Editing Measurement (section)

=== Quantum mechanics ===
In [[quantum mechanics]], a measurement is an action that determines a particular property (such as position, [[momentum]], or energy) of a quantum system. Quantum measurements are always statistical samples from a [[probability distribution]]; the distribution for many quantum phenomena is discrete, not continuous.<ref name=messiah>{{Cite book|last=Messiah|first=Albert|title=Quantum Mechanics|date=1966|publisher=North Holland, John Wiley & Sons|isbn=0486409244|language=en}}</ref>{{rp|197}} Quantum measurements alter [[quantum states]] and yet repeated measurements on a quantum state are reproducible. The measurement appears to act as a filter, changing the quantum state into one with the single measured quantum value.<ref name="messiah"></ref> The unambiguous meaning of the [[measurement problem|quantum measurement]] is an unresolved fundamental problem in [[quantum mechanics]]; the most common interpretation is that when a measurement is performed, the [[wavefunction]] of the quantum system "[[wavefunction collapse|collapses]]" to a single, definite value.<ref>{{cite book | last = Penrose | first = Roger | title = The road to reality : a complete guide to the laws of the universe | publisher = Vintage Books | location = New York | year = 2007 | isbn = 978-0-679-77631-4 }} "The jumping of the quantum state to one of the eigenstates of '''Q''' is the process referred to as ''state-vector reduction'' or ''collapse of the wavefunction''.  It is one of quantum theory's most puzzling features&nbsp;..."  "[T]he way in which quantum mechanics is used in practice is to take the state indeed to jump in this curious way whenever a measurement is deemed to take place." p 528 Later Chapter 29 is entitled the Measurement paradox.</ref>