- In a new study, scientists have discovered that flocks of birds can develop a certain order after a chaotic start.
- The formation of swarms and other “collective movements” has long been a mystery to scientists.
- Understanding physics in this way can help us understand trends in the cells of the human body.
In many ways, particles are one of the most vibrant parts of popular and scientific culture in 2024. Our video games and movies are characterized by how many particles are depicted to mimic reality. Our science is driven by supercomputer models that can go through billions of individual points in a simulation. These models allow scientists to combine models from different disciplines and calculate probabilities rather than certainties.
Real life is also full of “particles” – and things that behave surprisingly similarly to particles – that we are understanding better and better. For example, new research has been published in the peer-reviewed journal of the Institute of Physics. Journal of Statistical Mechanics: Theory and Experiment explains why the “transition to collective motion” – the catalytic moment when discrete (separate) things with individual directions all rotate together and act as one – is so elusive.
Collective motion is a well-known phenomenon in particle physics, but it is not limited to the microworld. Fish, for example, are examples of collective motion, and scientists believe that their highly evolved ability to change direction is the result of better sensory organs and sharper senses than we previously thought. But that still doesn’t fully explain the physics of how the groups maintain their order, react shockingly quickly, and form groups in the first place.
Is a flock of birds in some ways the same as the plasma in a fusion tokamak reactor? The fascinating answer is yes. And both have properties common to fluids like water and wind. The why behind these similarities is still a matter of debate. How can it be that “self-propelled” things – like fish, birds, individual cells, and even magnets – model the same physical ideas as particles?
In a statement from the SISSA Lab in Italy, Julien Tailleur of the Massachusetts Institute of Technology (MIT) said his professional curiosity about the subject dates back to an observation made by his colleague Hugues Chaté 20 years ago – namely that group reactions of magnetic particles depend on pressure and temperature. When scientists observed fish and birds, they thought that something like this property of magnetism could explain how this Groups also form. Within the right parameters, the magnetized particles behave in an orderly manner, and animals certainly look orderly as they change, group, and regroup.
But in this new experiment, scientists found that the groups instead realigned themselves based on fluctuations.
In statistical mechanics—a field that studies large groups such as water molecules using methods from probability and statistics—fluctuation is a special formula that shows the increase in entropy (tendency toward disorder) in a system such as a flock of birds or a bloodstream. This entropy comes in forms such as “noise,” which (in this case) is just one variable for what affects one part of a flock or system to cause a change. In their experimental setup, even magnets responded to the noise in a discontinuous or less orderly manner, resulting in an ordered grouping. Despite the result of an orderly flock or grouping, the catalyst for movement into these groups was disordered.
This result did not hold true for all the models the scientists tested, but it ultimately held true even for seemingly much more complex scenarios – such as pigeons forming groups based on what they can see and process in their heads rather than on their other senses.
By developing their own novel model to calculate similar results to those already known about collective motion, the scientists hope they can stimulate future work using this or other novel models to advance our knowledge. “To illustrate the usefulness of our approach,” they explain, “we highlight that it has been successfully applied to the case of active particles experiencing nematic torques.”
Nematic torques are a reaction in the unusual phase of matter known as time crystals, when atoms can line up neatly in parallel and become synchronized (torque in this case is the torsion or rotation of these atoms as a collective motion). The scientists found that fluctuations make the particles more responsive to each other and in some ways make this reaction more persistent. This means that this collective behavior could explain the formation of time crystals themselves, as atoms can slow down and synchronize. And this is just one step on the long road to understanding these phenomena.
As Tailleur said in the statement: “We are making progress!”
Caroline Delbert is a writer, avid reader, and editor at Pop Mech. She’s also passionate about pretty much everything. Her favorite topics include nuclear energy, cosmology, the mathematics of everyday things, and the philosophy behind them.
