Hello everyone. Below are two problems I have been thinking about lately. Let’s consider two cases: we have a spaceship surronded by an utter void - nothing outside which the spaceship’s pilot could refer to. The pilot (in his robotic body, allowing him to withstand enormous G-forces) turns on engines and starts accelerating at 100,000g and after few minutes reaches 270,000km/s (0.9c) - or is he? If there is no reference frame is the spaceship moving at all since speed is relative? we have two spaceships and nothing else, as described above. Spaceship A starts accelerating and after some time it reaches 0.9c (acceleration phase 1), so it is moving 270,000km/s with relation to the spaceship B. Now spaceship B turns on its engines and starts accelerating at even higher rate than previously spaceship A and it is doing that as long until it catches up with the first spaceship and then it kill its thrusters; now the ships are moving next to each other with the same speed - their relative speed is equal to 0. Next, the spaceship A turns on its engines and again starts accelerating to 0.9c (acceleration phase 2) and eventually it is moving again at 270,000km/s with relation to the spaceship B. The question - would the energy expenses on acceleration to the same speed be higher during one of the phases; which one? Or would they be equal? Besides, what about relativistic masses of the spaceships? Is mass of the spaceship A bigger after acceleration to 0.9c first phase than before that phase? Is it even bigger after acceleration during phase 2? Thank you.