Gravity Well
Every dot you fling here obeys one rule: mass pulls mass, and the pull falls off with distance. That single rule builds orbits, slingshots, collisions, and black holes. Drag to launch something and watch the rule do the rest.
The rules behind the dots
The simulator is a toy, but the physics is real. Here’s what’s happening under each thing you can do — and what to try in the console to see it.
Mass pulls mass
F = G · m₁m₂ / r²Every body tugs on every other body. The tug grows with mass and weakens with distance — and not gently: double the distance and the pull drops to a quarter. That “inverse square” falloff is why a body whips around hard when it’s close and barely feels anything when it’s far.
Try Drop two planets near each other and watch them fall together.
Falling, but missing
An orbit isn’t a balance of forces — it’s a body falling toward another and continually missing because it’s also moving sideways fast enough. Too little sideways speed and it spirals in. Too much and it flings off into the dark, past escape velocity, never to return.
Try Load Binary, then drag a planet with a gentle sideways flick to thread an orbit.
The slingshot
Fly a light body close past a heavy one and it leaves faster than it arrived — borrowing a sliver of the big body’s motion. Real spacecraft do exactly this to reach the outer planets without carrying the fuel it would otherwise take. Nothing is free: the heavy body slows by a fantastically tiny amount.
Try Aim a fast planet to skim just past a star, not into it.
When two become one
Get two bodies close enough and they merge. The new body keeps the combined mass and the combined momentum, so it heads off in a direction set by which one was heavier and faster. The flash of particles is the energy that can’t be kept — collisions are messy that way.
Try Launch two bodies straight at each other, head-on.
Black holes
forms when mass piles past ~1500 unitsPile enough mass into one place and the escape speed climbs past the speed of light itself — so nothing, not even light, gets back out past the edge called the event horizon. The glow around it isn’t the hole; it’s the superheated wreckage of everything spiralling in, the accretion disk.
Try Drop a Black hole, or merge several heavy stars until one forms.
Lagrange points
When a small body orbits a big one, there are five spots where all the pulls cancel just right, letting a third tiny body ride along without drifting. Two of them are genuinely stable — which is why spacecraft and clouds of asteroids actually park there in real systems.
Try Load Lagrange and watch the two small bodies hold station 60° ahead and behind.
The three-body problem
Two bodies are predictable forever. Add a third of similar mass and the future becomes genuinely unknowable — tiny differences explode into wildly different outcomes. There are a handful of rare, exact solutions where three equal masses chase each other in a stable loop, but they’re so delicate that the smallest nudge tears them apart.
Try Drop three stars of similar size close together and watch them refuse to settle into any repeating path.
This is a toy, not the cosmos
Real gravity plays out in three dimensions, over millions of years, with relativity bending the rules near anything heavy. This is a flat, sped-up, softened sketch in arcade units, built so the behavior is visible in seconds. Trust the patterns you see here — orbits, slingshots, runaway chaos — but not the exact numbers.
Try Anything. Breaking it is how you learn where the edges are.
Where this comes from
- Newton, I. (1687). Philosophiæ Naturalis Principia Mathematica — the inverse-square law of gravitation.
- Poincaré, H. (1890s) — established that the three-body problem has no general closed-form solution; the seed of modern chaos theory.
- Chenciner, A. & Montgomery, R. (2000). “A remarkable periodic solution of the three-body problem in the case of equal masses.” Annals of Mathematics — the figure-eight orbit.
- NASA — explainers on Lagrange points and gravity-assist (slingshot) trajectories.
The whole show — orbits, slingshots, mergers, the black hole at the center of a galaxy — runs on one short sentence about mass and distance. Most of the universe is like this: a small rule, repeated, building something that looks impossibly complicated.
— ThinkingDeeper