Why is High Energy Physics Lorentz Invariant?
  Despite the tremendous empirical success of equivalence principle, there are
several theoretical motivations for existence of a preferred reference frame
(or aether) in a consistent theory of quantum gravity. However, if quantum
gravity had a preferred reference frame, why would high energy processes enjoy
such a high degree of Lorentz symmetry? While this is often considered as an
argument against aether, here I provide three independent arguments for why
perturbative unitarity (or weak coupling) of the Lorentz-violating effective
field theories put stringent constraints on possible observable violations of
Lorentz symmetry at high energies. In particular, the interaction with the
scalar graviton in a consistent low-energy theory of gravity and a (radiatively
and dynamically) stable cosmological framework, leads to these constraints. The
violation (quantified by the relative difference in maximum speed of
propagation) is limited to $\lesssim 10^{-10} E({\rm eV})^{-4}$ (superseding
all current empirical bounds), or the theory will be strongly coupled beyond
meV scale. The latter happens in extended Horava-Lifshitz gravities, as a
result of a previously ignored quantum anomaly. Finally, given that all
cosmologically viable theories with significant Lorentz violation appear to be
strongly coupled beyond meV scale, we conjecture that, similar to color
confinement in QCD, or Vainshetin screening for massive gravity, high energy
theories (that interact with gravity) are shielded from Lorentz violation (at
least, up to the scale where gravity is UV-completed). In contrast, microwave
or radio photons, cosmic background neutrinos, or gravitational waves may
provide more promising candidates for discovery of violations of Lorentz
symmetry.
