Me too. I was never comfortable with the concept of relative motion. A simple thought experiment makes the problem clear. A friend of mine in physics class raised this and the teacher simply ducked it. If you have a bar, say 200 million light years long, and you push it, assuming incompressibility, you will get the same amount of motion on the other end. This becomes an argument for motion as absolute, not relative. As stupid and silly as this simple experiment appears, put in the frame of light years, it becomes different.
Also, dark matter brings back the idea of an ether.
Yes it's all in the assumptions...
Why?
Ignoring compressibility in your example is like saying “Suppose for a moment that water flowed up hill...”.
If you have a bar 200 million light years long, it’s impossible for it to be uncompressible, so your example is not really relevant. Because of compressibility (there may be deeper physics at work), almost any “bump” can be modeled in wave theory. Incompressibility rules out using wave theory which I would guess, make large parts of physics as we know it invalid as well. Luckily for us, incompressibility is not a real property of anything.
When matter reaches a true state of incompressibility, things either bounce off of it, or the matter in question will reorganize at the nuclear level (like fusion), and shed energy, and form something more dense -— which is really saying you compressed it :)
There's your mistake. That's like saying "assuming light is infinitely fast..." - well, it isn't. The movement on the bar must propagate from one end to the other (as information if nothing else), which cannot propagate faster than light.