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

Don’t think of time as something measurable in and of itself. What we experience as time is the effects of entropy. Entropy is how things change (super simplified version) and time is how much things change.

In places where time doesn’t move as quickly, entropy sort of slows down. For example, if you have a nail that is rusting, it will rust more slowly in places with higher gravity or speed than others.

If you have two rusting nails and one of them is near you and the other is 1000 lightyears away, it makes no sense to say “The entire universe is x amount of rust on my nail.” and expect all of the nails in the universe have rusted the same amount. All of the nails have been rusting at different rates.

Instead, you would have to say, “My nail rusts at this rate, my nail has rusted this much, therefore the local ‘time’ is x.” You would then say that the universe is x years old compared to your nail.