I realise that a light year is the measure of distance a particle of light can travel in a year from the point of view of an external observer, but this is highly subjective. If Einstein's theory of relativity is correct, the closer you get to light speed the more time slows down. So why isn't a light year measured taking relativity into account and going by a year from the point of view of the light particle? I'm interested to hear your thoughts on this.