Dynamic systems are those whose conditions change according to ... what?

Imagine a swinging pendulum, the orbit of a planet, or the weather patterns in the atmosphere. These systems are never static; their state is constantly evolving from one moment to the next. The crucial, unchanging variable that governs this evolution is the forward march of time. The position and velocity of the pendulum at this very second are a direct consequence of where it was a fraction of a second ago. This continuous progression is the defining characteristic of a dynamic system.

This concept extends far beyond simple physics. Ecologists model how animal populations change over the course of years, economists track market fluctuations from day to day, and neurologists study the firing of neurons over milliseconds. In each case, scientists develop mathematical rules that describe how the system's state changes as time progresses. The goal is often to predict the future state, whether it's tomorrow's weather or the future trajectory of an asteroid.

The opposite of a dynamic system is a static one, which remains in a state of equilibrium unless acted upon by an outside force, like a book resting on a table. For any system to be considered dynamic, from the beating of a human heart to the swirling of a galaxy, its behavior must be described as a function of this single, fundamental dimension.