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What are the two postulates of special relativity?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

In his theory of special relativity, Albert Einstein established two fundamental postulates that redefined our understanding of space and time:12

The Principle of Relativity: The laws of physics maintain the same form for all observers in inertial frames of reference, meaning one cannot distinguish between such frames through mechanical experiments.12
The Constancy of the Speed of Light: The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source or the observer.1

These postulates depart from classical mechanics, where velocities are typically additive. By accepting these as true, Einstein concluded that the speed of light acts as a universal limit that cannot be reached by any material object.12