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


{{Quantum book backlink|Conceptual and interpretations}}
{{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>
{{Quantum article nav|previous=Physics:Quantum Uncertainty principle|previous label=Uncertainty principle|next=Physics:Quantum Bell's theorem|next label=Bell's theorem}}


This raises the fundamental question: how does a superposition of many possible outcomes reduce to a single observed reality?
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[[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.]]
 
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'''Measurement problem''' quantum Measurement problem is a central conceptual issue in quantum mechanics concerning how definite outcomes arise from probabilistic quantum states. While the wave function evolves deterministically according to the Schrödinger equation, measurements yield single, definite results rather than superpositions. This raises the fundamental question: how does a superposition of many possible outcomes reduce to a single observed reality? Quantum Measurement problem is a central conceptual issue in quantum mechanics concerning how definite outcomes arise from probabilistic quantum states. While the wave function evolves deterministically according to the Schrödinger equation, measurements yield single, definite results rather than superpositions. This raises the fundamental question: how does a superposition of many possible outcomes reduce to a single observed reality? In quantum theory, the state of a system is described by a wave function that evolves deterministically:
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[[File:Quantum_measurement_problem.jpg|thumb|280px|Quantum Measurement problem.]]
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== Deterministic evolution vs measurement ==
== Deterministic evolution vs measurement ==
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* A single definite outcome   
* A single definite outcome   
* Apparent discontinuity (often called [[Physics:Wave function collapse|wave function collapse]])   
* Apparent discontinuity (often called 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>
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>
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== Schrödinger’s cat paradox ==
== Schrödinger’s cat paradox ==


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


* A quantum event (e.g., radioactive decay) determines the fate of a cat   
* A quantum event (e.g., radioactive decay) determines the fate of a cat   
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=== Copenhagen-type interpretations ===
=== Copenhagen-type interpretations ===


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


* Measurement causes collapse of the wave function   
* Measurement causes collapse of the wave function   
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=== Many-worlds interpretation ===
=== Many-worlds interpretation ===


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


* The universal wave function always evolves deterministically   
* The universal wave function always evolves deterministically   
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* All outcomes occur in separate branches   
* 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>
A key challenge is deriving the 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 ===
=== de Broglie–Bohm theory ===


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


* Particles have definite trajectories   
* Particles have definite trajectories   
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=== Objective-collapse models ===
=== Objective-collapse models ===


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


* Collapse occurs spontaneously   
* Collapse occurs spontaneously   
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* Predict experimentally testable deviations   
* 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>
Example: 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 ==
== Role of decoherence ==
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It remains one of the most important unresolved issues in the foundations of physics, closely linked to:
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]]  
* Interpretations of quantum mechanics   
* [[Physics:Quantum decoherence|Decoherence theory]]   
* [[Physics:Quantum decoherence|Decoherence theory]]   
* [[Physics:Quantum information theory|Quantum information]]   
* [[Physics:Quantum information theory|Quantum information]]   
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[[author|Harold Foppele}}
[[author|Harold Foppele}}
{{Author|Harold Foppele}}
{{Sourceattribution|Physics:Quantum Measurement problem|1}}
{{Sourceattribution|Physics:Quantum Measurement problem|1}}

Latest revision as of 12:22, 20 May 2026



← Back to Conceptual and interpretations
← Previous : Uncertainty principle
Next : Bell's theorem →

Measurement problem quantum Measurement problem is a central conceptual issue in quantum mechanics concerning how definite outcomes arise from probabilistic quantum states. While the wave function evolves deterministically according to the Schrödinger equation, measurements yield single, definite results rather than superpositions. This raises the fundamental question: how does a superposition of many possible outcomes reduce to a single observed reality? Quantum Measurement problem is a central conceptual issue in quantum mechanics concerning how definite outcomes arise from probabilistic quantum states. While the wave function evolves deterministically according to the Schrödinger equation, measurements yield single, definite results rather than superpositions. This raises the fundamental question: how does a superposition of many possible outcomes reduce to a single observed reality? In quantum theory, the state of a system is described by a wave function that evolves deterministically:

Quantum Measurement problem.

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 wave function collapse)

This mismatch between continuous evolution and discrete measurement outcomes defines the measurement problem.[1]

Schrödinger’s cat paradox

The measurement problem is famously illustrated by 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 Copenhagen interpretation posits:

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

However, the mechanism of collapse remains undefined.[2]

Many-worlds interpretation

The 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 Born rule for probabilities.[3]

de Broglie–Bohm theory

The pilot-wave theory introduces hidden variables:

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

No fundamental collapse occurs.[4]

Objective-collapse models

Objective-collapse theories modify quantum dynamics:

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

Example: GRW theory.[5]

Role of 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.[6]

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:

See also

Table of contents (198 articles)

Index

Full contents

9. Quantum optics and experiments (5) Back to index
Experimental quantum physics: qubits, dilution refrigerators, quantum communication, and laboratory systems.
Experimental quantum physics: qubits, dilution refrigerators, quantum communication, and laboratory systems.
14. Plasma and fusion physics (8) Back to index
Conceptual illustration of plasma physics in a fusion context, showing magnetically confined ionized gas in a tokamak and the collective behavior governed by electromagnetic fields and transport processes.
Conceptual illustration of plasma physics in a fusion context, showing magnetically confined ionized gas in a tokamak and the collective behavior governed by electromagnetic fields and transport processes.

References

  1. ↑ Weinberg, Steven (2005). "Einstein's Mistakes". Physics Today 58 (11): 31–35. doi:10.1063/1.2155755. 
  2. ↑ Schlosshauer, Maximilian; Kofler, Johannes; Zeilinger, Anton (2013). "A snapshot of foundational attitudes toward quantum mechanics". Studies in History and Philosophy of Science Part B 44 (3): 222–230. doi:10.1016/j.shpsb.2013.04.004. 
  3. ↑ Kent, Adrian (2010). "One world versus many". One world versus many. Oxford University Press. 
  4. ↑ Goldstein, Sheldon (2017). Bohmian Mechanics. Stanford Encyclopedia of Philosophy. 
  5. ↑ Bassi, Angelo; Lochan, Kinjalk; Satin, Seema; Singh, Tejinder P.; Ulbricht, Hendrik (2013). "Models of wave-function collapse". Reviews of Modern Physics 85 (2): 471–527. doi:10.1103/RevModPhys.85.471. 
  6. ↑ Schlosshauer, Maximilian (2005). "Decoherence, the measurement problem, and interpretations of quantum mechanics". Reviews of Modern Physics 76: 1267–1305. doi:10.1103/RevModPhys.76.1267. 

[[author|Harold Foppele}}


Author: Harold Foppele


Source attribution: Physics:Quantum Measurement problem

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