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
We can’t know in the present what events are happening simultaneously great distances away because the information takes some time to reach us, similarly to how lightning appears near-instantly to our eyes but the information from the sound takes time to reach us. But if we know the distance between galaxies and the speed at which information travels, we can determine what we were doing at the moment the information left its source, and can say those two instances occurred simultaneously.
For example, if you aim a telescope at Mars, you’re seeing Mars 3 to 22 minutes ago (depending on its distance from us). You can’t know what is happening there exactly now because that information is still travelling towards you and hasn’t arrived yet, but you do know what you were doing 22 minutes ago. So whatever happened then occurred “simultaneously” to what you’re seeing through the telescope now.
In terms of extremely far distances, the rate of passage of information (time) is affected by things like relative velocity and gravity. If you take those things into account, you can say that two stars exploding “simultaneously” to your eye through the telescope may be occurring at the same time, or at different times, but definitely not “now”.
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