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physics
Interactive Course
Interactive chapters from intuition to mastery
Structured Lessons
Eight modules with formulas and self-checks
Quantum Brain
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Hands-on circuits that teach one idea each
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Circuit Lab
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Quick reference for all quantum gates
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Quantum FoundationsThe Uncertainty Principle0/6 mastered
Module overview
Quantum Foundations0/6 mastered
Qubits and State VectorsCore
Superposition and MeasurementCore
The Wavefunction
Operators, Eigenstates, Eigenvalues
The Schrodinger Equation
The Uncertainty Principle
Next module
Measurement, Phase, and Interference
Module 1
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Quantum Foundations
Qubits and State VectorsCore
Superposition and MeasurementCore
The Wavefunction
Operators, Eigenstates, Eigenvalues
The Schrodinger Equation
The Uncertainty Principle
Next module
Measurement, Phase, and Interference
Home/Lessons/Quantum Foundations/The uncertainty principle
6Lesson 6 of 6inQuantum Foundations

The uncertainty principle

Uncertainty is not a flaw of instruments. It is a fundamental limit on how precisely certain pairs of properties can be defined at the same time.

The uncertainty principle explains why wave packets spread over time, why perfect classical trajectories fail at small scales, and why basis choice matters so much in quantum measurements. It sets hard limits on what any technology can achieve.

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CommutatorsLaw / equationUncertainty PrinciplePrinciple
1
The intuition
Start with the plain-language idea

A quantum state can be very sharp in position or very sharp in momentum, but not both at once. The sharper one becomes, the more spread out the other must be. This is not about clumsy measurements disturbing the system -- it is a property of the quantum state itself. Some pairs of properties (called non-commuting observables, meaning the order you measure them in matters) are subject to this tradeoff.

2
See it concretely
A real example before the abstraction

Picture squeezing a balloon: compressing it in one direction makes it bulge in another. A quantum state that is tightly localized in position needs many different momentum components to build that localization, which means the momentum is spread out. You cannot flatten both directions at once.

3
Tempting but wrong
The mistake most people make
Tempting but wrong
It is tempting to think uncertainty means anything can happen or that quantum mechanics is inherently unpredictable. That is not right. The spreads are governed by strict mathematical rules. Uncertainty tells you precisely how much spread you must accept -- it is a quantitative prediction, not a statement of chaos.
4
The precise version
Now with the formal detail
ΔxΔp≥2ℏ​

For any two observables whose operators do not commute (meaning A-hat B-hat is not equal to B-hat A-hat), there is a lower bound on the product of their statistical spreads. For position and momentum, this bound is hbar/2. This is a mathematical property of the quantum state, not a story about experimental limitations. Non-commuting observables include position/momentum, and different components of spin.

Check your understanding
Explain why the uncertainty principle is about the quantum state itself, not about measurement instruments.
Think about this against what you just read.
What does it mean for two observables to not commute?
Think about this against what you just read.
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↗ MIT OCW 8.04: lecture notes↗ Griffiths and Schroeter, Introduction to Quantum Mechanics
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How quantum states evolve over time
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