There are nuances to particle movement and energy at tiny scales that one of Einstein's equations did not capture, according to a paper published in Science this week. Researchers were able to measure the instantaneous velocity of a tiny glass bead undergoing Brownian motion, or making tiny random movements, and found that the particle was not always governed by the forces that Einstein predicted. Knowing how Brownian motion works at these short intervals may allow researchers to study these tiny particle systems for quantum effects.
Barring extreme cold, particles are usually not sitting still, but exhibit a kind of hyperactivity, always moving tiny distances in random directions. This is called Brownian motion, and it can be described by equations that relate the distance moved to the time involved, the drag of the medium, and the temperature of the particle's environment.
Einstein himself developed the equations we typically use to predict Brownian motion. However, scientists had begun to suspect that there was more involved with Brownian motion than these equations indicated.
Science Magazine wrote, “Brownian motion of particles impacts many branches of science. We report on the Brownian motion of micron-sized beads of glass held in air in an optical tweezer, over a wide range of pressures, and measure the instantaneous velocity of a Brownian particle. Our results provide direct verification of the energy equipartition theorem for a Brownian particle. For short times, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. We discuss the applications of these methods towards cooling the center of mass motion of a bead in vacuum to the quantum ground motional state.”