) is constant for all observers, regardless of their motion.
Before Einstein, physics was dominated by the Galilean view: space was an empty container, and time ticked away at the same rate for everyone. Special Relativity (SR) upended this by introducing two core postulates: in all inertial frames. The speed of light ( Special Relativity and Classical Field Theory
CFT is built on the Action Principle . We define a Lagrangian density ( Lscript cap L ) is constant for all observers, regardless of their motion