well my physics teacher taught us some stuff about it...
what happens is...if a planet is 1 light year away from Earth then it takes light(traveling at the speed of light) to reach that planet if it started from Earth. hmmm a more simple example would be the sun...if it went out it would take 8 minutes for the last ray of light to disapear from sight on earth, because it takes the light 8 mins to travel to Earth from the sun

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the broccoli must die