The thing with travelling to the future by going close to the speed of light always bothered me. Maybe someone in the comments  smarter than me can answer this for me. If you imagine a universe with no other point of reference, just earth and the spaceship. I get on the spaceship, accellerate to 99.9999% the speed of light, and travel at that constant speed for some amount of time (presumably the longer, the more into the future I travel). But, once I've accelerated to my final velocity, and my velocity is constant for the rest of the journey,  with earth being the only other frame of reference, there is no way to tell if I'm the one travelling, or the earth (or is there? does that not apply at relativistic speeds?) The only difference between me in the spaceship, and the earth, is that I have accelerated, and then, at the end of the trip, decelerated, which makes me think that the "magic" of traveling into the future would happen then, and not during the actual trip. Which is clearly wrong. What am I missing here? Apologies for a dumb question.