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Glossary.
Short, careful definitions. Where the site uses one of these terms, its definition appears on hover or focus — the full entry is here.
- Superposition
- A state holding several alternatives at once, each with an amplitude, relative to a chosen set of alternatives. Not “one of them, unknown” — the amplitudes can interfere, which mere ignorance could never do.
- Amplitude
- The complex number the theory assigns to each possibility. Its squared magnitude gives the probability of the corresponding outcome (the Born rule).
- Wave function
- The mathematical object (ψ) encoding a system's amplitudes. Whether it is a real physical thing, a tool for prediction, or an agent's expectations is exactly what the interpretations dispute.
- Born rule
- The rule connecting amplitudes to measurement outcomes: the probability of an outcome equals the squared magnitude of its amplitude, |ψ|².
- Measurement problem
- The equation evolves states smoothly and never picks an outcome; experiments end with exactly one. Saying precisely what happens in between — and what counts as a measurement — is the central unsolved puzzle of quantum foundations.
- Collapse
- The apparent jump from many weighted possibilities to one actual result at measurement. Interpretations disagree on whether it is a physical process, a bookkeeping update, or nothing at all.
- Decoherence
- What happens when a quantum system couples to an environment: its many degrees of freedom carry the interference away where it cannot be recovered. This explains why superpositions become undetectable in practice — but not, by itself, why one outcome occurs.
- Entanglement
- A state of two or more systems that cannot be described system-by-system. Measuring one is instantly informative about the other, however far away — though no usable signal travels.
- Nonlocality
- Correlation between distant events that no local mechanism can reproduce, as Bell's theorem makes precise — given that settings are chosen freely and each run has a single outcome. Interpretations differ over which of those assumptions to give up, and over whether anything acts at a distance.
- Unitary evolution
- The smooth, reversible, deterministic change of the wave function described by the Schrödinger equation — what the orthodox account says happens between measurements. Several interpretations say it is all that ever happens.
- Which-path information
- Any physical record of which route a particle took. The sharper the record, the fainter the interference — Englert's relation makes the trade exact — and erasing the record can bring the fringes back. No person needs to look.
- Interference
- Amplitudes adding and cancelling like overlapping ripples, producing patterns (bright and dark bands) that probabilities alone could never make.
- Framework (consistent histories)
- A family of alternative histories whose amplitudes do not interfere, so ordinary probability rules apply within it. Different frameworks can be individually valid yet impossible to combine.