Or should it be “pendula”? Probably not, because that sound vaguely sexual in a rather creepy way.
I am now assuming that this video is showing the same phenomenon as the wobbling of the Millennium Bridge when it first opened.
Tweet-commenter Alma Cook also mentions how periods in groups of women get synchronised.
And yes, I found what I was looking for. Tweet-commenter Morris Jasper says:
This is essentially what happened with the ‘wobbly’ Millennial Bridge.
But as several tweet-commenters say, it’s not right to call any of this “spontaneous”, if by that you mean happening for no reason. The pendulums are all resting on the same oscillating platform. Just as all those people on the Millennium Bridge were walking on the same wobbling bridge.
Talking of pendulums, I am fond of György Ligeti’s Poème Symphonique For 100 Metronomes. They don’t synchronise themselves, because the structure they rest on is not wobbling. They just stop. One by one. It takes just over eight minutes for this to happen. (I also like Ligeti’s piano music. (But now I really digress.))