  We demonstrate that long-range interaction in a system can lead to a very
strong interaction between long-wavelength quasiparticles and make them heavily
damped. In particular, we discuss magnon spectrum using 1/S expansion in 3D
Heisenberg ferromagnet (FM) with arbitrary small dipolar forces at T<<T_C. We
obtain that a fraction of long-wavelength magnons with energies e_k<T has
anomalously large damping G_k (ratio G_k/e_k reaches 0.3 for certain k). This
effect is observed both in quantum and classical FMs. Remarkably, this result
contradicts expectation of the quasiparticle concept according which a weakly
excited state of a many-body system can be represented as a collection of
weakly interacting elementary excitations. Particular materials are pointed out
which are suitable for corresponding experiments.
