Why does a measurement produce one definite outcome?
In plain words
When a detector interacts with a particle in a superposition (a blend of states), the Schrodinger equation predicts a blended detector, yet one reading is always seen. The question is what physical process or rule selects that single reading.
Precise statement
For a system $S$ in state sum_k c_k |k> coupled unitarily to apparatus $A$ and environment $E$, unitary evolution gives sum_k c_k |k>|A_k>|E_k>; decoherence makes the reduced density matrix of $S+A$ diagonal but selects no single $k$. Required: a modified dynamics, an added ontology (hidden variables, branches) or a principled account that yields single outcomes with frequencies $\mid c_k\mid^2$, together with either predictions that differ from standard quantum mechanics or a proof of empirical equivalence. This is the root problem of the topic; its workable sharper forms are listed under related.
What would settle it
An experiment detecting a deviation from unitary evolution predicted by one class of proposals, or a proof that a specified account reproduces all quantum predictions using only its stated assumptions.
See also
- Related How large can a Wigner friend be while violating Local Friendliness?
- Related Optimal overlap of epistemic states in $\psi$-epistemic hidden-variable models
- Related A pilot-wave theory for interacting quantum fields without preferred foliation
- Related Objective collapse models and their experimental tests
- Related Does any parameter window of white-noise CSL remain viable?
- Related Can the Born rule be derived from unitary quantum mechanics alone?