Gravitational lens

A telescope can be empty until an unseen mass sits between it and a star. The same star can then show up on both sides. The light is following a null geodesic through curved spacetime. It is not being sucked in.

In the weak field the deflection is α = 4GM/(c²b). More mass, or a closer pass (a smaller impact parameter b), bends the ray more. When the source, the lens, and the observer line up, the images join into an Einstein ring. That ring is how astronomers weigh mass they cannot see.

The picture used by the demo is a 2D slice. Near the photon sphere it is only qualitative: image count and brightness are not a measurement. Dragging the lens and aiming the telescope happen on the Chinese demo.