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Six things to find out for yourself. Each one sets up a real question; “Set it up” does the first step for you.
“Spaghettification” is the stretching. The popular picture is a noodle so thin its atoms line up in single file. Here is what the numbers say for falling into .
Pieces keep breaking where the tidal pull across them beats what holds them together: material strength for solids, a chemical bond for molecules, the electron’s pull for atoms. Times are on the falling piece’s own clock. Inside the edge, “distance from the centre” is really a countdown: the centre is a moment in the future, not a place you can avoid. Near the very centre known physics probably stops working.
Solids break where the tidal stress across them, about density × (2GM/r³) × length² / 8, reaches their strength. Planets, stars and clouds break at the standard tidal-disruption radius, R × (M/m)1/3. Before that point the object falls in one piece; after it, every piece follows its own orbit with the speed it had at the moment of breakup (the “frozen-in” approximation used in research on stars torn apart by black holes).
Each piece falls under the Paczyński–Wiita gravity of a still black hole. It is not full general relativity, but it gets three of its key numbers exactly right: the closest stable orbit (3 × the edge), the closest bound orbit (2 ×) and the capture limit for anything falling from far away. Pieces do not pull on each other, and light-travel time is ignored. See the checks →
The black hole and the stars behind it are traced exactly through the Schwarzschild solution when you are close; far away the standard weak-field bending is used. Debris passing behind the hole is bent by the point-lens formula, so you see its second image and Einstein rings. Near the edge this is an approximation. Pieces dim as their light climbs out of the hole’s gravity. The optional glowing disk is a chosen look: its brightness and temperature are picked, while its motion, Doppler shift, redshift and bending are calculated.
Small things are shown bigger than life so you can see them (the readout says by how much); where they break and how they move are not changed. Colours, the glow of hot debris and how fast gas settles into a disk are chosen. Time speeds up far out and slows near the hole so you can watch; the clock shows the real time that has passed.
With spin, the sky and the disk are traced through the Kerr solution (the same tracer as the Event Horizon page). The pieces feel a pull whose edge and closest stable orbit match Kerr exactly for orbits turning with the spin (Artemova, Björnsson and Novikov, 1996), plus the Lense–Thirring push that drags everything around with the spin. Both are approximations close to the hole. Debris behind the hole is still bent as if it were not spinning.
With more than two holes, each pulls on the others by Newton’s law, and every pair loses energy to gravitational waves at the rate Peters (1964) gives for its separation. Holes merge when their edges nearly touch, keeping their momentum and losing the energy the merger fits predict. Light is bent by adding up each hole’s weak-field bending. Full relativity for several holes at once needs supercomputers.
Movie mode makes a galaxy small and light enough to be eaten, which no real black hole can do. Its stars follow the hole’s gravity plus the galaxy’s own (a Plummer sphere moving with the galaxy), the method Toomre and Toomre used in 1972 to explain tidal tails.
The orbit shrinks by Peters’ 1964 gravitational-wave formula, and the energy radiated and the final spin come from fits to numerical-relativity simulations. The ripple grid shows the wave pattern, travelling at light speed, with its height exaggerated by a factor of about 10²⁰. The sound plays the wave’s real frequency for small black holes; heavy pairs are shifted up into hearing range (the readout says so). Light bent by two holes is approximated by adding up their bending.