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{{Short description|Fundamental problem concerning the emergence of definite outcomes in quantum measurements}}

{{Quantum book backlink|Conceptual and interpretations}}
'''Quantum Measurement problem''' is a central conceptual issue in [[Physics:Quantum mechanics|quantum mechanics]] concerning how definite outcomes arise from probabilistic quantum states. While the [[Physics:Wave function|wave function]] evolves deterministically according to the [[Physics:Schrödinger equation|Schrödinger equation]], measurements yield single, definite results rather than superpositions.<ref name="Weinberg1998">{{cite book |title=The Oxford History of the Twentieth Century |first=Steven |last=Weinberg |year=1998 |publisher=Oxford University Press |isbn=0-19-820428-0}}</ref><ref name="Zurek2003">{{cite journal |last=Zurek |first=Wojciech H. |title=Decoherence, einselection, and the quantum origins of the classical |journal=Reviews of Modern Physics |volume=75 |issue=3 |pages=715–775 |year=2003 |doi=10.1103/RevModPhys.75.715}}</ref>

This raises the fundamental question: how does a superposition of many possible outcomes reduce to a single observed reality?
[[File:Quantum_measurement_problem.jpg|thumb|400px|Conceptual illustration of the quantum measurement problem: a superposed quantum state yielding a single classical outcome upon observation, often represented by Schrödinger’s cat.]]

== Deterministic evolution vs measurement ==

In quantum theory, the state of a system is described by a wave function that evolves deterministically:

* Continuous, unitary evolution governed by the Schrödinger equation
* Linear superposition of multiple possible states

However, measurement introduces:

* A single definite outcome
* Apparent discontinuity (often called [[Physics:Wave function collapse|wave function collapse]])

This mismatch between continuous evolution and discrete measurement outcomes defines the measurement problem.<ref name="Weinberg2005">{{cite journal |last=Weinberg |first=Steven |title=Einstein's Mistakes |journal=Physics Today |volume=58 |issue=11 |pages=31–35 |year=2005 |doi=10.1063/1.2155755}}</ref>

== Schrödinger’s cat paradox ==

The measurement problem is famously illustrated by [[Physics:Schrödinger's cat|Schrödinger’s cat]]:

* A quantum event (e.g., radioactive decay) determines the fate of a cat
* Before observation, the system exists in a superposition
* The cat is simultaneously “alive” and “dead” in the formalism

Yet observation always yields a definite state, raising the question:

→ How do probabilities become actual outcomes?

== Major interpretations ==

Different interpretations of quantum mechanics provide distinct resolutions:

=== Copenhagen-type interpretations ===

The [[Physics:Copenhagen interpretation|Copenhagen interpretation]] posits:

* Measurement causes collapse of the wave function
* The wave function encodes probabilistic knowledge

However, the mechanism of collapse remains undefined.<ref>{{cite journal |last1=Schlosshauer |first1=Maximilian |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |title=A snapshot of foundational attitudes toward quantum mechanics |journal=Studies in History and Philosophy of Science Part B |volume=44 |issue=3 |pages=222–230 |year=2013 |doi=10.1016/j.shpsb.2013.04.004}}</ref>

=== Many-worlds interpretation ===

The [[Physics:Many-worlds interpretation|many-worlds interpretation]] removes collapse entirely:

* The universal wave function always evolves deterministically
* Measurement creates branching worlds
* All outcomes occur in separate branches

A key challenge is deriving the [[Physics:Born rule|Born rule]] for probabilities.<ref>{{cite encyclopedia |last=Kent |first=Adrian |title=One world versus many |year=2010 |publisher=Oxford University Press}}</ref>

=== de Broglie–Bohm theory ===

The [[Physics:De Broglie–Bohm theory|pilot-wave theory]] introduces hidden variables:

* Particles have definite trajectories
* The wave function guides motion
* Apparent collapse emerges dynamically

No fundamental collapse occurs.<ref>{{cite book |last=Goldstein |first=Sheldon |title=Bohmian Mechanics |year=2017 |publisher=Stanford Encyclopedia of Philosophy}}</ref>

=== Objective-collapse models ===

[[Physics:Objective-collapse theory|Objective-collapse theories]] modify quantum dynamics:

* Collapse occurs spontaneously
* Governed by stochastic nonlinear terms
* Predict experimentally testable deviations

Example: [[Physics:Ghirardi–Rimini–Weber theory|GRW theory]].<ref>{{cite journal |last1=Bassi |first1=Angelo |last2=Lochan |first2=Kinjalk |last3=Satin |first3=Seema |last4=Singh |first4=Tejinder P. |last5=Ulbricht |first5=Hendrik |title=Models of wave-function collapse |journal=Reviews of Modern Physics |volume=85 |issue=2 |pages=471–527 |year=2013 |doi=10.1103/RevModPhys.85.471}}</ref>

== Role of decoherence ==

[[Physics:Quantum decoherence|Quantum decoherence]] provides a partial resolution:

* Interaction with the environment suppresses interference
* Quantum probabilities become classical probabilities
* Explains emergence of classical behavior

However:

* Decoherence does **not** produce actual collapse
* It does not fully solve the measurement problem

It instead explains why classical outcomes appear stable.<ref name="Schlosshauer2005">{{cite journal |last=Schlosshauer |first=Maximilian |title=Decoherence, the measurement problem, and interpretations of quantum mechanics |journal=Reviews of Modern Physics |volume=76 |pages=1267–1305 |year=2005 |doi=10.1103/RevModPhys.76.1267}}</ref>

== Conceptual significance ==

The measurement problem highlights a deep divide:

* Quantum reality: superpositions and probabilities
* Classical reality: definite outcomes

It remains one of the most important unresolved issues in the foundations of physics, closely linked to:

* [[Physics:Interpretations of quantum mechanics|Interpretations of quantum mechanics]]
* [[Physics:Quantum decoherence|Decoherence theory]]
* [[Physics:Quantum information theory|Quantum information]]

== See also ==
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}

==References==
{{reflist|3}}

[[author|Harold Foppele}}
{{Sourceattribution|Physics:Quantum Measurement problem|1}}

Physics:Quantum Measurement problem - Revision history

imported>WikiHarold: Repair Quantum Collection B backlink template

imported>WikiHarold: Repair Quantum Collection B backlink template