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.
It is, in fact happening at the same time, but if you look at it it will take millions of years for you to see it
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”.
Kind of. The speed of light is the speed in which that information travels.
Take our sun for example. It is around 8 light minutes away, meaning that it takes light about 8 minutes to travel from the sun to the Earth. Let’s say that the sun quit shining at exactly 3:00, and at that exact time, you sneezed. The sun going out and you sneezing would have happened at the same time. But it would take 8 minutes for the information that the sun went dark to get to you. So from your perspective, you sneezed at 3:00 and the sun went out at 3:08.
Now, if someone was twice at far from the sun as you and could somehow see you sneeze, from their perspective, you sneezed at 3:08 and the sun went out at 3:16, even though both events happened at the same time at 3:00. That’s why there are theories of Relativity. Because the time in which events are noticed are relative to where the observer is and how fast the observer is moving
On a cosmic scale, time and distances are enormous. So a lot of the stars that you see at night have blown up or burnt out hundreds or thousands of years ago, it’s just that the light of those explosions haven’t reached your eyes yet.
Yes, you will hear a lot about relativity. But at the end of the day observers don’t alter the situation.
Two events happening at the same time happened at the same time regardless of distance apart. It’s that simple.
If you are seeing it happen (in the sky / through a telescope) it happened in the past relative to you at the time you saw it. If it’s just you thinking about something happening now across the universe then it’s happening now, at the same time.
Yeah but the distance between you two is so much that to see each other’s actions will take forever.
You’d both jump at the same time but it’s gonna take 50 billion years to see it lol
Latest Answers