Hi All

Peter & FrankC, Special Relativity applies to unaccelerated reference frames. However to get to 0.999 c requires acceleration and that is what causes the time-dilation to start with according to General Relativity. From the point of view of two unaccelerated observers passing each other at high speed ship-time seems normal, but dilated for the other observer.

An aside: Of the two who is stationary? Both and neither. Without an external reference and no acceleration both would perceive, via LIDAR say, the other as moving and themselves as stationary.

To get to 0.999 c takes quite a lot of acceleration time too. At one gee you'd travel 20.7 light-years distance and take 21.7 years earth-time and 3.7 years ship-time getting up to speed. The time distortion factor, or gamma, is 22.4 which is also the multiplier for the ship's relativistic mass. Thing about continuous acceleration is that the ship-time is logarithmic, hence if you accelerate twice as long (7.4 years) earth-time would be 1053 years and travel distance ~ 1020 light years. Your gamma is now over 2100. Almost an hour Earth-time for every ship second. Accelerate for 10.6 years ship-time and you'd cross the 26,000 light-years to the Galactic Core. Gamma ~ 27,000...

Adam