Bouncing beads outwit Feynman

A life-size thought experiment machine performs work

View a video of the newly designed machine

Researchers have built a machine that harnesses energy from the random motion of bouncing beads to perform work. The machine, a modified re-creation of a system dreamt up nearly a century ago in a captivating thought experiment, dances around physicist Richard Feynman’s dictum that work can’t be extracted from such a system. 

In 1912, Polish physicist Marian Smoluchowski proposed a thought experiment in which tiny moving particles spin a windmill-type paddle, which then spins a toothed wheel. A pawl prevents the wheel from slipping backwards, forcing the wheel to move in one direction only. But as Feynman later pointed out in his famous lectures on physics, the original calculations — which seemed too good to be true — missed something. If everything in the system was the same temperature, the pawl would occasionally slip off the wheel, resulting in no net movement, he showed.

By skirting some of the rules of the original system, the new machine, described in a paper to appear in Physical Review Letters, keeps the wheel spinning in one direction. “It’s an amusing play on a classical problem,” comments physicist Bob Behringer of Duke University in Durham, N.C. “By changing an assumption you can actually make this work.”

In the new study, Devaraj van der Meer of the University of Twente in the Netherlands and his colleagues designed a vigorously shaking platform that sends glass beads flying up like popcorn dancing off a popper. The beads smash into windmill-like vanes, which start turning a rod, which rotates a sensor. If this spinning is directional, it can be put to good use. 

When the paddles had the same kind of surface on each side, there was no net rotation — the machine swung back and forth evenly, van der Meer and colleagues found. But when they coated one side of each paddle with duct tape, the vanes spun in one direction. The beads lost more energy when they hit the softer duct-taped side of the vanes, causing the system to rotate in one direction.

As the vanes began to turn in one direction, their motion created a roiling pattern in the beads, the researchers saw. “There’s a back interaction between the vanes and the surroundings,” van der Meer says. This reciprocal give-and-take — where the beads move the vanes and the vanes move the beads too — could also happen for very tiny molecular ratchets, such as those in the body, he says. 

Small molecular workhorses perform critical jobs in the body by ratcheting along tracks inside cells: RNA polymerase inches across DNA strands in a single direction as it makes RNA from a DNA template, and the cargo-carrying protein kinesin steps down cellular paths in a similar way. Having a large system to poke and prod may translate to better understanding about how ratchets work at such tiny scales. “The things that we see in this macroscopic system could also be there in very small systems,” van der Meer says. “The idea is to learn more about how these systems work.”

Since the new machine requires many vigorously shaking beads to work, it loses most of its energy through heat and sound. “If you think about the amount of energy you need to put into the shaker, it’s an extremely inefficient device,” van der Meer says. “In terms of the second law of thermodynamics, there’s no problem whatsoever.”

Behringer says that the new machine won’t change the way people think about physics, but the study will have appeal for people who are intrigued by classical thought experiments. “In some sense, it puts the spotlight back on an old problem in a new way,” he says.



Newly created machine from Science News on Vimeo.

In a newly created machine, the random motion of 2,000 beads spins a ratchet almost exclusively in the counterclockwise direction (from the sensor’s view).

Credit: Physics of Fluids group/University of Twente

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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