Cited research: Science doi:10.1126/science.1189403 (2010)

In 1907, Albert Einstein predicted that the kinetic energy of a particle in a gas would depend solely on its temperature, not its size or environment. The 'equipartition theorem' has been a mainstay of statistical physics ever since, but physicists have been unable to check it for an individual particle, because doing so would require a measurement of that particle's instantaneous velocity.

Now Mark Raizen and his colleagues at the University of Texas at Austin have succeeded in measuring the speed of a micrometre-sized glass bead suspended in air. Raizen and his colleagues used two lasers to hold the bead aloft (pictured). Feedback from the beam allowed them to make snap measurements of its velocity. The findings agree with the theorem and could help future studies in statistical mechanics. G.B.

Credit: SCIENCE/AAAS