The free field is in fact necessary for the formal consistency of the theory. In particular, it is necessary for the preservation of the commutation relations, which is required by the unitary of time evolution in quantum theory: [ z ( t ) , p z ( t ) ] = [ U † † ( t ) z ( 0 ) U ( t ) , U † † ( t ) p z ( 0 ) U ( t ) ] = U † † ( t ) [ z ( 0 ) , p z ( 0 ) ] U ( t ) = i ℏ ℏ U † † ( t ) U ( t ) = i ℏ ℏ