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

Not an physicist – just like learning this stuff as a hobby.

I think the answer is both yes and no, because you need to include a lot of context.

Let’s take two points in spacetime. A and B. Both have a clock that started ticking at the same time. These two points have experienced time at the same rate without deviation.

Then things happening at those two points can be said to occur “at the same time.”

But another point – C – may have experienced time dilation due to gravity if they passed by (or through – dark matter?) objects massive enough – their clock would read an earlier period. So something wouldn’t occur “at the same time” for A/B vs C.

I’m sure it gets crazier complicated if you want to dive deeper into it, and someone more knowledgeable might have better answers.