Mosh pit dynamics

From The Met to Mardi Gras, Glastonbury to Concertgebouw, music syncs groups of people together. Getting people moving together and feeling e.g. joy, sublimity or nostalgia together is one of many virtues of music (of course, you can also see it as a vice if you look at how music is used in preparation for battle or as a propaganda tool).

In studying music and entrainment, the focus is often in the musicians, who need to time their notes at a millisecond level. This is of course partly due to convenience, as it is easier to get a quartet into the lab than the 1000 people in their audience. In studying such phenomena, music researchers often borrow methods from natural sciences. Physics and especially the field of nonlinear dynamics have been very helpful in these studies, as musicians or dancers performing rhythmic movements can pretty well be modelled as oscillators. The interactions and entrainment of these oscillators can then be studied, e.g. producing simulations such as this one on how a group of metronomes entrain together or not, depending on small changes in the parameters.

Physics have also been applied to studying the audience, as Neda et al. showed in their paper on how a concert audience intermittently self-organises and claps in unison, to lose the synchrony again a bit later. They noticed that simple parameters drive this process: the rate of clapping (how fast or slow you clap) and how loud the clapping sounds like. To increase the loudness of the clapping (to communicate their appreciation to the performers) audience members are first locally (either subconsciously or on purpose) synchronising their claps with their neighbours, often in a slower tempo than what they’d normally clap at. This increases the loudness of the subgroup as the sound energies generated by their claps are now more effectively summed together. This brings in more people, again both through intentional synchronisation processes as well as via automatic entrainment (a similar process by which we start to tap our feet to a groovy piece of music without thinking about it).

Now, the whole audience is clapping together and typically starts to speed up, to increase the loudness even more, at which point the synchrony breaks down as the rate gets too fast to maintain. So, to be louder you can clap in sync and clap faster, but there is a trade-off: you can clap faster when you don’t need to sync your clapping with others, so there is a tradeoff between the two, and an equilibrium can be found only at a narrow range of these parameters. The audience constantly changes these parameters in search for the optimal combination and as a result keeps falling in and out of sync.

That study looked at an audience in a classical music concert. People there are generally sitting down still and only making themselves seen or heard after the music has stopped. A study came out this week that looked at a different kind of an audience: the mosh pit of a heavy metal concert.

Now, while the audience in the classical music concert could be modelled as an orderly set of metronomes, the mosh pit is a different beast, and so Jesse Silverberg and colleagues from Cornell looked at the moshers as molecules in a gas! They are bouncing about and to each other in a confined space, forming vortices and eddies in the pit. They analysed video data from mosh pits around the world, and also simulated them as MASHers (mobile, active, simulated humanoids).

In their modelling approach, they only needed a few parameters, just as has been done in looking at various kinds of “flocking behaviour“, people moving together in large crowds, or birds flocking, fish schooling etc. Fascinatingly, the model predicted the “birth” of local vortices in the pit, as the crowd “phase separates” so that those moving more flock together; the same phenomenon was observed in the videos, the so-called “circle pit”, where a subgroup of the moshers start moving together in a circle, usually rotating counter-clockwise. The modelled MASHers had a 50-50 chance of rotating clockwise or counter-clockwise, but the actual moshers rotate counter-clockwise about 95% of the time. This, the authors suspect, is because the real life moshers have dominant hands and feet, whereas such a parameter was not programmed into the MASHers.

Isn’t it fascinating how the behaviour of a group of smart, thinking and feeling people is so alike that of molecules or metronomes?

(for those interested in these topics, I can warmly recommend these books: Strogatz, S. (2004). Sync: The Emerging Science of Spontaneous Order and McNeill, W. (1997). Keeping Together in Time: Dance and Drill in Human History. Both are very readable and aimed for a general audience.)

Picture by wetwebwork @Flickr.

Néda, Z., Ravasz, E., Brechet, Y., Vicsek, T., & Barabási, A. (2000). The sound of many hands clapping Nature, 403 (6772), 849-850 DOI: 10.1038/35002660

Jesse L. Silverberg, Matthew Bierbaum, James P. Sethna, & Itai Cohen (2013). Collective Motion of Moshers at Heavy Metal Concerts Physics and Society arXiv: 1302.1886v1