eli5 Is it correct to think of events happening on the far side of the universe or in a different galaxy as occurring “at the same time”?
In: Physics
Depends on what you mean by time.
But if something happens somewhere, where it can not interact with us (the observer) then in all practical purposes it didn’t happen at all.
But time is a tricky thing, because it is and isn’t. It is a thing that exists between things that exist. If you have an empty cube with absolutely nothing in it, no particles no interacting forces, there is no time in it. Time exists between two things that interact.
For an observer to say that something has happened, it can only measure it from it’s perspective so time comes in to existence from our perspective. If it can’t interact with us or we can’t observe it, the time starts existing in practical sense. Yes it can exist between other things, but only thing that matters is the observer and the other thing. Even if there isn’t a direct link to observe or for interaction, there can be indirect thing, in which case time comes in to existence through the chain that leads all to way to the even itself.
I don’t know if it makes it any easier, but stuff like this becomes way easier to understand when you accept the fact that time is not a thing, while also being a thing. Time is a thing between things, it can not exist by itself. You can not tell time from a still image, but you can tell time from two images, as in you can say that time has happened because things have changed.
But in short. If there is no observation or interaction, whatever just happened might as well not have ever existed.
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