  Trying to combine standard quantum field theories with gravity leads to a
breakdown of the usual structure of space-time at around the Planck length,
1.6*10^{-35} m, with possible violations of Lorentz invariance. Calculations of
preferred-frame effects in quantum gravity have further motivated high
precision searches for Lorentz violation. Here, we explain that combining known
elementary particle interactions with a Planck-scale preferred frame gives rise
to Lorentz violation at the percent level, some 20 orders of magnitude higher
than earlier estimates, unless the bare parameters of the theory are
unnaturally strongly fine-tuned. Therefore an important task is not just the
improvement of the precision of searches for violations of Lorentz invariance,
but also the search for theoretical mechanisms for automatically preserving
Lorentz invariance.
