"A year is a unit of time."
"A light year is a unit of distance, representing the total length light travels at the speed of light in a year."
"Assuming the speed of light is unchangeable, then the length of 1 light year is constant."
"An event occurred at point A. Point B1, which is 1 light year away from point A, will observe this event at A 1 year later."
"Similarly, points B2, B3, and B4, which are 2 light years, 3 light years, and 4 light years away from point A, will observe the event at A two years later, three years later, and four years later, respectively."
"The farther away from point A, the more of A's history we can see."
"By relying on this theory, we can observe the past but cannot predict the future."
"But just helping us observe the past is already enough."
"Because the questions we most want to know are, where does life come from, what is the origin of everything, what was the beginning of the universe like."
