Isaac Newton imagined a bucket of water suspended on a fine (ideally torsionless) rope, set spinning. Friction eventually causes the water to rotate along with the bucket. The surface develops a dip in the middle and rises at the edges owing to the water's inertia. (You see this effect every time you stir a cup of tea.) Einstein considered this 'thought experiment' too, and asked what this rotation might be relative to. Was it relative to nearby masses, or to the entire mass of the universe surrounding the bucket? Or to an absolute 'stationary' ether (as Newton did)? And he wondered whether this inertia effect would occur if the bucket of water were spinning in an otherwise empty universe devoid of any other masses. My own 'Newtonian/relativistic' thoughts on this are as follows. The situation can be regarded as equivalent to a 'stationary' bucket with the rest of the universe spinning round it at the same rotational speed but in the opposite direction. The fact that the mass of the universe is orbiting the bucket of water implies that it is accelerating towards it (like the Moon accelerating in its orbit towards Earth). This implies that the bucket is exerting a force on the 'rotating' universe. But this in turn implies that the rotating universe is exerting an equal and opposite force on the bucket and the water within it. It is this force that causes the water to pile up at the edges and dip in the middle, and give the impression of possessing 'inertia'. It follows from this, that in an otherwise empty universe the inertia effect would not occur. That is to say, the spinning water would not develop a dip in the middle and rise at the edges, unless empty space has mass. I cannot believe for an instant that these thoughts are new. Would more knowledgable forum members please refer me to where in the literature my thought experiment has been described. And also, I'd be very interested to hear what YOUR thoughts are on this?