Causal linear response and transport inference
Research context
research method · conditional · As of 2026-10-10
For perturbation H′(t)=−f(t)B, the retarded response of A is χ_AB(t)=(i/ħ)θ(t)⟨[A(t),B(0)]⟩ in an equilibrium reference state.
Key knowledge
For perturbation H′(t)=−f(t)B, the retarded response of A is χ_AB(t)=(i/ħ)θ(t)⟨[A(t),B(0)]⟩ in an equilibrium reference state. Its correlation spectrum can determine small-signal transport under specified limits. The equilibrium fluctuation–dissipation relation is not generally valid for a strongly driven nonstationary state.
Values and equations
χ_AB(t)=(i/ħ)θ(t)⟨[A(t),B(0)]⟩; δ⟨A(t)⟩=∫χ_AB(t−t′)f(t′)dt′
- A,B
- Observable operators with their own physical units
- f
- Perturbing field with units energy divided by B
- χ_AB
- Response kernel; units A divided by field and time
- ħ
- Reduced Planck constant J s
- t,t′
- Times s
- θ
- Causal step function
Conditions and scope
- Small perturbation, calibrated conjugate field f and chosen Fourier/sign convention; equilibrium/stationary reference state.
- DC transport can depend on thermodynamic, zero-frequency and relaxation-limit order; finite simulations need convergence and conserved-current checks.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Inspected §§4.1–4.4: causal response, Kubo commutator and equilibrium fluctuation–dissipation relation; ħ restored explicitly; source check dated 2026-10-10. Scope is limited to the cited statements, equations and assumptions, not the whole publication.
2026-10-10 · Author source inspection of the displayed claim, model assumptions and equations; independent exact-content and branch-inventory review is a separate gate. Contemporary status is a dated snapshot, not a promise of ongoing monitoring.
Entry details
Permanent link to this entry ↗- Record identity
- research-physics-kubo-transport
- Revision
- research.1
- Structured reference record
- Postgraduate & research reference