The famous twin paradox of special relativity involves a scenario where one twin (he) rockets away from the home twin (her), coasts to a far-away turnpoint, reverses course, coasts back, and comes to a halt when they are reunited. At the reunion, both twins agree (by inspection) that she is older than he is.
There is no dispute about the outcome at the reunion. But physicists DO differ about what HE concludes about HER current age DURING his trip. One school of thought is that he says that she is ageing more slowly than he is, on both the outbound leg and on the inbound leg, but that he concludes that she instantaneously ages by a large amount during the instantaneous turnaround. But that conclusion can be shown to imply that he will have to conclude that it is possible for her to instantaneously get YOUNGER when he changes speed in certain other ways. THAT result is abhorrent to many (maybe most) physicists. The most extreme reaction is to conclude that simultaneity at a distance is simply a meaningless concept. Other physicists react by embracing alternative simultaneity methods, that don’t result in instantaneous ageing (either positive or negative).
So what does the above have to do with philosophy? For many physicists, there is no place for philosophy in special relativity. Philosophical arguments are usually banned on physics forums. But philosophy has always played a role in my thinking on the subject, even though I’m a physicist, not a philosopher (and I have only a VERY limited knowledge of philosophy). Philosophy has entered into my thinking about the twin paradox in this way: when some physicists contend that simultaneity at a distance is meaningless, I have a philosophical problem with that. IF I were that traveler, I don’t think I would be able to believe that she no longer EXISTS whenever I am not co-located with her. (And I doubt that many other physicists believe that either). BUT many physicists DO believe that she doesn’t have a well-defined current AGE when he is separated from her (at least if he has accelerated recently). THAT’S the conclusion that I can’t accept philosophically: it seems to me that if she currently EXISTS right now, she must be DOING something right now, and if she is DOING something right now, she must be some specific AGE right now. So I conclude that her current age, according to him, can’t be a meaningless concept. That puts me at odds with many other physicists.
What say the philosophers on this forum?