If we could build a spacecraft that travels at 0.999998c, it could travel to the neighboring galaxy, Andromeda, which is 2.9 million
light-years away, in only 10 years elapsed time as measured on the clocks aboard the ship. If the ship turns around, the astronauts will return home aged by 20 years, while 5 million years will have passed on Earth.
Should something like – *the time needed for acceleration and negative acceleration (deceleration)* be included here?
Let’s see. At 1g acceleration it would take 3×10^8/9.8 = 3×10^7 s = 1 year Earth time to accelerate and decelerate ay 1 g, which is negligible on the scale of 3 million years. Ship time would be 1/3×10^9 = 3×10^-8 y = 0.1 seconds.
“Maybe I’m lucky to be going so slowly, because I may be going in the wrong direction.”
– Ashleigh Brilliant