The orthodox view01
Copenhagen
The wave function is a tool for predictions. Measurement makes outcomes real.
See it explain the experimentAn interactive guide to quantum foundations
One equation.
Different realities.
Every interpretation of quantum mechanics agrees on the mathematics. They disagree about what reality is.
One quantum state evolves smoothly — until a measurement. What happens next depends on who you ask.
01 — The problem
The math works. The meaning doesn't.
A century of flawless predictions rests on three facts that refuse to fit everyday reality.
Superposition
Before you look, a system holds several outcomes at onceNot “one of them, unknown” — the amplitudes interfere, which a mere lack of knowledge could never do., in definite proportions.
Measurement
Looking seems to change what is there. And nothing in the theory says what counts as looking.
Nonlocality
Measure here, and something is instantly settled far away — with no signal travelling in between.
02 — One equation
All of it follows from one line.
Written in 1926. Never once contradicted by an experiment. Take each term in turn — hover, focus or tap it.
iħ ∂t |ψ⟩ = Ĥ |ψ⟩
Everything that follows on this site is an argument about what this equation means — never about what it predicts.
03 — Many interpretations
Seven stories about the same evidence.
Each one keeps the predictions and pays a different price. Pick the price you can live with.
04 — The experiment room
Same experiment. Switch the reality.
The double slit below never changes. Only the explanation does. This is the whole idea of the site — try it.
Fig. 01 — Double slit
Copenhagen: The wave function is a tool for predictions. Measurement makes outcomes real.
Copenhagen: the wave function is a prediction tool; measurement makes one outcome real.
05 — Side by side
Where they part ways.
Eight questions separate the seven interpretations. Three of the seven, previewed.
| Question | Copenhagen | Many Worlds | Pilot Wave |
|---|---|---|---|
| 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. Full definition → | YesThe textbook projection postulate | NoNever | NoNever |
| Deterministic? | No | YesGlobally | Yes |
| Hidden variables?Additional facts, beyond the wave function, that settle what a measurement will find — either outright or as probabilities. Pilot-wave theory adds them (particle positions); Bell showed that no such theory, deterministic or not, can be local and still match quantum mechanics. Full definition → | No | No | YesParticle positions |
| Multiple universes? | No | YesBranching worlds | No |
| Observer dependent? | DependsNeeds a classical side | No | No |
| Adds new physics? | No | No | DependsGuidance equation |
| Explicitly nonlocal?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. Full definition → | DependsCorrelations, no story | NoNo action at a distance, supporters argue | YesGuidance is instantaneous |
06 — A century of argument
No one has ever conceded.
The experiments keep agreeing with the equation. The argument about reality keeps sharpening.
- 1926Schrödinger writes the wave equationThe mathematics everyone will agree on — the argument starts immediately.
- 1927Bohr and Heisenberg set the toneComplementarity and uncertainty. The name comes much later — Heisenberg coined it in 1955.
- 1935Einstein and the cat push backEPR and Schrödinger's box argue the orthodoxy leaves reality undefined.
- 1952Bohm revives the pilot waveParticles with definite positions, steered by the wave. Dismissed in 1927, workable after all.
- 1957Everett proposes many worldsDrop collapse entirely. Everett called it relative states; DeWitt named it many worlds in 1970.
- 1964Bell's theoremLocal hidden variables make different predictions. The question becomes testable.
- 1984Consistent historiesGriffiths, then Omnès, Gell-Mann and Hartle: probabilities for whole histories — measurement not required.
- 1986GRW: collapse as physicsMaybe the wave function collapses on its own — rarely for atoms, instantly for cats.
- 1996Relational quantum mechanicsRovelli: states are relations. A fact for the screen isn't yet a fact for you.
- 2002QBismFuchs and colleagues: the wave function is an agent's betting book.
- 2015Loophole-free Bell testsDelft, Vienna, Boulder: no local hidden-variable theory can reproduce the correlations.
07 — Keep going
Further reading.
Book
What Is Real?
Adam Becker
The history of the argument — and how Copenhagen won by default.
Book
Something Deeply Hidden
Sean Carroll
The case for many worlds, made carefully and in plain language.
Book
Quantum Mechanics and Experience
David Z Albert
A short, sharp tour of the measurement problem itself.
Every interactive model on this site is a conceptual representation — a teaching picture, not a simulation of the underlying physics. Where interpretations disagree, the site says who claims what; where experiments can decide, it says that too.
Sources & disclaimers →