Einstein-aether Theory *

Could there be an æther after all and we have just not yet noticed it? By an “æther” of course we do not mean to suggest a mechanical medium whose deformations correspond to electromagnetic fields, but rather a locally preferred state of rest at each point of spacetime, determined by some hitherto unknown physics. Such a frame would not be determined by a circumstance such as the moon’s gravitational tidal field, or the thermal cosmic microwave background radiation, but rather would be inherent and unavoidable. Considerations of quantum gravity have in multiple ways led to this question, and it has also been asked in the context of cosmology, where various puzzles hint that perhaps something basic is missing in the standard relativistic framework. Lorentz symmetry violation by preferred frame effects has been much studied in non-gravitational physics, and is currently receiving attention as a possible window on quantum gravity. But what about gravity itself? General relativity is based on local Lorentz invariance, so if the latter is violated what becomes of the former? It is hard to imagine, both philosophically and technically, how we could possibly give up general covariance, the deep symmetry finally grasped through Einstein’s long struggle. Thus the