Editing Quantum indeterminacy (section)

{{Short description|Apparent lack of definite state before measurement of quantum systems}}
{{Use American English|date=January 2019}}{{Quantum mechanics}}{{More citations needed|date=December 2008}}
'''Quantum indeterminacy''' is the apparent ''necessary'' incompleteness in the description of a [[physical system]], that has become one of the characteristics of the standard description of [[quantum physics]]. Prior to quantum physics, it was thought that
{{ordered list | list-style-type = lower-alpha
| a physical system had a determinate [[Classical mechanics|state]] that uniquely determined all the values of its measurable properties, and
| [[conversely]], the values of its measurable properties uniquely determined the state.
}}

'''Quantum indeterminacy''' can be quantitatively characterized by a [[probability distribution]] on the set of outcomes of [[Measurement problem|measurements]] of an [[observable]]. The distribution is uniquely determined by the system state, and moreover quantum mechanics provides a recipe for calculating this probability distribution.

Indeterminacy in measurement was not an innovation of quantum mechanics, since it had been established early on by experimentalists that [[Observational error|errors]] in measurement may lead to indeterminate outcomes. By the later half of the 18th century, measurement errors were well understood, and it was known that they could either be reduced by better equipment or accounted for by statistical error models. In quantum mechanics, however, [[Uncertainty principle|indeterminacy]] is of a much more fundamental nature, having nothing to do with errors or disturbance.