PerspectivePhysics

Brownian Motion Goes Ballistic

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Science  13 May 2011:
Vol. 332, Issue 6031, pp. 802-803
DOI: 10.1126/science.1192222

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Summary

When Einstein explained the origin of Brownian motion in 1905, he described the erratic movement of a microscopic particle driven by the thermal motion of liquid molecules as a random walk with sharp changes of direction between each step (1). He realized that this picture—the one we seem to see if we watch a particle under the microscope—must break down if we were to look more closely. A moving object would require an infinite force to change its speed or direction discontinuously. The particle actually moves “ballistically” along a smooth trajectory (24), as if it were a microscopic ocean liner on an erratic course (see the first figure, panels A and B). It has taken more than a century to observe this ballistic motion. The studies of Li et al. (5) were conducted on particles in air; its low viscosity allowed ballistic motion to be followed accurately for extended periods and showed that a particle's instantaneous velocities along its path obey a statistical distribution consistent with thermal motion. Huang et al. (6) used liquid water; in this higher-density medium, the transition from ballistic motion at short times to diffusive motion at longer ones could be studied in detail.