Watching Brownian Motion
“At its heart, Brownian motion is still described by classical Newtonian physics, even if we cannot define a classical velocity and can only measure mean square displacement.
Einstein said, “It is therefore impossible… to ascertain the root mean square velocity by observation” because the timescales of the instantaneous velocity are vanishingly short.
Einstein calculated that the time for a particle to decelerate significantly is about ~100ns, impossibly fast to measure at the time.
But that was then.
Now researchers have been able to probe time scales an order of magnitude faster. By holding a micron-sized sphere in a optical trap and measuring the scattered light with a high speed position detector (75MHz), a team was able to measure the motion of the spheres with a resolution of 0.2 angstroms and a temporal resolution of ~10ns.
At these resolutions, they were able to track the inertial Brownian motion of the sphere.”