The theorem, which states that the kinetic energy of particles is proportional to its temperature, but is independent of its mass or composition, is one of the key tenets of statistical mechanics and has never before been tested for a single particle.

The investigation, by researchers at the Center for Nonlinear Dynamics and Department of Physics, University of Texas at Austin, examined the Brownian motion of glass beads that were three micrometers across held in air in an optical tweezer, over a wide range of pressures. The study of the Brownian motion of glass beads in air and vacuum helps an understanding of the dynamics of their system.

The results verify the energy equipartition theorem, as for short periods the ballistic regime of Brownian motion can be observed, as opposed to the usual diffusive regime. Published in Science [Li et al., Science (2010) doi: 10.1126/science.1189403], the study disproved the view of Albert Einstein, who thought the instantaneous velocity of tiny particles would never be observed because of their Brownian motion.

Team leader Mark Raizen points out that “This is the first observation of the instantaneous velocity of a Brownian particle. It’s a prediction of Einstein’s that has been standing untested for 100 years. He proposed a test to observe the velocity in 1907, but said that the experiment could not be done.”

The Brownian motion of aerosols, such as powder, dust particles, viruses and spores, is?crucial in many areas, including industry, climate, public health and even national security. If you can measure instantaneous velocities, then the real-time monitoring of the masses of airborne particles is possible.

The team now hope to push these limits, and open the way for future tests of the equipartition theorem at the quantum level, by applying these methods to cooling the center of mass motion of a bead in vacuum to the quantum ground motional state, which is fundamentally important in the study of quantum mechanics. If the optically trapped glass beads were replaced by a virus or a cell, then this could enable the study of the quantum superposition of living organisms.

At this point, Raizen expects the equipartition theory to break down, leading to new problems and solutions surrounding the quantum mechanics of small particles composed of many atoms.