One hallmark of Albert Einstein’s genius is his 1905 theory that the kinetic energy of pollen grains, dust, and other similarly sized objects in thermal equilibrium depends solely on temperature—the classic definition of Brownian motion. Einstein then concluded that the instantaneous velocity of such particles would be impossible to physically measure, and for more than a century, it seemed that he was right. But now, Mark Raizen and his colleagues at the University of Texas at Austin have used optical tweezers in a vacuum chamber to trap a 3-µm-diameter silica bead, observe its ballistic (inertial) motion at short time scales, and determine its instantaneous velocity. The bead is held at the focal point of two noninterfering laser beams, similar to the setup in the image. When the bead makes a random move, it deflects the beams, which allows its position to be traced and the instantaneous velocity to be measured. From those measurements, the researchers calculated root mean square velocities; even when taken at varying air pressures, the results agreed with each other and with the theoretically predicted value, proving that in the ballistic regime, the bead’s mean velocity is solely dependent on temperature and not on pressure or the inertial effects of the surrounding air molecules. Raizen says they will next attempt to cool the particle’s motion to the quantum ground state and confirm that the kinetic energy will be nonzero even at 0 K. (T. Li et al., Science, in press, doi:10.1126/science.1189403.)—Jermey N. A. Matthews
