Entrainment in Physics

Saw a curious YouTube video, courtesy of Slipped Disc, Norman Lebrecht’s blog at Arts Journal:

I puzzled for a short while about how independent mechanical devices could sync up. The first commentator at Slipped Disc identifies the phenomenon as entrainment, which is accurate except that the comment refers to music therapy. With metronomes, however, there is no nervous system at work as with entrainment in humans. Rather, this video merely demonstrates a property of physics, also called entrainment, whereby interacting oscillating systems achieve mode lock or sync to the same period. In fact, the Wikipedia link in the previous sentence includes a CBS News report assuring everyone that it’s merely physics. This property was observed 350 years ago. Let me draw attention to the fact that the floating tray on which the metronomes sit moves sufficiently (left and right in the video) to allow the devices to interact. In truth, it took me only a little poking around to uncover the physics of it.