The theory of special relativity is based on two fundamental principles:
The Principle of Relativity: The first principle of special relativity states that the laws of physics are the same in all inertial reference frames. An inertial reference frame is a coordinate system in which an object not subject to any external forces remains at rest or moves with a constant velocity. In simpler terms, this means that the laws of physics and the outcomes of experiments should be the same for all observers moving at a constant speed relative to each other. This principle emphasizes that no particular inertial reference frame is privileged or preferred over others.
The Invariance of the Speed of Light: The second principle of special relativity is that the speed of light in a vacuum is constant for all observers, regardless of their relative motion. This means that the speed of light, denoted by 'c,' is the same for everyone, regardless of whether the observer is moving toward or away from the source of light. This postulate contradicts classical Newtonian mechanics, where the velocity of an object relative to an observer depends on the relative velocities of the object and the observer. The constancy of the speed of light is a fundamental feature of special relativity and has profound implications for our understanding of space, time, and the behavior of objects moving at high speeds.
These two principles, the Principle of Relativity and the Invariance of the Speed of Light, are the cornerstone of special relativity. They provide the framework for understanding how space and time are interconnected, and how they are affected by motion and the propagation of light. From these principles, many other important results and concepts in special relativity, such as time dilation and length contraction, can be derived.