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University Of Lisbon Scientists Solve Pendulum Clock Mystery


And now a vexing problem solved.

HENRIQUE OLIVEIRA: Tick-tock, tick-tock.

BLOCK: This is mathematician Henrique Oliveira. And for a while, he's tried to solve a problem that scientists have pondered for 350 years.


If you put two pendulum clocks next to each other on a wall, eventually they'll sync up.

BLOCK: As one pendulum swings to the left...

CORNISH: The other swings to the right.

OLIVEIRA: They swing in opposite directions forever (laughter). It's amazing. It's mesmerizing.

BLOCK: The question is why?

CORNISH: In a new study published in the journal Scientific Reports, Oliveira offered the answer.

BLOCK: Which is essentially sound waves - insistent little pulses that travel from one pendulum to the other.

OLIVEIRA: They swing as they were talking one to each other, so each one of the clocks is going to perturb the second one.

BLOCK: And yes, the word is perturb.

OLIVEIRA: Perturb, exactly, the exact term is perturbation.

BLOCK: Perturbation is what happens when sound pulses move through a beam on the wall that holds the two clocks. Each sound pulse is like a little kick, and those kicks add up.

OLIVEIRA: They listen to each other. They listen to the ticks and the tocks of each other and that adjusts each one to a proper antiphase swing.

BLOCK: That's phase opposition, and when the clocks reach that state, one pendulum swings right...

BLOCK: As the other swings left.

CORNISH: They stay that way.

OLIVEIRA: Tick-tock.

CORNISH: Mesmerizing.

OLIVEIRA: Tick-tock, tick-tock.

CORNISH: Henrique Oliveira, a mathematician at the University of Lisbon and co-author of the study that appeared this month in the journal Scientific Reports. Transcript provided by NPR, Copyright NPR.