I stumbled upon this point several times, the latest beeing this question: Connection between conserved charge and the generator of a symmetry
I want to understand, why Quantum fields transform under symmetry transformations as
(g⋅ϕ)(y)=T−1gϕ(y)Tg=e−tXϕ(y)etX=[1+t[X,⋅]+O(t2)]ϕ(y)
Two ideas from me:
1) Fields are treated as operators in the Heisenberg picture, and instead of transforming the states with Tg|x>, the states stay as they are and all operators, including the quantum fields transform as T−1gOTg
2) There is a good reason why quantum fields live in Te, i.e. the tangent space at the identity of the group (=the Lie algebra), onto which the group acts with the adjoint action: Adg(x)=T−1gXTg ∀X∈Te
Any ideas or reading tips would be awesome!
Answer
Your idea 1) is the right idea: It's just the law of transformation of matrices generalized from the transformation of matrices:
If we apply a general linear transformation U:V→V on a vector space, the matrices/operators on it transform as M↦U†MU
For unitary operators U†=U−1, so the transformation law becomes M↦U−1MU. Since Tg is the representation of the group upon our space of states, the quantum fields as operators transform according to this law.
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