Thermal noise limitations on micromechanical experiments

Thermal motions of microscopic probes limit the possibilities of experiments that are designed to resolve single-macromolecule dynamics in aqueous conditions. We investigate theoretical strategies for maximizing signal-to-noise ratios or resolution in typical situations, illustrating our discussion with examples from optical tweeters and atomic force microscopy experiments. A central result is that the viscous drag on a micromechanical probe is more important than the compliance of the probe. Within limits, increased stiffness of an AFM cantilever or of an optical trap does not increase resolution, and decreased stiffness does not provide the possibility of less invasive measurements.