> Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on

> Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.) Oh? But if there is no ether, and no absolute dead reckoning for motion in the vacuum, then for all we know, the Earth system is moving at 0.8c in one direction and the spacecraft would be moving at 0.2c in the opposite direction. Or something like that.