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”?

Question

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

Answer 1

You have probably noticed that there are a lot of answers here. Yes, No, Maybe, and Depends are all here. Most of them are right, given a certain way of looking at things.

Let me ask you this: suppose something happens somewhere, and you never know. In fact, this event affects literally nothing, even theoretically, about your life. Further, it never will affect anything, and I do mean *anything* about your reality.

Did it happen?

This is the “if a tree falls…” question, but turned to 11.

I personally answer this with “no”. As such, it makes no sense to talk about events having happened until they actually affect you. But then, all those “distant” events are really happening to you, at that moment you are in, in that place you are standing. Other people will receive that information at different times with slightly different perspectives, so you won’t even be able to agree with them after the fact how to assign the time values to distant events. If you move, you might even end up not agreeing with yourself anymore. So what was the point of assigning arbitrary time values again?

This is really difficult to “eli5”, because it gets at the very nature of the universe. Good analogies don’t really exist except to give the most vague outline of what is going on.

Merely changing the direction you are walking can have drastic implications about how the “nows” all line up at distant locations. If you stick to a single frame of reference, you can make it work, but the moment you change references, you get completely different answers.

For all these reasons, I don’t think it is generally useful to consider such distant events happening at the same time in any meaningful way. For a certain reference frame you can make a definition like that work, but that frame is fleeting, making the entire exercise moot.

(as an aside, I love the way the word “moot” sounds. Moot. moot. moooooooot. Great word.)