Several approaches to quantum gravity suggest violations of Lorentz symmetry as low-
energy signatures. This article uses a concrete Lorentz-violating quantum field theory to
study different inertial vacua. We show that they are unitarily inequivalent and that the
vacuum in one inertial frame appears, in a different inertial frame, to be populated with
particles of arbitrarily high momenta. At first sight, this poses a critical challenge to the
physical validity of Lorentz-violating theories, since we do not witness vacuum excitations by …