Beyond the Everyday
By 1900, classical physics seemed complete until relativity and quantum mechanics overturned it at extremes. Special relativity (1905) rests on one postulate: the speed of light is the same for all observers. Space and time must adjust to keep this true.
| Phenomenon | Definition |
|---|---|
| Time dilation | A moving clock runs slow relative to a stationary observer. |
| Length contraction | A moving object is shortened along its direction of motion. |
| Mass-energy equivalence | where is energy, is mass, and is the speed of light. |
General relativity extends this by reinterpreting gravity as the curvature of spacetime by mass. These effects dominate near light speed but are negligible at everyday speeds.
The Quantum Realm
Quantum mechanics governs the atomic scale with stranger rules. Energy comes in discrete packets called quanta, and light and matter exhibit wave-particle duality.
| Quantum Principle | Rule & Implication |
|---|---|
| Uncertainty principle | You cannot know a particle's position and momentum both precisely. |
| Probability | The theory predicts statistical outcomes, not certainties, using a wavefunction. |
Common pitfall: The uncertainty principle is a fundamental limit of nature, not a flaw from clumsy measurement. Quantum mechanics is inherently probabilistic, a departure from deterministic classical physics.