[Moderator's note: This is a portion of a post originally made in another thread, which has been moved due to being off topic there.] Einstein's Equivalence principle is based on the realization that, "An observer in a windowless room cannot distinguish between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g(2)". I'm not sure why the tidal effects quoted below in NOTES would not register in his elevator, as I thought I had worked that out with SR here before and the analogous fields actually appear indistinguishable, just as he said. I'm fairly confident that a link between electrodynamics and GR will eventually be published, but since I'm not aware of any present peer reviewed submission, it's not open for me to discuss here. Keep in mind that Einstein's original Unified Field idea did not include the unknown strong and weak 'forces' (interactions), and today, 'unifying all' might better be called a Theory of Everything (T.O.E.). Wes NOTES (1) From Wikipedia, "The special principle of relativity states that physical laws should be the same in every inertial frame of reference, but that they may vary across non-inertial ones. This principle is used in both Newtonian mechanics and the theory of special relativity. Its influence in the latter is so strong that Max Planck named the theory after the principle." (2) From wikipedia, "So the original equivalence principle, as described by Einstein, concluded that free-fall and inertial motion were physically equivalent. This form of the equivalence principle can be stated as follows. An observer in a windowless room cannot distinguish between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g. This is not strictly true, because massive bodies give rise to tidal effects (caused by variations in the strength and direction of the gravitational field) which are absent from an accelerating spaceship in deep space. The room, therefore, should be small enough that tidal effects can be neglected."