[PatrickHenry]

If we were flatlanders, living in a 2D plane, and a 3D rod were perpendicular to our plane, we'd see it only as a 2D circular cross-section of the rod. I see no possibility for simulating mass. If the rod were in motion, passing through our plane, I still see no illusion of mass. I'm obviously missing something. Perhaps if we could, in our 2D way, handle the circle, and test its mass, it would then reveal the effect of the motion, but we wouldn't know about that motion. We'd just think it was massive, and we wouldn't know why. Is that it?

[longshadow]

I'm obviously missing something.

Objects can have motion components in any or ALL dimensions, not just the ones we can see, or just the ones we don't see.

[Physicist]

If we were flatlanders, living in a 2D plane, and a 3D rod were perpendicular to our plane, we'd see it only as a 2D circular cross-section of the rod.

The rod isn't perpendicular to the plane (although it could be). The extra dimension is perpendicular to the plane.

If the rod were in motion,

Full stop. The "rod" only represents the trajectory of the particle over time. The particle is pointlike and massless. The particle moves through the space as a massless object. The shadow of the particle on the plane moves on the plane as if it had mass.