World Thrift Day is observed on October 31 to promote saving money and developing a sense of financial prudence. We’re taught to save for a rainy day, as we grow up from being the owners of a piggy bank to the possessors of a bank account. It so happens that our vast universe’s scheme of operations also has an economical character. That is, our universe is thrifty as well.
Physicists generally attribute this to the principle of least action. Action in physics is defined by the change in energy of a system over time. The conservation laws in physics follow from the principle of least action. They imply that all energy is conserved, as is the total momentum. Nothing is deleted or destroyed, only conserved. All the phenomena that happen, from the subatomic world to the galaxies, follow the path of least action.
The word ‘least’ here doesn’t mean minimality. Instead, it means that a physical system between any two points in space-time evolves along a path that minimises or maximises the action depending on the outcome of the process.
