Instead of proceeding in chronological order — Planck, Einstein, Bohr, de Broglie, Heisenberg — it is more illuminating to organise quantum mechanics around four great themes, closely linked to one another. Every experiment, every formula, every quantum “oddity” can be traced back to one (or more) of these four areas.
The four themes are:
- Quantisation of physical observables — energy, angular momentum, spin, polarisation take only discrete values.
- Wave-particle duality — matter and light together show both wave-like and particle-like behaviour.
- Uncertainty principle — not all quantities can simultaneously have a precise value.
- Measurement theory — the act of measurement does not passively reveal a property, but changes it.
The strength of this framework lies in the fact that these areas are not independent: each pair is linked by experiments and logical deductions. Entering from any one of them inevitably leads to the other three, as we shall see in the concrete examples that follow — from the hydrogen atom to the polarising filter.
The four themes of quantum mechanics. The golden arrows indicate that each pair is linked by experiments and logical deductions: the structure is a network, not a list.
Note on terminology
The first theme is not just “quantisation of energy” but of physical observables in general (energy, angular momentum, spin, polarisation…). The fourth theme is not “probability” or “the Copenhagen interpretation” but measurement theory: the measurement process itself is the conceptual content.
Links
Topics: Quantum physics Concepts: Wave-particle duality · Uncertainty principle
Related exercises: Detector at the slits and interference · Problem — Wave-particle complementarity · Connecting Malus’s law and a single photon