Srednicki — Quantum Field Theory · Problem 3.3

Canonical Quantization of Scalar Fields — Mark Srednicki

Physics — QFT · canonical-quantization-scalar-fields

Problem

Use U(Λ)−1φ(x)U(Λ)=φ(Λ−1x)U(\Lambda)^{-1}\varphi(x)U(\Lambda) = \varphi(\Lambda^{-1}x) to show that U(Λ)−1a(k)U(Λ)=a(Λ−1k) ,U(Λ)−1a†(k)U(Λ)=a†(Λ−1k) ,(3.34)\begin{aligned} U(\Lambda)^{-1}a(\mathbf{k})U(\Lambda) &= a(\Lambda^{-1}\mathbf{k})\,, \notag\\ U(\Lambda)^{-1}a^\dagger(\mathbf{k})U(\Lambda) &= a^\dagger(\Lambda^{-1}\mathbf{k})\,, \tag{3.34} \end{aligned} and hence that U(Λ)∣k1…kn⟩=∣Λk1…Λkn⟩ ,(3.35)U(\Lambda)|k_1\ldots k_n\rangle = |\Lambda k_1\ldots\Lambda k_n\rangle\,, \tag{3.35} where ∣k1…kn⟩=a†(k1)…a†(kn)∣0⟩|k_1\ldots k_n\rangle = a^\dagger(\mathbf{k}_1)\ldots a^\dagger(\mathbf{k}_n)|0\rangle is a state of nn particles with momenta k1,…,knk_1,\ldots,k_n.

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