Special Relativity: Space, Time and Mass–Energy
Why the speed of light rewrote our ideas of space, time and mass
Big Idea: At the end of the 19th century physics rested on two pillars: Newton's mechanics and Maxwell's electromagnetism. They quietly disagreed about one thing — the speed of light. Michelson's experiment showed that light always travels at the same speed c = 3 × 108 m/s no matter how the observer moves. In 1905 Einstein took this seriously and built the Special Theory of Relativity (SR), in which moving clocks run slow, moving rulers shrink, and mass is a form of energy: E = mc2.
By the end you will be able to:
- describe Michelson's experiment and why its null result destroyed the aether idea,
- state Einstein's two postulates of special relativity,
- use time dilation and length contraction to compare space and time in different frames,
- explain relativistic mass and momentum,
- apply the mass–energy relation E = mc2 to real numbers.