If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

Question

For some parts relative to us, only a billion years would have passed, for others maybe 20?

In: Physics

Answer 1

Simple, because time is actually just a construct. It isn’t a physical thing, merely a label attached to relativistic perception.

So while the interaction between molecules is effected by their relativistic speed and thus “age” differently, the pure time of existence of the components is the same.

If a 20 year old human departs earth on a round trip spacecraft and returns 300 years later with the body of a 21 year old, it doesn’t matter that the human only perceived 1 year, his body still exists for those 300 years and so it is 320 years old from the earth standard.