ELi5: How does the whole light year thing work? How does something happen a long time ago that we know of yet we can still see it and have to wait for it to happen again (like those exploding star things on tt)
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Imagine you and I live a few towns apart. It takes a letter three days to travel from me to you.
On January 11, I stub my toe. It pisses me off, so I write you a letter. On January 12, that letter is still in the mail. Me stubbing my toe hasn’t reached you yet – as far as you can observe, my toe is still un-stubbed. It’s only a couple days later, on January 14, that news of my toe-stubbing reaches you.
In this analogy, you live 3 “letter-days” away from me. So the fastest you can know about anything that happens in my town is 3 days, when the first letters from that event arrive.
Light is much faster than a letter, but the distances to the stars are incredibly vast. So it takes light years to reach us from stars. When something happens far away from us, we only know about it when the light (and other signals) from that event reach us; since other signals can never move faster than light, the light always gets here first.
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