In a previous thread, reference was made to an entertaining "defense" of relativity by Einstein, which can be found here: https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity One of the arguments Einstein makes in this dialog is that the twin paradox can be analyzed using "pseudo gravitational fields"--the traveling twin can explain the fact that his clock shows less elapsed time when he and the stay-at-home twin meet again, by saying that, when he fires his rockets to turn around, a gravitational field exists, and since the stay-at-home twin is at a much higher altitude in the field, his clock runs much faster. The Usenet Physics FAQ article on the twin paradox includes an analysis based on this same idea: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html In this post in a recent thread, harrylin mentions a criticism of this argument of Einstein's, by Builder. An actual quote from his criticism is given in that thread, but here I will try to paraphrase it to get at the essence of the argument: the problem with the traveling twin's explanation in terms of a "gravitational field" is that, when he fires his rocket to turn around, this "field" must instantly propagate to the stay-at-home twin, and when he completes his turnaround and turns off his rocket, the disappearance of the "field" must likewise propagate instantly to the stay-at-home twin. But this violates causality: no field can propagate faster than light. So this "field" explanation cannot be physically correct. The question I want to pose to the forum is: is this criticism correct? Does it invalidate the "GR analysis" of the twin paradox that is presented by Einstein and in the Usenet Physics FAQ? I have my own answer, but I'll put it in a separate post.