Hi all - hope I'm not beating a dead horse here, but I'm following up on at least two other threads (made sense to consolidate): There are theories of quantum gravity (or the Standard Model Extension) that allow for local Lorentz violation. So, my first question: is there any reason why there has to be one preferred frame (across the whole universe). Could there not be multiple preferred frames, if, e.g., the values of fundamental 'constants' changed in both space and time? My second question follows from another thread in which I asked whether, in a universe where Lorentz symmetry is broken, one would expect to see different vacuum energies when moving in different directions. The relevant thread is here: https://www.physicsforums.com/threads/lorentz-violation-and-the-physical-vacuum.900243/ Now, if one did see different vacuum energies (non-zero vacuum expectation values), one would expect to see particles when moving in certain directions, while not in others, no? Haelfix kindly commented on this: I'm wondering if anyone has any input on this? Final question: given the constraints on Lorentz violation established so far, it must the be case that, were it true that differences in vacuum energy existed, these differences must be very small or they would have been detected in laboratory experiments (i.e., I should not expect that, by moving in a certain direction, I run into a bunch of electrons or protons, etc.). Thanks all - sorry for the lengthy post. It seems the more you dig, the more interesting things become.