Tiny gelatinous droplets of water swirl perfectly in the water— their movements stir up a force that attracts their neighbors. When enough of them come together, this synchronized dance arranges them in precise hexagonal, ordered, repetitive patterns, like carbon atoms forming a graphene crystal structure.
But these are not atoms or any kind of inanimate matter controlled only by physical forces—they are living, self-moving embryonic starfish
The spinning starfish embryos gather into a living crystal.
"We know we have crystals of many materials, but we never really associate the formation of crystals with what is actually living," Nikta Fakhri, a physicist at the Massachusetts Institute of Technology, told Nature.
"This is an absolutely remarkable phenomenon that has never been reported before."
Fakhri, MIT physicist Tzer Han Tan and colleagues study active substances — systems in which each individual component (such as a bird in a flock of birds, or a cell in a water droplet) uses energy to move, throwing the entire population out of balance with its surroundings.
Like ordinary matter, these systems can have surprising, sudden properties. Starfish's "crystals" are no exception.
Each embryo rotates itself in the left-handed direction using its hair-like cilia. When enough individuals gather on the surface of the water, spontaneous ripples begin to flow through the structures they form.
They are very large-scale waves, as large as the crystals themselves, Fakhri explains.
A corrugated model with arrows and color coding indicating the direction of each embryo.
"We could see this crystal spinning and shaking over a long period of time, which was absolutely unexpected," said Alexander Mitek, a theoretical biophysicist and study author at the Massachusetts Institute of Technology.
"You'd expect these ripples to disappear quickly because the water is sticky and will suppress these oscillations." This tells us that the system has some strange elastic behavior. "
The researchers modeled the movement of the embryos to see how their rotation pulled the water toward themselves. This, in turn, pulls their rotating neighbors toward them as well, creating a mutually attracting force.
However, the water that eventually flows below also creates another horizontal force that does not interact within the system. This causes the entire crystal structure to rotate as well.
Both the overall rotation and the waves exhibit the physical characteristics of singular elasticity. This is strange because, according to the classical laws of physics, simply deforming an isolated solid object, such as a crystal floating in water, should not, in a given case, pass through the forces of the environment.
However, if an object is composed of active ingredients, the overall deformation of this shape may have some interesting mechanical properties.
At least, that's the theory. In reality, this is still a field of physics that is being explored.
Some intriguing possibilities arise in nature. A similar phenomenon has been observed in algae before.
The spinning crystal-like structure disappears after about 30 hours.
Once a small starfish has formed a floating living crystal structure, it holds itself together for several days during embryonic development until they change enough to break the pattern.
A better understanding of the physical properties of these lives can help us develop new technologies.
"Imagine building a soft, spinning swarm of robots that can interact like these embryos," Fahri said.
"They can be designed to be self-organizing, rippling and crawling in the sea, doing useful work." These interactions open up a new and interesting scope for exploration in physics. "
As for why the little starfish is a galaxy – yes, starfish has the most perfect collective noun!? - The crystal structure that forms when they gather is currently a mystery.
However, this phenomenon of living crystals is unlikely to occur naturally in the wild, as starfish embryos do not normally wander on the surface of the water.
The study was published in the journal Nature.