One of the (probably many) things I don't understand about relativity is how examples can be shown from a travellers POV which include relatavistic effects. If I may borrow part of a reply which Grant Hutchinson made in another thread..
Relativity means that we can never reach lightspeed, but can get pretty close to it for a fairly short elapsed time aboard our spaceship. After one year accelerating at 1g, we've reached 0.75 lightspeed; at two years, 0.96; five years, 0.9999; ten years, 0.99999999. (Unfortunately, after ten years of acceleration, 9,000 years would have passed at home on Earth.)
How could these speeds apply to the traveller himself as he, his ship, engines, fuel etc are all in the same reference frame for the whole journey?