First observation of Brownian instantaneous velocity

A hundred years after Albert Einstein said we would never be able to observe the instantaneous velocity of tiny particles undergoing Brownian motion, a group of Texan physicists led by Mark Raizen has done so. “This is the first observation of the instantaneous velocity of a Brownian particle,” says Raizen, a professor of physics at The University of Texas at Austin. “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.”

In all fairness, Einstein likely did not foresee a time when dust-sized particles of glass could be trapped and suspended in the air by optical tweezers. Nor would he have known that ultrasonic vibrations from a plate-like transducer would shake those glass beads into the air to be tweezed and measured as they moved in suspension.

Published in Science, Raizen’s work is the first direct test of the equipartition theorem for Brownian particles, one of the basic tenets of statistical mechanics. The equipartition theorem states that a particles’ kinetic energy – the energy it possesses due to motion – is determined only by its temperature, not its size or mass.

The new study proves that the equipartition theorem is true for Brownian particles; in this case, glass beads that were three micrometers across. Raizen says he and his colleagues can now push the limits, moving the particles closer to a quantum state for observation.

“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.” There, he expects that equipartition theory will break down, leading to new problems and solutions surrounding the quantum mechanics of small particles composed of many atoms.

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Source: University of Texas at Austin
Pic courtesy Tongcang Li, the University of Texas at Austin

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