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The comparison of form factors calculated from a single-particle current indifferent relativistic quantum mechanic approaches evidences tremendousdiscrepancies. The role of constraints coming from space-time translations isconsidered here with this respect. It is known that invariance under thesetranslations implies the energy-momentum conservation relation that is usuallyassumed to hold globally. Transformations of the current under thesetranslations, which lead to this result, also imply constraints that have beenignored so far in relativistic quantum mechanic approaches. An implementationof these constraints is discussed in the case of a model with two scalarconstituents. It amounts to incorporate selected two-body currents to allorders in the interaction. Discrepancies for form factors in differentapproaches can thus be removed, contributing to restore the equivalence ofdifferent approaches. Results for the standard front-form approach ($q^+=0$)are found to fulfill the constraints and are therefore unchanged. The relationwith results from a dispersion-relation approach is also made.

Form factors in relativistic quantum mechanics: constraints from space-time translations