Science › Quantum world
The measurement problem
1927· Open question
Open questionNobody knows the answer.
The unresolved question of why measuring a quantum system gives one definite result when the theory describes a blend of possibilities.
See it on the Science map →How it works
Quantum theory has two rules. Between measurements, the wave function evolves smoothly and stays a blend. At measurement, it seems to jump to one outcome. But measuring devices are made of atoms too, so why should they follow different rules, and what counts as a measurement? Interpretations answer differently: collapse is real, the world branches, or hidden details decide. All predict the same lab results.
Why it matters
It is the deepest open question about what quantum theory says reality is, and it shapes how physicists think about observers and the universe as a whole.
How we know
No experiment yet distinguishes the main interpretations. Decoherence experiments explain why blends become invisible, and tests of 'objective collapse' theories using ever-larger objects have so far found no collapse.
Still a mystery
Is there a physical collapse, branching worlds, hidden variables, or is the wave function only information? Surveys of physicists show no consensus.
Connections (6)
▸The double-slit experiment shows watching changes the result, posing The measurement problemProven
The relationship
Put a detector at the slits to learn which one each particle uses and the stripes disappear: particles behave as if they took one path. Remove the detector and the stripes return. Learning the path destroys the superposition. Why gaining information has this effect, and what counts as 'watching', is the measurement problem.
How the link was found
Debated by Bohr and Einstein from 1927; real which-path experiments became possible from the 1990s, for example with atoms by Dürr, Nonn and Rempe in 1998.
Why it's proven
Experiments confirm that path information, not a human looking, kills the interference. Any physical record of the path does it, so 'watching' means any recording interaction.
Open questions and mysteries
Why one definite outcome appears is unexplained. 'Quantum eraser' experiments, where erasing path data restores stripes, are often misread as backward-in-time effects; standard analysis shows they are not.
▸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.
▸The measurement problem is answered with "collapse" by Copenhagen interpretationSpeculative
The relationship
Copenhagen answers the problem by adding a rule: on measurement, the wave function collapses to one outcome, with odds given by Born's rule. It treats the measuring apparatus as classical and declines to describe the collapse physically. Some versions treat the wave function as only a calculation tool, so collapse is just updated knowledge.
How the link was found
Bohr and Heisenberg from 1927; John von Neumann gave collapse its mathematical form in 1932.
Is it proven?
Not proven. Collapse-based predictions match every experiment, but so do rival interpretations. It is one answer among several.
Open questions and mysteries
What counts as a measurement, and what physically happens during collapse, remain unanswered in this view.
▸The measurement problem is answered with "branching" by Many-worlds interpretationSpeculative
The relationship
Many-worlds answers by denying collapse. The measuring device and observer become entangled with the particle, and the universal wave function splits into branches, one per outcome. Each branch contains an observer who sees one definite result. Decoherence keeps the branches from interfering, so each looks like the only world.
How the link was found
Hugh Everett III, in his 1957 Princeton PhD thesis supervised by John Wheeler; Bryce DeWitt named and popularized it around 1970.
Is it proven?
Not proven, and possibly untestable. It reproduces standard predictions; its support comes from mathematical economy, not experiment.
Open questions and mysteries
How probabilities make sense when every outcome occurs, and what exactly counts as a 'world', are still debated.
▸The measurement problem is partly explained by Decoherence: why big things look normalProven
The relationship
Decoherence shows that when a quantum system interacts with its environment, its different possibilities quickly lose the ability to interfere. To any observer, the system then looks as if it is in one definite state, with standard odds. This explains why measurements look classical, without saying why one particular outcome is picked.
How the link was found
H. Dieter Zeh (1970), Wojciech Zurek (early 1980s), and Erich Joos with Zeh (1985).
Why it's proven
Decoherence rates have been measured, for example by Haroche's group in 1996 and in Vienna molecule-interference experiments in 2003–2004, matching theory.
Open questions and mysteries
It handles the 'why do things look classical' half. The 'why this outcome' half still needs an interpretation.
▸The measurement problem raises the role of Observers and consciousnessOpen question
The relationship
Quantum theory describes a particle as a spread of possibilities until it is measured, then gives one definite result. The theory never says exactly what counts as a measurement. That gap invites the question of whether an observer, and maybe a conscious one, has a special role, or whether any physical interaction counts.
How the link was found
John von Neumann (1932) showed the cut between system and observer could be placed anywhere. Fritz London and Edmond Bauer (1939) and Eugene Wigner (1961) suggested consciousness might complete it; Wigner later dropped the idea.
Is it proven?
Decoherence theory (Zeh 1970, Zurek in the 1980s) explains, with experiments to back it, why interactions with surroundings hide quantum effects. Photon experiments in 2019 tested Wigner's-friend scenarios. None show consciousness is needed.
Open questions and mysteries
Decoherence explains why we never see blends, but not why one specific outcome occurs. Interpretations such as many-worlds and pilot waves still answer that differently.