Frames & Postulates
Special relativity begins with inertial frames and two commitments: the laws of physics take the same form in every inertial frame, and light in vacuum has the same speed c for every inertial observer.
What breaks if light refuses to obey ordinary velocity addition?
In Newtonian mechanics, speeds simply add between moving frames. That works well at ordinary speeds. But applying it to light would make different inertial observers measure different light speeds, contradicting the relativistic postulate.
Below, all velocities are shown as fractions of , so the formula uses .
Launch the same signal from a moving frame.
Special relativity compares frames in uniform relative motion. No inertial frame is physically privileged as the universal frame at rest.
Set the signal to light and move the source frame. Relativistic velocity addition keeps the observed vacuum light speed at exactly c.
If c is invariant, observers cannot all share Newtonian simultaneity, lengths, and time intervals. Lorentz transformations replace Galilean ones.
The laws of physics have the same form in all inertial frames. Uniform motion alone cannot reveal a preferred inertial frame.
Every inertial observer measures the same vacuum light speed c, independent of the source or observer motion.