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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Light travels at a certain speed in a vacuum (like space). A lightyear is the distance that light in a vacuum (space) travels in the time it takes the Earth to make a lap around the sun (a year). That turns out to be 9.46 x 10^(12) km or 5.88 trillion miles.
So, if something is really far away, like a star exploding 10 lightyears away, it takes 10 years for the light to reach us. If we “see” the star explode, it’s because we see the light from the explosion, which takes 10 years to get to us. We’re used to these sorts of delays in communications. If we send a letter, it takes days to arrive. If we watch an interview on the news with a reporter on the other side of the planet, we see a few seconds delay as we wait for the radio signals to move back and forth. This is the same thing, a delay while we wait for the message (the light) to travel some distance.
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