Two postulates: relativity and light constancy

Summary

Special relativity is built on two postulates: (1) the principle of relativity, which states that the laws of physics are the same in all inertial frames (frames moving at constant velocity relative to each other), and (2) the principle of light constancy, which states that the speed of light in vacuum is the same for all observers, regardless of the motion of the light source or the observer. Together, these postulates replace the Newtonian idea of absolute time and absolute simultaneity. They imply that measurements of time and space depend on the observer’s relative motion, leading to effects such as time dilation and length contraction. They also require a new transformation rule between inertial frames—the Lorentz transformation—under which a spacetime interval remains invariant and the ordering of events can differ between observers when events are spacelike separated.

Key Points

• 1

  The principle of relativity says physical laws are invariant across inertial frames.

• 2

  The principle of light constancy says all inertial observers measure the same light speed in vacuum.

• 3

  Combining the postulates forces the use of Lorentz transformations and leads to time dilation, length contraction, and relativity of simultaneity.