  Extending general relativity by adding extra degrees of freedom is a popular
approach for explaining the accelerated expansion of the Universe and to build
high energy completions of the theory of gravity. The presence of such new
degrees of freedom is, however, tightly constrained from several observations
and experiments that aim to test general relativity in a wide range of scales.
The viability of a given modified theory of gravity, therefore, strongly
depends on the existence of a screening mechanism that suppresses the extra
degrees of freedom. We perform simulations, and find that waves propagating in
the new degrees of freedom can significantly impact the efficiency of some
screening mechanisms, thereby threatening the viability of these modified
gravity theories. Specifically, we show that the waves produced in the
symmetron model can increase the amplitude of the fifth force and the
parametrized post-Newtonian parameters by several orders of magnitude.
