Science › Quantum world
Superposition: many states at once
1926· Idea
ProvenTested many times; part of working physics.
A quantum system can be in a blend of several possible states at once until it is measured.
See it on the Science map →How it works
An electron can be in a mix of 'here' and 'there', or spin up and spin down, at the same time. This is not mere ignorance of which one it really is: the options interfere with each other, which a hidden single answer could not do. A measurement always gives one definite result, with odds set by the blend. Superpositions are fragile and get scrambled by contact with the surroundings.
Why it matters
It is the core difference between quantum and classical physics, and the resource that quantum computers and quantum sensors exploit.
How we know
Interference in double-slit experiments with electrons, atoms and large molecules; superconducting loops carrying current both ways at once (2000); qubits in working quantum computers.
Still a mystery
How large can a superposition get? Experiments keep pushing to bigger objects; some theories predict a size limit, so far untested.
Connections (6)
▸The double-slit experiment demonstrates Superposition: many states at onceProven
The relationship
The stripes appear only if each particle's two possibilities, through the left slit and through the right, coexist and interfere. If each particle secretly took one slit, the result would be just two bands. So the experiment directly shows a particle in a superposition of two paths.
How the link was found
Implicit in quantum mechanics from 1926; Feynman made the double slit the standard teaching example in his 1960s lectures.
Why it's proven
Interference vanishes exactly when which-path information becomes available, even in principle, as quantum theory predicts. Shown with electrons, atoms and molecules, and in 'quantum eraser' experiments.
Open questions and mysteries
Whether the superposition is physically real or just bookkeeping for knowledge depends on the interpretation.
▸The wave function describes Superposition: many states at onceProven
The relationship
Schrödinger's equation is linear: if two wave functions are valid solutions, so is any blend of them. So a wave function naturally describes a system as a weighted mix of possibilities, such as passing through both slits at once. The weights set the odds of each outcome when measured.
How the link was found
Built into quantum mechanics from 1926 (Schrödinger, Born); Dirac's 1930 textbook made the superposition principle a founding rule.
Why it's proven
Interference effects, which require weighted blends adding and cancelling, are observed in every quantum system tested.
Open questions and mysteries
Whether the wave function is a real thing in a superposition, or a summary of information, is unresolved.
▸Superposition: many states at once is scaled up to absurdity in Schrödinger's catProven
The relationship
Superposition is routine for atoms. Schrödinger linked one atom's superposition, decayed and not decayed, to a poison trigger and a cat. If superposition applies to everything, the cat must be in a blend of alive and dead. Scaling up shows how bizarre the rule looks for large objects.
How the link was found
Schrödinger, 1935, prompted by EPR and his correspondence with Einstein, who had suggested a similar example involving gunpowder.
Why it's proven
Superpositions of ever-larger systems, from big molecules to a 16-microgram crystal (2023), have been made in labs. No cat-sized superposition has, because decoherence destroys it almost instantly.
Open questions and mysteries
Is there a size limit where superposition fails? Some collapse theories say yes; experiments are still searching.
▸Superposition: many states at once raises The measurement problemOpen question
The relationship
If a particle is in a blend, and the measuring device obeys the same quantum rules, the device should end up in a blend too, showing both readings. Yet every measurement shows one result. Reconciling superposition with single outcomes is exactly the measurement problem.
How the link was found
John von Neumann formalized it in his 1932 textbook, separating smooth evolution from collapse; Einstein and Schrödinger pressed the issue in 1935.
Is it proven?
That superpositions exist is proven, and that measurements give single outcomes is observed. The conflict itself is not disputed; how to resolve it is.
Open questions and mysteries
No experiment yet tells which resolution, collapse, branching or hidden variables, is correct.
▸Superposition: many states at once shared between particles becomes Quantum entanglementProven
The relationship
When two particles interact, their joint state can become a superposition of combinations, such as 'first up, second down' plus 'first down, second up'. Neither particle then has a state of its own; only the pair does. That shared superposition is entanglement, and its correlations persist after the particles separate.
How the link was found
Schrödinger named it 'Verschränkung', entanglement, in 1935, calling it the characteristic trait of quantum mechanics.
Why it's proven
Created and measured directly in labs since the 1970s with photons, atoms, ions and superconducting circuits; Bell tests confirm the correlations are genuinely quantum.
Open questions and mysteries
How entanglement fades into ordinary correlation as systems grow, and its possible role in building space itself, are active research.
▸Superposition: many states at once is the resource of Quantum computingWell supported
The relationship
A qubit can be a blend of 0 and 1. Algorithms manipulate these blends so that paths leading to wrong answers cancel and paths to right answers reinforce, like interfering waves. This interference across many possibilities, not simply 'trying every answer at once', is the source of a quantum computer's potential speedup.
How the link was found
Richard Feynman (1981) and David Deutsch (1985) proposed quantum computers; Peter Shor's 1994 factoring algorithm showed a dramatic potential use.
Is it proven?
Small quantum processors run algorithms as theory predicts. Large speedups are proven mathematically for some problems but not yet demonstrated on useful real-world tasks.
Open questions and mysteries
Exactly which problems gain big speedups, and whether superpositions can be kept stable at large scale, remain open.