Theory that gases are made up of a bunch of small billiard balls that don't interact with each other.
This theory attempts to deduce/explain properties of matter such as the equation of state in terms of classical mechanics.
This field is likely both ugly and useless.
OK, in 2D they've achieved some cute rational number results. But still.
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Statistical mechanics is a branch of theoretical physics that connects the microscopic properties of individual atoms and molecules to the macroscopic properties of materials and systems. It provides a framework for understanding thermodynamics in terms of the behavior of large numbers of particles, allowing for predictions about bulk properties based on the statistical behavior of microscopic states.