So, here's an example. Suppose you're in a toroidal space. It's an Asteroids world: you're in a cube, and if you pass through one wall you find yourself passing back into the cube from the opposite one.

If you look in the right place, you will see the back of your head- several times! So there's an illusion of seeing yourself in several places at one time. But toroidal space really is different from normal space (even if it's basically flat and Euclidean, the geodesics are different).

So it seems to me that you can have warped spacetime that produces illusions like duplicate galaxies.

Yea thanks, that is exactly my point; warped space produces illusory concepts of reality.

So, an apparent illusion of disagreement seems to have occurred ;-)

Right, but you seemed to be arguing that illusory images meant that the curvature itself was illusory, whereas in my toy universe space really is toroidal, and anyone using the existence of multiple images to suggest otherwise would be wrong (because I set up the universe that way). In fact, such images would be convenient evidence that space was actually toroidal. I don't see how Einstein rings are any different- they can't be evidence against actual curvature of actual spacetime. You'd expect double and triple images if spacetime was curved.

One prediction of GR is that if there were enough mass in the universe, the global curvature would be positive everywhere and the universe would close in on itself and be connected more like a sphere than a plane. If you moved in a straight line you would eventually return to your starting point. That doesn't seem to be the universe we live in (as far as anyone can tell, it's flat) but that certainly seems like a prediction about the actual curvature of actual spacetime.

I don't seem to be able to get my point across, but thanks for playing anyway. I really believe this point is of some philosophical and ultimately scientific value, however still largely misunderstood, but didn't really expect a forum comment (albeit on HN) to make a significant dent. Have a good day!

"Seem" refers to the lens like nature or the warped space.