Two professors at the University of Lisbon say they have discovered why the pendulums of clocks set on the same surface will eventually swing together in opposing directions.
MELISSA BLOCK, HOST: And now a vexing problem solved.</s>HENRIQUE OLIVEIRA: Tick-tock, tick-tock.</s>MELISSA BLOCK, HOST: This is mathematician Henrique Oliveira. And for a while, he's tried to solve a problem that scientists have pondered for 350 years.</s>AUDIE CORNISH, HOST: If you put two pendulum clocks next to each other on a wall, eventually they'll sync up.</s>MELISSA BLOCK, HOST: As one pendulum swings to the left...</s>AUDIE CORNISH, HOST: The other swings to the right.</s>HENRIQUE OLIVEIRA: They swing in opposite directions forever (laughter). It's amazing. It's mesmerizing.</s>MELISSA BLOCK, HOST: The question is why?</s>AUDIE CORNISH, HOST: In a new study published in the journal Scientific Reports, Oliveira offered the answer.</s>MELISSA BLOCK, HOST: Which is essentially sound waves - insistent little pulses that travel from one pendulum to the other.</s>HENRIQUE OLIVEIRA: They swing as they were talking one to each other, so each one of the clocks is going to perturb the second one.</s>MELISSA BLOCK, HOST: And yes, the word is perturb.</s>HENRIQUE OLIVEIRA: Perturb, exactly, the exact term is perturbation.</s>MELISSA BLOCK, HOST: 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.</s>HENRIQUE 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.</s>MELISSA BLOCK, HOST: That's phase opposition, and when the clocks reach that state, one pendulum swings right...</s>MELISSA BLOCK, HOST: As the other swings left.</s>AUDIE CORNISH, HOST: They stay that way.</s>HENRIQUE OLIVEIRA: Tick-tock.</s>AUDIE CORNISH, HOST: Mesmerizing.</s>HENRIQUE OLIVEIRA: Tick-tock, tick-tock.</s>AUDIE CORNISH, HOST: Henrique Oliveira, a mathematician at the University of Lisbon and co-author of the study that appeared this month in the journal Scientific Reports.