Light years is a measure of distance not time. As a result 10,000 light years measured from either parties perspective is 10,000 light years. ie a human on the ship would still experience 10,000 years of travel. No human can currently live that long.
Now change your hypothetical numbers to 10 light years and you're not misunderstanding anything. The human aboard the ship would have experienced 10 years of travel. However, any observer from Earth would have experienced 10*(some large factor) years, such that making a return trip "home" is impossible.
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So.. you're definitely missing a bit.
Light years is a measure of distance not time. As a result 10,000 light years measured from either parties perspective is 10,000 light years. ie a human on the ship would still experience 10,000 years of travel. No human can currently live that long.
Now change your hypothetical numbers to 10 light years and you're not misunderstanding anything. The human aboard the ship would have experienced 10 years of travel. However, any observer from Earth would have experienced 10*(some large factor) years, such that making a return trip "home" is impossible.
The flip side of time dilation is length contracation. If you are traveling at relativistic speeds, then space also contracts.