University of Hawaii, Honolulu
A physicist and biogeochemist gets a kick out of the problem of Brownian motion and diffusion.
The movement of a particle in a gas or fluid, known as Brownian motion, exhibits two different regimes: the ballistic and the diffusive. For illustration, imagine a drunken sailor staggering back to his ship. While taking a few rapid steps, his instantaneous velocity may be quite high (ballistic regime), but his average ‘random walk’ velocity may be rather low (diffusive regime). If we were to monitor the sailor with a coarse-resolution Global Positioning System device, we would conclude that he is walking leisurely towards the docks, but we wouldn’t be able to detect his rapid motions on much shorter timescales.
Until recently, a similar problem applied to observing a Brownian particle’s instantaneous velocity. Now, Mark Raizen and his colleagues at the University of Texas at Austin have followed the ballistic motion of micrometre-sized particles on microsecond timescales, using lasers (T. Li et al. Science 328, 1673–1675; 2010). Their results not only confirm the equipartition theorem, but may also be critical to observing certain quantum effects.
My interest in the story is more practical. I am currently using molecular dynamics to calculate ionic diffusion coefficients. It was a great pleasure to see that the underlying theory and the new experimental results agree flawlessly.
In response to the authors’ observation, the media stated that Einstein had been wrong because he had predicted such an observation to be impossible. He wasn’t. As a German-speaker, I have been able to read the early landmark papers in physics, often originally in German. They include Einstein’s 1907 paper on Brownian motion. He stated that observing the instantaneous velocity of ultra-microscopic particles is impossible. He didn’t rule out the possibility of studying microscopic particles — as Li et al. have done.