I don't understand why/how for instance in the twin paradox the twin that comes back to Earth will feel a proper acceleration/deceleration when turning back to Earth. I do know that this twin will suffer an acceleration since an accelerometer will tell him he's not in an inertial frame. If I am over the Earth I will "see him" turning back and it's accelerating with respect to the Earth, I understand this. However if I was this twin and I look at my accelerometer, I'd say "wow, I'm accelerating... but with respect to what?!!!!". On one side, I know I'm obviously accelerating with respect to the Earth but the Earth isn't a "privileged reference frame" in special relativity. So it seems like there's an "ether", in other words, a privileged reference frame from which I'm accelerating... This is how I see it. I know that the ether doesn't exist but I still don't understand what's going on. There's this accelerometer that tells me I'm not in an inertial reference frame and that I'm "accelerating". I have no idea with respect to what I'm accelerating. It seems like in the twin paradox the Earth is treated as an inertial reference frame and not the flying twin when he's turning back, I don't understand why either. Another thing I don't understand is that if I was born on a constantly accelerating spaceship I'd feel a force in a certain direction. But I don't understand with respect to what I'm accelerating. Does it matter? I mean, I don't see why the existence of the stars/massive matter would influence noticeably my proper acceleration if I'm between 2 galaxies. And yet it seems that this "accelerometer" will tell me in what direction I'm accelerating. So eventually I could use an accelerometer as a compass if I don't see any stars in my spaceship. I'd appreciate if someone could eradicate my doubts. Namely, when I'm accelerating it seems that there's somehow a privileged reference frame and I'm accelerating with respect to it. I know this is wrong, but where is it wrong? Thanks in advance.