8. Interpretations of Quantum Mechanics
Reading time: ~25 minutes | Pages: 5
All agree on predictions, but differ on what QM tells us about reality.
1. Copenhagen Interpretation
Main idea: $|\psi|^2$ represents knowledge/information, not objective reality
- Wave function collapse upon measurement
- Complementarity: wave-particle duality
- No objective reality before measurement
2. Many-Worlds Interpretation
Main idea: No wave function collapse; universe splits at each measurement
- All outcomes occur in parallel universes
- Deterministic evolution (unitary)
- Observer becomes entangled with system
3. de Broglie-Bohm (Pilot Wave)
Main idea: Particles have definite trajectories guided by wave function
- Nonlocal hidden variable theory
- Deterministic, but depends on initial conditions
- Reproduces all QM predictions
4. Consistent Histories
Main idea: Assign probabilities to histories (sequences of events)
- Framework to talk about unobserved events
- Decoherence selects consistent sets
5. QBism (Quantum Bayesianism)
Main idea: Wave function represents personal beliefs/expectations
- Subjective probability interpretation
- Measurement updates beliefs (Bayesian)
Key Questions
- Is the wave function real or epistemic?
- Does wave function collapse actually happen?
- Is QM complete or are there hidden variables?
- What happens in measurement?
Note: All interpretations agree on experimental predictions—this is philosophy, not physics.