If I accelerate to .9c I am gaining mass, but if I reverse thrust and decelerate why aren't I still adding mass since I'm still putting energy into the system? I thought it wasn't about speed but about changing speed. It takes energy to slow down as well. Or does deceleration lower mass? Or does the decrease in kinetic energy from the lower speed make up for it? I'm sure I just don't understand it properly. I mean, one observer might think you're accelerating and another might think you're decelerating. Isn't it irrelevant whether you are slowing down or speeding up? How do you know which one you are really doing? What if you and a planet are both moving at the same speed in the same direction but you are in front? For me to get to the planet I'd need to reduce speed, but from my perspective I'd be increasing speed. Thanks!