Do gravitational waves create particles?

Solving the Klein-Gordon equation for a free scalar field in a curved spacetime in the weak-field approximation we rederive the results of Gibbons et al: no particle creation from the interaction between scalar fields and gravitational waves. We show, however, that particle creation would really occur when the matter field is constrained in space by some boundaries. Such an effect is likely to be related to the uncertainty principle. in connection with a sort of dimensional reduction (as in the Kaluza-Klein theory) that causes the field to acquire a mass term, also responsible for conformal symmetry breaking in a spatially flat FRW spacetime. Although derived in the weak-field approximation, the results seem also to be reasonably valid in the presence of exact gravitational waves.