If water creeps up the walls of the bucket, obviously it’s because the water is being pushed inward
I wouldn’t put it like that, but I guess it doesn’t matter.
More to the point: from what I remember of GR, both 1) an empty universe with one bucket with a flat water surface, and 2) an empty universe with one rotating bucket with water on the sides, can be solutions to GR’s equations. At least I’m fairly sure it’s like that for black holes (Schwarzschild and Kerr metrics). You seem to be saying that all rotation of matter in GR is relative to other matter, but if I have my physics right, that simply isn’t true.
If water creeps up the walls of the bucket, obviously it’s because the water is being pushed inward
I wouldn’t put it like that, but I guess it doesn’t matter.
More to the point: from what I remember of GR, both 1) an empty universe with one bucket with a flat water surface, and 2) an empty universe with one rotating bucket with water on the sides, can be solutions to GR’s equations. At least I’m fairly sure it’s like that for black holes (Schwarzschild and Kerr metrics). You seem to be saying that all rotation of matter in GR is relative to other matter, but if I have my physics right, that simply isn’t true.