Why does a measurement produce one definite outcome? · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
Why does a measurement produce one definite outcome?
Plain
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 · changed · core
Revision 1: 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.
Revision 2: 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.
Settled by
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.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
open
Status note
(empty)
Posed since
(empty)

Decisions