Talk:Time-travel theories/@comment-39.41.165.137-20151109145218/@comment-68.80.170.97-20170409141009

Time fot the craft would slow down, as the passage of time slows down as you reach light speed. 600 years on earth would equal for the traveler only 60 years, and they would only age 60 years as time itself is slowing down for them. This has already been proven by haveing 4 identical atomic clocks, 2 of which they sent into flight, and they came back out of sync enough that it would not be attributable to menchanical error. So no, you wouldn't age 600 years in your hypothetical situation, you would age 60, and nothing more. https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment

These sort of time slows are even incorporated into current technology, as GPS systems have to account for this.