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
No. All clocks run at different speeds. Some faster, some slower. In fact, Mr. Bird’s clock on the roof is faster than Mr. Worm’s clock on the ground.
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.)
Time is relative to the observer. Some say that time isn’t how you perceive it, but rather that everything is happening at once. If you look at a supernova for 1000 light-years away, that explosion isn’t happening now as you watch it. It happened 1000 years ago but you are just now seeing it.
Yes. When we observe a supernova always acknowledge that it has already occurred, because it has. We never discuss it as though it was just happening, only that we are just now able to observe it.
The eli5 are more ask science than eli5. So here is my best attempt.
Yes there is a universal now. Now exists everywhere in the universe. But no one can observe now. You can observe events that happen “now” after some time has passed. This is how long it takes the light to reach your eyes.
Depending on how far away the event is. It might take you a while to observe the event. So long in fact that it no longer appears to have happened “now”.
Also because of relativity no one agrees on the delta between the event and the observation.
Yes two events on opposite side of the galaxy can happen at the “same time”, meaning simultaneously, but that does not mean you will observe them happening at the same time where you are. You will see the one farther from you happen later due to the light from the event having to travel farther.
From your perspective…Yes. From the distant object/incident’s perspective…Yes. From somewhere in between…No.
Based on the distance and amount of time it’s takes to travel that distance …. No.
An easier way to imagine it…. Sound travels slower than the speed of light. Imagine The sound of thunder versus the lightning bolt. You see the lightning but hear the sound later. Did they happen when you saw it, or only when you heard it?
Think of a thunderstorm. We know that the lightning and thunder occur “at the same time”, however we on earth observe the lightning first, and the thunder a few seconds later depending on how far away it originated.
It’s all relative.
For any two events where there is enough time for light from event A to reach event B (whether it actually does or not), then event A precedes event B for all observers.
For events where this is not the case, events A and B will be simultaneous for some observers, event A will occur before event B for some observers and event B will occur before event A for some observers.
The closer together in time and further apart in space two events are, the wider this window is, but any two events that can be described as truly simultaneous will not be from some perspective, even if very close together.
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.
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