Historical research guide · 0048 · 1998
Precision measurement of the Casimir force (AFM)
Mohideen · Roy
In one minute
Hendrik Casimir predicted in 1948 that two conducting surfaces facing each other across empty space must attract, because not all of the vacuum's own field modes fit into the gap between them. Umar Mohideen and Anushree Roy measured it. They fixed a polished sphere 196 microns across to the tip of an atomic force microscope cantilever, coated the sphere and a sapphire plate with 300 nanometres of aluminium and a thin gold-palladium layer, and brought them together from 0.9 microns down to 0.1, recording the force at 256 separations. The measurement matches theory to a root-mean-square deviation of 1.6 piconewtons — one percent at the closest approach — and it is sharp enough to resolve the corrections for the metal's finite conductivity and its surface roughness, which the bare Casimir formula leaves out. They repeated the run with different cantilevers, spheres and plates.
Connection
This is a second independent instrument measuring the Casimir force, a year after Lamoreaux, in a different laboratory by a different method — which is what carries the structured vacuum from prediction to settled physics on chapter 1's ladder. Everything built in chapter 6 rests on a force measured this well.
From the source
An atomic force microscope was used to make precision measurements of the Casimir force between a metallised sphere of 196 microns diameter and a flat plate, for sphere-plate separations from 0.1 to 0.9 microns.Abstract
The experimental results are consistent with present theoretical calculations once the finite conductivity, roughness and temperature corrections are included; the root-mean-square deviation between theory and experiment over 256 data points is 1.6 piconewtons, a 1 percent deviation at the smallest separation.Abstract; results and Figure 4
The measurement is precise enough to distinguish the corrections themselves: the bare Casimir force without any of them deviates by 6.3 piconewtons (5 percent at the smallest separation), the finite-conductivity correction alone leaves 5.5 piconewtons, and the roughness correction alone leaves 48 piconewtons, a 40 percent deviation at the closest separation.Results; Figure 4
The sphere and the optically polished sapphire plate were coated with 300 nanometres of aluminium, chosen for its high reflectivity at the relevant wavelengths, then with a layer under 20 nanometres of 60 percent gold and 40 percent palladium to prevent space-charge effects from patch oxidation of the aluminium.Experimental method
The experiment was repeated for different cantilevers, spheres and plates.Results
With lithographically fabricated cantilevers of large radius of curvature, interferometric detection of the cantilever deflection and lower temperatures to reduce thermal noise, the authors judge that a factor of over 1000 improvement in precision is possible with this technique — enough, in their words, for careful checks of the mechanical properties of vacuum.Conclusion
The way in
https://arxiv.org/abs/physics/9805038How to cite it
Mohideen, Roy (1998) Precision measurement of the Casimir force (AFM) doi:10.1103/PhysRevLett.81.4549