Pendulum waves. Credit: Harvard Natural Sciences Lecture Demonstrations; You Tube.
Pendulums are fun to watch. They are hypnotizing and mesmerizing. This video is both. Apparently, it’s not a new demonstration concept, but it continues to fascinate.
The 15 bobs start together, and as their periods go in and out of phase, they appear to engage in a struggle of order versus chaos.
This demonstration was created by the Harvard Extension School as part of their natural science lecture demonstrations. It is worth reading their write-up about it before watching, but here is their brief description to get you started.
What it shows: Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce visual traveling waves, standing waves, beating, and random motion. One might call this kinetic art and the choreography of the dance of the pendulums is stunning! Aliasing and quantum revival can also be shown.
How it works: The period of one complete cycle of the dance is 60 seconds. The length of the longest pendulum has been adjusted so that it executes 51 oscillations in this 60 second period. The length of each successive shorter pendulum is carefully adjusted so that it executes one additional oscillation in this period. Thus, the 15th pendulum (shortest) undergoes 65 oscillations. When all 15 pendulums are started together, they quickly fall out of sync-their relative phases continuously change because of their different periods of oscillation. However, after 60 seconds they will all have executed an integral number of oscillations and be back in sync again at that instant, ready to repeat the dance.
And, in case you were wondering about the broader implications, the website offers this, “Here at Harvard, Prof Eric Heller has suggested that the demonstration could be used to simulate quantum revival. So here you have quantum revival versus classical periodicity!”