Light in a vacuum behaves like a stubborn metronome, not a negotiator. Its speed refuses to track with any moving source or detector, and that defiance, codified as a postulate, sets a trap for everyday intuition.
The sharp claim is this: once you accept that postulate and the demand that all inertial frames obey identical dynamical laws, you have already spent the credit that kept time and space separate. Galilean transformations, which simply add velocities, no longer preserve the constant speed of light. To rescue both postulates at once, the coordinate change between inertial frames must become Lorentzian, with space and time mixed in each other’s equations.
From that mix, the weird effects are not optional quirks but accounting requirements. A moving clock must tick slower by the Lorentz factor, or different observers would disagree about how far light travels in a given interval, breaking the postulate about its speed. A moving rod must contract along its direction of motion for the same reason, or light would cross its length in incompatible times for different observers. Proper time and spacetime interval stay invariant; separate notions of absolute time and absolute length do not survive that constraint.
