- In special relativity, the speed of light in a vacuum is postulated to be the same for all inertial observers, regardless of their motion or the motion of the light source.
- This constancy of the speed of light contradicts classical mechanics, where speeds add linearly. In relativity, even if an observer moves toward a light source, they still measure the light’s speed as exactly cc.
- Because of the fixed speed of light, observers moving relative to one another perceive time and space differently. This leads to effects such as time dilation, where a moving clock appears to run slower compared to a stationary one.
- Length contraction occurs because maintaining the same speed of light requires space itself to appear shorter in the direction of motion for a moving observer.
- The relativity of simultaneity arises because light takes time to reach observers. Two observers moving relative to each other may disagree on whether two distant events happened at the same time.
- The invariance of the speed of light necessitates the use of Lorentz transformations instead of Galilean transformations to convert coordinates between inertial frames.
- The speed of light serves as a cosmic speed limit. No information, matter, or energy can travel faster than cc, ensuring causality is preserved.
- The mass–energy equivalence formula E=mc2E = mc^2 uses the speed of light squared to relate mass and energy, showing how even small amounts of mass correspond to large energy.