That's interesting, thanks for explaining.

This means outcomes need to be symmetric.

I guess I'm just confused as to how you can have situations where someone ages faster than another due to time dilation. That doesn't seem to be a symmetrical outcome. Like this scene in Interstellar (which, as I understand it, is scientifically accurate). In this scene Matthew McConaughey goes away for 3 hours and finds that, due to time dilation, his kids have aged 23 years. There doesn't seem to be a symmetry here. Because Matthew McConaughey aged slower than his children, it would be weird if, from his point of view, his kid's clocks were ticking slower than his (his kids can't also think that he has aged 23 years in 3 hours). So how do you resolve this lack of symmetry with the requirement that outcomes need to be symmetrical from both these reference frames?

Granted, in this scene Matthew McConaughey ages due to gravitational time dilation. Is that somehow make things different? Would a similar scenario not be possible with time dilation solely caused by travelling at very fast speeds?

(Please let me know if I'm not making sense with these questions, and I'll try to reword them)