several routes to unifying gravity with the other forces predict an extra, short-range force alongside Newton’s — parametrised by a range and a strength — and the micrometre scale is where the predicted window is still open. But at that scale the dominant force between two surfaces is the Casimir force, the attraction that quantum vacuum fluctuations produce between conductors. So any hunt for new gravity in the micrometre range is really a contest with Casimir metrology. Onofrio surveys the six successful Casimir measurements, the limits on new forces they set, and the systematic effects that can imitate a signal. He then proposes a way to compare experiments of different geometries: convert each measurement into a determination of the constant formed from the reduced Planck constant and the speed of light, then compare it with the value known to far higher precision from microscopic physics.
Connection
Chapter 2 rests on the Casimir force being measured, and this is the paper that says exactly how well it is measured and what still limits that accuracy — the finite-temperature correction above all. It also gives chapter 1 a clean piece of evidence discipline: turn every experiment, whatever its geometry, into one number that can be checked against an independently known value, so a claim of new physics has to survive a comparison it cannot arrange for itself.
From the source
The Casimir force — a macroscopic force originating from quantum vacuum fluctuations, predicted in 1948 alongside the measurement of the Lamb shift and the invention of renormalisation — has been solidly demonstrated by the last decade of experiments, in configurations spanning parallel plates, sphere-plane and dielectric surfaces.Section II, Casimir forces and quantum vacuum; Section III; Abstract
A short-range force added to Newtonian gravity is parametrised by a Yukawa range and a coupling strength relative to gravity, and such forces are expected in many attempts to unify gravity with the other interactions. Even a minimal unification argument predicts them: if spontaneous symmetry breaking is the universal path, it should produce intermediate gauge bosons with couplings between gravity and the other interactions, exactly as electroweak unification produced the neutral Z boson.Section I, Introduction, equation (1) and Figure 1
In the micrometre range the Casimir force, or its variants for dielectric rather than conducting materials, is the major expected background to any search for a gravitational-strength short-range force. Constraints on extra-gravitational forces in the submillimetre range do not match the far more stringent limits at intermediate distances, which is why this window remains open.Abstract; Section I, Introduction
Onofrio proposes a geometry-independent way to compare all the data: once every correction is accounted for, each Casimir force measurement amounts to a determination of the product of the reduced Planck constant and the speed of light, which can then be compared against the far more precise value known from microscopic measurements. A significant deviation between the two would be the signature of new physics — and the same protocol supports blind tests and open data sharing between groups.Abstract; Section IV, Precision and accuracy in Casimir force experiments; Section VI, Conclusions
Three lines of hardware are named as the next generation. Cryogenic atomic force microscopy aiming at a sensitivity of the order of ten to the minus eighteen newtons per square root hertz; improved torsion balances, under development at Los Alamos for the sphere-plane geometry and at the Institut Laue-Langevin in Grenoble for parallel plates; and the cylinder-plane geometry at Dartmouth, whose first data acquisition on an upgraded apparatus aimed at the two to five micrometre range was underway as the review was written.Section V, Future experimental directions
No Casimir force measurement so far shows, within its claimed accuracy, any residual signalling non-Newtonian physics — and the urgent open item is the finite-temperature correction, without a solid understanding of which the limits on non-Newtonian forces remain weak. Onofrio expects a definitive result to come from complementary evidence across radically different classes of experiment operating at different length scales, naming neutron states in the Earth’s gravitational field, surface excitations in superfluid helium, and nanotube dynamics.Section VI, Conclusions
The way in
https://doi.org/10.1088/1367-2630/8/10/237How to cite it
Roberto Onofrio (2006) Casimir forces and non-Newtonian gravitation doi:10.1088/1367-2630/8/10/237