I now have a general understanding of relativity but it threw up a few issues. I understand that there are no 'fixed' points of reference in the universe, which of course begs the question as to why inertia exists. The way I imagine it is if I am the only matter in the universe, then why should up be up, down be down, if I turn left or right, or rotate, why should there be inertia? If I stand in my living room and spin, why should my arms fly out.. It seems that, of course, there shouldn't be inertia if the universe is truly relative. I know that Einstein grappled with this and he referred to Mach's principle to vaguely explain this problem in that matter over there (far away) has an unexplainable effect on matter here, so in a way, we can say there are reference points in the universe... sort of. But then I got thinking more about it, and then it also threw up another 'observation'. I understand the universe lacks any real geometry.. as in, it's not like space is a room and I can pin point my location X,Y,Z in relation to the walls... Its better to imagine it as a void with 'stuff' inside it. But do you notice that the Mach principle applies exactly to light too? In that, if I fly in a 'straight line' (or a particular direction), then light waves also follow the same 'straight' line that matter does.. What I mean is that if I 'veer' from the course that light follows with a left or right turn, I feel inertia. (could it be that inertia is caused by us 'crossing' the path of light) I guess my question is WHY is 'straight' is straight for matter, as well as for light? Also, I wander what would happen if all the matter in the universe disappeared, except for me in my small rocket ship. I guess if we extrapolate the Mach principle to this situation, my rocket ship dictates the geometry of all of space, and if I 'spin' I won't feel anything at all (as I am not really spinning). I wander if there is zero inertia in this situation? But I also wander, if I shine a light beam in front of me, and I rotate.. will I perceive the light fly away from rotating around me? If I use a 'left thruster' to spin to the right, would the light follow the same path as me? Would I perceive it spinning around in a outward spiral, as I should be dictating the geometry of all of space no? but then this created a new problem which goes to the heart of this issue for me.. Why should my rotation or lack of dictate the 'geometry' of space. If we apply that to the universe as a whole, WHY has matter dictated what is a 'straight line'. I just cannot see any explanation or plausible reason why it would.... Light and matter should be going in crazy random directions if the universe is truly relative and inertia should either not exist, or be a completely random force that isn't consistent.... Is gravity and inertia the same thing? Wait a minute.... is gravity defining space, and the 'pull' I would feel near a massive object is really just 'inertia'? This is all bugging me so any explanation or thoughts are welcome please. Anything!