One of the issues that remains controversial in special relativity is whether or not simultaneity at a distance has any meaning. In other words, when the traveling twin (he) in the twin paradox says that the home twin's (her) current age "right now" is such-and-such, is his conclusion true, real, and meaningful? Personally, I've always thought (purely for philosophical reasons) that the answer is "yes": his conclusion about her current age IS "meaningful", "real", and "true". I base that on my belief that she doesn't cease to exist just because they are separated by a vast distance. And if she DOES currently EXIST, then she must currently be DOING something specific. And if she is currently doing something specific, her brain must currently be in a specific and unique state, which implies that she is currently some specific age. But I THINK I may have discovered a proof that his conclusion about her current age is meaningless. If that proof is valid, that is obviously very disturbing to me (because of the above philosophical argument). I discovered the proof while investigating a possible new simultaneity method (different from the one that I wrote about some months ago). In the new method, I decided to assume a much stronger version of the causality principle than I had been using. The weaker causality principle that I had been using just says that how the traveling twin chooses to accelerate in the future can't influence the home twin's current age "right now". Under that (weak) causality principle, both my previous simultaneity method, and the CMIF simultaneity method, are (weakly) causal, but the Dolby and Gull method, and Minguizzi's method, are NOT (weakly) causal. The strong causality principle that I've decided to impose says that the home twin's (her) current rate of ageing (relative to the traveling twin's (his) rate of ageing) can't change for some period of time after he changes his velocity. Specifically, when he changes his velocity, he immediately sends a light pulse to her, and strong causality says that her relative rate of ageing can't change before that light pulse reaches her. I.e., strong causality says that his velocity change can't cause his conclusion about her current relative ageing rate to instantaneously change. So I took a specific twin paradox example, and constructed an age correspondence diagram (ACD), using the above reasoning. The outbound portion of the trip before his velocity change, as usual, gives a slope of 1/gamma for the first segment of the ACD ... he says she is ageing gamma times slower that he is. But, because of strong causality, that slope continues for a while after he changes his velocity. Then finally, after his transmitted pulse reaches her, the slope of the ACD changes to a value greater than one (and just enough to make her age be the required value at their reunion). The trouble is, when I tried to do that, I found that his age at the beginning of the steep segment is GREATER than his known age at their reunion, which is of course nonsense. There can be no doubt about the correctness of the outcome of the twin paradox at the reunion, and the strong causality assumption is inconsistent with that outcome. Therefore strong causality can't be correct. IF his conclusion about her current age is meaningful, real, and true, that would seem to me to REQUIRE that suddenly changing his velocity COULDN'T instantly change her current relative rate of ageing. Specifically, her current relative rate of ageing CAN'T change before the pulse reaches her. So meaningfulness REQUIRES that the strong causality principle be obeyed. But strong causality ISN'T obeyed in the (correct) twin paradox reunion outcome. Therefore his conclusion about her current age isn't meaningful, real, or true.