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

For 99.9% of the conversation, the only perspective that everyone understands is that of humans living on earth. So it makes sense to use that perspective as a measurement point. It would not make a whole lot of sense to say it is xx “years” old from the perspective of someone living on another planet since pretty much no one understands anything from that perspective.

We can say that a “year” on Jupiter is about 12 of earth years. Would it make much sense to say that someone is 2 Jupiter years old? It is perfectly definable and measurable, but such a measure is pretty meaningless to everyone. Science is about discovery and the communication of discovery. So, where possible, it is logical to choose a way of communication that is relatable to most people.