Introduction: What Does It Mean?
Quantum mechanics works spectacularly well, but what does it mean? Different interpretations offer different answers to this question, all consistent with the mathematics.
Copenhagen Interpretation
- The wavefunction is a tool for predicting observations
- Measurement causes real collapse
- Don't ask what happens between measurements
- "Shut up and calculate" (attributed to Feynman)
Many-Worlds Interpretation
- No collapse—all outcomes occur
- The universe branches at each measurement
- We only experience one branch
- Solves measurement problem but implies infinite worlds
Other Interpretations
Pilot Wave (Bohmian): Particles have definite positions guided by wavefunction
QBism: Wavefunctions represent subjective beliefs
Consistent Histories: Probabilities assigned to histories, not single times
Objective Collapse: Collapse is physical (GRW, Penrose)
The Quantum Connection
Remarkably, all interpretations make identical experimental predictions—they're "empirically equivalent." The choice between them is (currently) philosophical. Yet working physicists often hold strong views, and quantum information theory has reinvigorated foundational questions. The interpretation problem remains quantum mechanics' deepest puzzle.