It has often been claimed that Brownian motion will prevent nanoscale machines from working. Brownian motion is the random travel of floating objects caused by thermal noise (random heat energy).
In a nanomachine of the kind Drexler described in Nanosystems, the objects would be fixed in place (just like in a large-scale machine) and could not float away. They could still vibrate, but Drexler calculated this vibration and allowed for it in his system designs. But scientists appeared to ignore this, and continued to assert that Brownian motion would pose an unsolvable problem.
A recent discovery may start to change that. Scientists have managed to mechanically sort one-micron particles, putting each one in its correct spot despite Brownian motion. What surprises me is not that they could do it, but that they didn't think they were going to be able to.
The device works in a unique way because the arrangement of pillars forces particles along completely predetermined paths, like pennies and dimes rolling through a child's coin sorter. Previous attempts required the particles to diffuse randomly so that bigger particles slowly drifted one way and smaller ones another.
Researchers had believed that fixed paths were not possible in part because small particles jiggle constantly, making them move in uncontrollable ways. Huang discovered that, with the proper arrangement of pillars, the particles could be made to slide in a tango-like dance forward or sideways at each obstacle depending precisely on the particle's size.
"To suddenly say that there is a deterministic (non-random) way to do this really flies in the face of conventional wisdom," said Austin. "It's something I never would have thought of."
Take a moment to remember the experienced computer scientist who wrote that an unfamiliar kind of digital computer couldn't work because entropy would make the information leak away (I wrote this up in our latest C-R-Newsletter). 'Entropy' is another common excuse for why molecular manufacturing supposedly won't work, and it's just as wrong as 'Brownian motion' as an excuse for why you can't sort small objects or build small machines.