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
If you involve relativity events “happening at the same time” depends on the observer.
So you can say some things are happening simultanously, but you have to say from wich perspective.
There is no universal neutral observer that could give you a “most correct” order of events.
There is no such thing as “at the same time” for things far away or moving very fast. It is perfectly possible for one observer to see A happening before B, and for another observer to see B happening before A.
I can’t really explain it, not even li50, but I recently read a book on this subject that explains it really well: [The Order of Time, by Carlo Rovelli](https://en.m.wikipedia.org/wiki/The_Order_of_Time_(book). Can’t recommend it enough.
It depends on your definition of “Time”
If you consider time as in the skyline positioning of Sun then no its not considered as the same time
If you consider time as in seconds/minutes then yes they are the same time imagining all galaxies were created in the exact same moment
We see distant galaxies as they were billions of years ago. If there’s intelligent life on one of those galaxies, they see our Galaxy as we see theirs, an ancient blob of stars without any proof of intelligent life beyond theirs. It’s fair to say if there are aliens, they also invented the nuclear bomb, invented new ways to travel their planet, and might even be searching for intelligent life like we are, and with the size of the universe being so big, it’s likely some species made their discoveries at the same time as we did, it’s also possible they are far more/less advanced than we are.
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.
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.
Everything in the universe is relativistic for our practical purposes. You can only know something in comparison to another, and that comparison makes things different based on what is being compared.
That means the answer is either “it depends” or “no”.
Some slightly too philosophical examples 😅:
What does it mean to be a man without first seeing a woman?
What does it mean for something to happen in New York if you don’t first know of that which is not New York?
What does it mean for it to be 5am if you do not know what it means to be 4am?
What does it mean for something to happen at one side of the universe without first knowing what the other half is?
So the question of absolute time is a relative question. In comparison to what? In comparison to two particles in superposition? To a person looking through a telescope? To a ship travelling through space?
The only absolute in the universe is the sum of the whole universe. If you account for everything, you have everything.
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.
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