The interaction picture does not always exist, though. In interacting quantum field theories, Haag's theorem states that the interaction picture does not exist. This is because the Hamiltonian cannot be split into a free and an interacting part within a superselection sector. Moreover, even if in the Schrödinger picture the Hamiltonian does not depend on time, e.g. H = H 0 + V, in the interaction picture it does, at least, if V does not commute with H 0, since H i n t ( t ) ≡ ≡ e ( i / ℏ ℏ ) t H 0 V e ( − − i / ℏ ℏ ) t H 0 .