  In contrast to Hamiltonian perturbation theory which changes the time
evolution, "spacelike deformations" proceed by changing the translations
(momentum operators). The free Maxwell theory is only the first member of an
infinite family of spacelike deformations of the complex massless Klein-Gordon
quantum field into fields of higher helicity. A similar but simpler instance of
spacelike deformation allows to increase the mass of scalar fields.
