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Insect Swarms Go Supercritical

A new study finds clouds of tiny bugs mirroring one of the weirdest non-states of matter.
​Image: Andrea Cavagna/University “La Sapienza” of Rome

Swarms of insects and birds behave in very similar ways to a realm of physics in which the phases of matter—solids, liquids, and gases—cease to exist. It's here, near what's known as the critical point, that very slight perturbations are able to extend instantaneously across vast collections of particles and, likewise, across entire flocks and swarms.

This revelation, which (most recently) comes c​ourtesy of physicists based at the University of Rome, might explain a unified driving force behind a vast kingdom of seemingly disconnected biological systems.

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Different materials have different phase-change diagrams (below), which are two-dimensional spaces relating the effects of temperature and pressure to a material's phase. Some region of the diagram will mark where the material is a gas, another region where it's a solid, and another where the material is a liquid. These lines are fuzzy, however, and there exist points of in-betweenness, where a material might behave as both a solid and a gas, etc.

Image: ​Wikipe​dia

In addition to these boundaries, there is a certain transition called the critical point. This is where both the temperature and pressure are high enough that the material doesn't really occupy a distinct phase at all. This sort of material is called supercritical and it behaves in all kinds ​of ways that seem just plain wrong—passing through solid materials like a gas, dissolving other materials like a liquid. A supercritical liquid can be extremely dense and heavy but still float around like a puff of smoke. Supercritical CO2, in particular, is widely used in industry.

Materials like this experience what's called "scale-free correlation." Basically, whatever I do to one part or particle I do to the whole. If I push against a block of ice, the effect will probably be the movement of the entire block; if I push against liquid water or water vapor, not so much. In a normal liquid or gas, "here" just isn't sufficiently correlated with "there."

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The Rome researchers, led by physicist Alessandro Attanasi, sought to extend this behavior to swarms of insects, building upon the promising theory—and g​rowing body of experimental results—that suggests natural systems of all sorts behave supercritically and experience scale-free correlations. Influencing one bird or bug influences the entire swarm or flock.

The results, as summarized in an A​merican Physical Society Viewpoint: "Attanasi et al.'s attractive message is that biological groups, such as swarms, coordinate their behavior so as to optimize the ability to react collectively (e.g., to avoid predators or to attract sexual partners). Their observations, in fact, provide stronger evidence in support of criticality in animal groups than the scale-free correlations found in more ordered groups like starling flocks."

Basically, swarms and other organizations in nature, including the human brain and gene expression networks, get all of the benefits of the tight organization of a solid (the block of ice), but with the fluidity needed to be able react to an environment. It's a shrewd adaptation.

"A 'critical' brain could have several functional advantages, such as enhanced response to stimuli, the ability to exist in many states, and optimal transmission and storage of information," physicists Hugues Chaté and Miguel Muñoz, who were tasked with reviewing the new study for the APS, write. "Bolstering this idea, researchers have argued that complex computations can only be performed by 'machines' operating at criticality. However, we are left with the challenge of understanding how a biological system arrives and stays at criticality."

Indeed, particles are particles: points in space. A molecule is only the slightest filament of physical organization. Birds and bugs and brains, meanwhile, are unfathomably large biological machines in comparison. Finding the connection will be a good time for all.