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

That’s not exactly how relativity works. It’s about differences in time flow. Like your perception of the time flow will not differentiate, but to your frame of reference other people’s would be aging much faster if you were moving at the speed of light. But that is a very hypothetical scenario. It does not mean that there necessarily are different times that exist at the same time, but simply that it’s possible to experience time at different rates.

The theory of general relativity does not really settle what now is, or if that question even makes sense.

It just explains how time flow changes based on mass, and speed.

However, what you are asking about in your question is essentially a time block (now’s) multiverse, and not only would that would mean that the universe is deterministic. Meaning there is no free will, and every action ‘will’ happen no matter what – because rules on a planck scale.

But also that what we perceive as “now” can be different for you, and your mother. Like theoretically she could be in her now (time block) as a 10 year old, where as your now (time block) is reading this message!

In terms of the universe a “now” is a really really complicated issue. We all have our own perceivable/observable universes (although on a universal scale earth is about the same) and now (time block) theoretically could be different for everyone in the universe, but for anything to interact in any way, the now must be synchronized – otherwise the universe is pre-determined.

Your true question is, what is “now”. And is it the same?

[I recommend you watch this](https://youtu.be/EagNUvNfsUI)

But this video is not an ELI5, and I’m definitely not clever enough to ELI5 this issue.