Einstein said it couldn't be done. But more than one hundred years later physicists at the University of Texas at Austin have finally found a way to witness “Brownian motion”; the instantaneous velocity of tiny particles as they vibrate. The “equipartition theorem” states that a particle's kinetic energy, that due to motion, is determined only by its temperature and not its size or mass, and in 1907 Einstein proposed a test to observe the velocity of Brownian motion but gave up, saying the experiment would never be possible.

More than a century later Mark Raizen and his team have finally proved this long-anticipated prediction by means of “optical tweezers”: a single laser beam was fired at a 5μm micrometer bead from below, suspending the bead in an “optical trap” mid-air using the force from the laser and the gravitational force on the bead. A plate-like transducer shook the beads to be tweezed and measured them as they were suspended, and the Brownian motion of the trapped bead was studied with ultra-high resolution.

Having noted that in this case glass beads were 3 micrometers across, Raizen and his team have proved that equipartition theorem is in fact true for Brownian particles. This is the first time in history that the equipartition theorem has been tested for Brownian particles, which forms one of the basic principles of statistical mechanics. They now intend to go further by moving the particles closer to a quantum state for observation. They also expect this to stimulate further research into cooling glass beads to a state where they could be used as oscillators or sensors.

As with much of quantum science, they don't expect the experiment to yield more answers than questions, however: “We've now observed the instantaneous velocity of a Brownian particle," says Raizen. "In some sense, we're closing a door on this problem in physics. But we are actually opening a much larger door for future tests of the equipartition theorem at the quantum level."

Mark Raizen is professor of physics at The University of Texas at Austin, and the Sid W. Richardson Foundation Regents Chair. His co-authors are Tongcang Li, Simon Kheifets and David Medellin of the Center for Nonlinear Dynamics and the Department of Physics at The University of Texas at Austin. Their paper is published in Science.