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University Quantum Mechanics I

University Quantum Mechanics I · The Free Particle · 5.05

Fourier decomposition: position space and momentum space

ψ(x) = (1/√(2π)) integral of φ(k) e(ikx) dk, inverted by φ(k) = (1/√(2π)) integral of ψ(x) e(−ikx) dx.

Course-map guide · not a complete lesson or simulation

Scope & orientation

What this subsection covers

ψ(x) = (1/√(2π)) integral of φ(k) e(ikx) dk, inverted by φ(k) = (1/√(2π)) integral of ψ(x) e(−ikx) dx.

A strong response uses re ψ, im psi and |ψ|² on one axis and states where the model stops being reliable.

Reasoning checklist

Evidence, assumptions and limits

01

Assumptions to state

State the system, observable, approximation, and conditions held fixed before using a model.

02

Evidence to collect

Does FFT evolution of a Gaussian packet, applying the free phase exp(−i ℏ k² t/(2m)) in k-space, reproduce σ₀ √(1 + (ℏ t/(2 m σ₀²))²), and where does the periodic grid break it?

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Does FFT evolution of a Gaussian packet, applying the free phase exp(−i ℏ k² t/(2m)) in k-space, reproduce σ₀ √(1 + (ℏ t/(2 m σ₀²))²), and where does the periodic grid break it? Useful evidence includes width fitted from |ψ|² at each step against the analytic curve, ⟨x⟩(t) against x₀ + ℏ k₀ t/m, the norm conserved to a stated tolerance, the time at which the packet reaches the grid edge and wraps, and one run started from a chirped packet that narrows before it spreads..

03

Limits to state

This GioPhysics course map is an adaptable learning sequence, not academic credit, accreditation, or a universal university syllabus.

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This GioPhysics course map is an adaptable learning sequence, not academic credit, accreditation, or a universal university syllabus. It is a first dedicated quantum-mechanics course, distinct from University Physics V, which covers quantum mechanics alongside atomic, nuclear and particle physics in twenty units: this course is narrower, slower, and teaches the linear algebra it needs rather than assuming it. Hydrogen appears here as an introduction, with the full radial derivation belonging to a second course. A midterm examination is assumed around week 8. Follow your institution's published scope, notation, laboratory programme, and assessment rules. A result should be checked against units, signs, limiting cases, and the conditions under which its model was derived.

Diagram & examples

Work the claim before choosing an equation

Interactive concept map

Follow the model from claim to evidence.

01 · Physical claimFourier decomposition: position space and momentum space

ψ(x) = (1/√(2π)) integral of φ(k) e(ikx) dk, inverted by φ(k) = (1/√(2π)) integral of ψ(x) e(−ikx) dx. One state, two descriptions, with Plancherel keeping the norm. It needs psi square-integrable.

02 · RepresentationThe subsection's claim

Model, evidence, and boundary

03 · TestPrediction before measurement

What is the strongest test of a claim about Fourier decomposition: position space and momentum space?

04 · Evidence & boundaryDecide, then qualify

Does FFT evolution of a Gaussian packet, applying the free phase exp(−i ℏ k² t/(2m)) in k-space, reproduce σ₀ √(1 + (ℏ t/(2 m σ₀²))²), and where does the periodic grid break it?

Read the complete note

Does FFT evolution of a Gaussian packet, applying the free phase exp(−i ℏ k² t/(2m)) in k-space, reproduce σ₀ √(1 + (ℏ t/(2 m σ₀²))²), and where does the periodic grid break it? Useful evidence includes width fitted from |ψ|² at each step against the analytic curve, ⟨x⟩(t) against x₀ + ℏ k₀ t/m, the norm conserved to a stated tolerance, the time at which the packet reaches the grid edge and wraps, and one run started from a chirped packet that narrows before it spreads..

Interactive diagram for Fourier decomposition: position space and momentum space: follow the physical claim through its representation, proposed test, evidence, and model boundary.

modelModel, evidence, and boundary turns the stated idea into a representation that can make a prediction.

Example questions

Try the reasoning before revealing the structure.

Diagram check

What is the strongest test of a claim about Fourier decomposition: position space and momentum space?

Quick check

Test the reasoning, not recall

What is the strongest test of a claim about Fourier decomposition: position space and momentum space?

Choose an answer to test the model.