CFT is built on the Action Principle . We define a Lagrangian density ( Lscript cap L

These were the first true relativistic field theories. In four-vector notation, electricity and magnetism are not separate forces but components of a single Electromagnetic Tensor ( Fμνcap F raised to the mu nu power Energy-Momentum Tensor ( Tμνcap T raised to the mu nu power

When you combine SR and CFT, you get the framework for almost all modern physics, including Electromagnetism and General Relativity. For a field theory to be "relativistic," its equations must be —meaning they look the same regardless of how fast you are moving.