Many physics students would tell you immediately off the bat what Newton's Third Law of Motion states. For those who have forgotten our high school physics, NASA informs us that the third law of motion says that every action has an equal, albeit opposite, reaction. Newton's laws of motion have been a constant in many of our lives, reassuring us over the years that the world is a predictable system. However, that's not always the case. While the math does stand on its own, the world around us shows that nothing is ever in proper equilibrium.

## Defying Newton's Third Law - Nonreciprocal Systems

Everything in nature is out of balance. The universe loves entropy and tries its hardest to ensure that everything is in balance, but while one part of the universe may be, other factors aren't. Even being alive is in denial of Newton's Third Law. If a body enters complete equilibrium, that person must be dead. Every firing of a neuron or the pumping of a heartbeat is an example of equilibrium changes within a living individual. From weather systems to solar phenomena and even the rain that falls outside on a cloudy day - all of these things demonstrate that while Newton's Third Law is mathematically sound, it's not usually found in nature. Statistical mechanics don't have a good way to represent phase changes and transitionary states for all of these reactions.

## Introducing Exceptional Points

In reciprocal systems, everything is based on a single moment. Mathematically, it's elegant, but realistically, it's impossible. With so many systems never being in equilibrium, it isn't easy to mathematically represent those systems the way they should be. However, phase shifts do have points where one or two characteristic properties can collapse down into a singularity. These points, termed "exceptional points," serve as a basis for measurement and interpretation. At an exceptional point, the system's behavior is quite different from anywhere else in the system. A new article in Nature has shown that these exceptional points may also control phase transition in those nonreciprocal systems previously mentioned.

It might be sensational to say that exceptional points are a "new" discovery, but they're well known in scientific circles. However, never before have they been associated so closely with phase transitions. Before this most recent study, no one had considered using them as a way to describe phase changes in nonreciprocal systems. Equipment already exists to measure and interact with these systems. Now it's just a matter of applying those observations to studying exceptional points within those systems.

## Breaking Symmetry Leading to a Breakthrough

As with most new discoveries, this one came from an event that didn't seem that momentous at the time. A few years ago, a couple of researchers were studying a quasiparticle called a polariton. According to Quanta Magazine, a quasiparticle isn't really an entire particle but is a set of quantum behaviors climbed together. Polaritons result when photons (light particles) combine with another quasiparticle called an exciton. Typically, polaritons have very low mass and can collapse into a single quantum state known as a Bose-Einstein condensate (BEC). It's challenging to make a BEC with polaritons, unfortunately. BECs are leaky structures, and some photons escape the structure, meaning that light must constantly be put back into the system to keep it stable. Immediately, astute readers would spot that a BEC is out of equilibrium. The researchers wanted to know how this behavior affected the symmetry of the BEC.

Symmetry is at the heart of all phase transitions. Liquids and gases are symmetrical. If you were to view them exploded, all the particles would look the same regardless of which direction you were looking at them from. Solids, on the other hand, are non-symmetrical. When something moves from a liquid or gas to a solid, its symmetry 'breaks.' Other breaks in symmetry could be something as simple as a car moving forward from rest. The car's continued forward motion is fueled by the gas it has in its tank. The internal source of energy allows the vehicle to behave differently from other natural systems. As far as the system is concerned, the energy it needs to keep moving is instantaneously created out of "nothing."

In researching this phenomenon, the team responsible noted the formation of critical points. A critical point occurs where there's no way to distinguish where one state starts and the other ends. As the team worked, they realized that they might be on to something even bigger than just a phenomenon applying quantum systems. This behavior could extend to everything, even classical physics.

## Syncing Up

The next step was to determine whether this phenomenon would be viable at larger scales. To achieve this, the researchers programmed a series of robots with no one robot being able to get what they wanted. The results were surprising, with all the robots rotating in the same area, trying to fulfill their programming. Next, the team looked at bifurcation simulations traditionally used to determine state changes in a system. In this case, results were similar, prompting the researchers to note that a lot of the language used for describing systems in flux may need to be revisited.

## Just The Beginning

So, what sort of repercussions can we expect from a study like this? At this stage, it could mean a complete rethinking of systems in physics, which might lead to applications in electronics and engineering. However, one of the researchers thinks it can go even further and reveal new insights into the human brain and how it works as a nonreciprocal system in flux. This research has the potential to be groundbreaking, forcing a rethink on things we've accepted as accurate for decades, even centuries.