It's possible that the confusion comes from the weirdness of non-inertial reference frames. Switching from [rotating bucket] to [universe rotating around the bu

It's possible that the confusion comes from the weirdness of non-inertial reference frames. Switching from [rotating bucket] to [universe rotating around the bucket] is not an intuitive operation. To build on the atom-level view, imagine you take a snapshot in time of a cross-section of the bucket and the universe. Then you put debuggers on four particles. These debuggers tell us the linear velocities of each particle. Two of the particles, A and B, are part of the water in the bucket. The other two particles are somewhere out in the universe, but aligned in a line with C and D. Here's the picture so far, with the size of the arrow indicating the linear velocity of each particle. The bucket+water has been rotating for a long time, so all the water is rotating normally. [center of bucket] A --> B ----> [edge of bucket] . . C D Now let's switch reference frames so the the universe (containing particles C and D) is rotating around the bucket. C an D are going faster since they are further from the center of rotation. [center of bucket] A B [edge of bucket] . . <------ C <------------ D In this new reference frame, A and B have no difference in velocity. This is very worrisome! The information containing the relative velocities of A and B (and, therefore, the forces on them) has essentially been destroyed. We might need to add new forces to get the information back - information which is crucial to explaining the water getting pushed outwards. This is a sketch, definitely not rigorous. But my main point is that you can't say "imagine the universe is rotating around the bucket" without introducing a lot of messiness. That's what it means to have a non-inertial/accelerating reference frame. On another note, I do think the Nautilus article is conflating two different points of confusion. First, as I described above, rotating reference frames are weird. But despite that weirdness, we can fully explain it in terms of the underlying physical concepts (acceleration, inertia, space, etc). Second, we haven't fully explained the origin of those concepts yet. But that's a very different matter. It seems like a stretch to say that we can't explain the water vortex, or that we can't explain the bulging planet.