Einstein's General Theory of Relativity reframes gravity not as a force, but as the curvature of spacetime caused by mass. The grid you see on screen is spacetime itself — each intersection is a point in four-dimensional fabric. As the black hole's mass warps the grid inward, any nearby object (light, planets, other black holes) simply follows the curved geometry. This is why gravitational lensing occurs: starlight appears to bend around a black hole not because it's being pulled, but because it travels in a straight line through bent space.

At the Schwarzschild radius (Rs = 2GM/c²), escape velocity equals the speed of light. Cross this boundary and nothing — not matter, not light, not information — can return. For a black hole with the mass of our Sun, Rs is about 3 km. In the simulation, any planet or debris that enters this radius is immediately absorbed. The gradient glow around each black hole visualises the accretion disk — superheated infalling matter radiating energy just outside the event horizon.

The stronger the gravitational field, the slower time flows — a direct consequence of GR confirmed by GPS satellites, which must correct for relativistic effects to stay accurate. The two clocks in the HUD illustrate this: Earth time ticks at a fixed rate, while the warped clock near the black hole runs at a fraction of that rate. The dilation factor approaches zero at the event horizon — from an outside observer's perspective, an infalling clock would freeze completely and never cross.

When two massive objects orbit each other, they radiate energy as ripples in spacetime called gravitational waves — first directly detected by LIGO in 2015. As energy bleeds away, the objects spiral inward (inspiral phase). In the simulation, the GW_DAMPING constant models this energy loss, causing orbiting black holes to slowly decay inward. At merger, roughly 5% of the combined mass converts to gravitational wave energy (E = mc²). The white burst of debris and the 'GW Energy' counter in the HUD represent this mass-energy conversion.

When a planet passes close enough to a black hole, the gravitational pull on its near side is vastly stronger than on its far side. If this tidal force exceeds the planet's own self-gravity, the object is torn apart — stretched into a stream of debris in a process called spaghettification. The Roche limit marks this critical distance. In the simulation, the particle burst when a planet enters the tidal radius visualises this violent disruption event, with debris following the momentum of the original object.