Everything is drag, click and slide, no setup.
Look around. Drag anywhere on the background to orbit the camera. Scroll, or use the + / − buttons, to zoom in and out.
Load a scene. Under Scenes, tap a starting point, Kepler (one planet round a star), Binary (two stars circling each other), System (a mini solar system), Slingshot (a flyby), Three-Body (a stable figure-eight), or Menagerie (everyday things in orbit).
Drop your own object. Under Drop an object, pick something, a cat, a fridge, a planet. Then click on the glowing sheet and drag: the direction you drag is the direction it flies, and the further you drag, the faster it goes. Let go to launch. Drag short for a tight orbit, long to fling it across the system.
Tune it. Gravity makes the pull stronger or weaker. Mass sets how heavy the next object is. Trail length draws the path behind each body. Time speeds up or slows down; tap Space to pause. Reset reloads the scene; Clear empties it.
About the numbers. The sliders are in sandbox units, scaled so the orbits fit the screen; the objects you drop carry real-world masses for flavour, mapped onto that same scale. It is the ratios that are physical, not the readings.
Reading the panel. bodies is simply how many objects are in play right now (so "bodies 3" means three): it rises when you add one and falls when two merge. energy drift (since last merge) is a live honesty check: an isolated system's total energy should never change, so this shows how far the simulation has wandered from its reference value. It stays tiny, a few thousandths of a percent, which is how you know the orbits are being computed faithfully. Merges are inelastic and genuinely lose energy, so the reference resets at each merge and the readout keeps measuring only the integrator's own error.
Good to know: two objects that drift too close merge into one (their masses add together). Switch off Spacetime well if you'd rather watch the bare orbits.
Each is a famous gravity setup, a real situation the same law produces.
Kepler. One planet circling one star in a clean ellipse. Named after Johannes Kepler, who worked out in the early 1600s that planets trace ellipses, not perfect circles. The simplest case, and the textbook place to start.
Binary. Two equal stars swinging around their shared midpoint (their "barycentre"), with no fixed centre. Not exotic: more than half the stars in the night sky come in pairs or larger groups.
System. One star with several planets at different distances, a miniature solar system. Watch the inner planets race around while the outer ones crawl: closer means faster.
Earth. Our own backyard, Earth with the Moon on a wide, slow orbit and a satellite skimming close and fast. The same "closer is faster" rule that keeps the space station low and quick.
Slingshot. A light probe falling past a heavy world on a single curved flyby. This is the geometry of a "gravity assist", the trick real spacecraft like Voyager use to steal a little speed from a planet and fling themselves deeper into space.
Three-Body. The famous figure-eight: the rare choreography where three equal masses share one stable orbit. It is the exception that proves the rule, because there is no general solution to the three-body problem. Nudge it, drop something into it, and watch the choreography tip into chaos. This is where the simulation earns its keep.
Menagerie. A star with everyday things, a person, a cat, a fridge, a car, an elephant, all in orbit, to make the point that gravity doesn't care what something is, only how much it weighs and where it is.
Empty. A blank stage: just the grid, ready for you to drop your own objects and build a system from scratch.
What you're actually watching, no maths required.
Gravity is just attraction. Every object pulls on every other one. Heavier things pull harder, and the pull grows stronger the closer they get. That single rule drives everything on screen.
An orbit is falling and missing. A planet is forever falling toward its star, but it's also moving sideways fast enough that it keeps missing. That endless fall-around-the-side is an orbit.
Gravity treats everything the same. Launch a cat and a fridge from the same spot at the same speed and they trace the identical path, being heavier doesn't make you fall faster. Galileo first spotted this; Einstein made it the cornerstone of relativity. Try it, it's the most surprising thing here.
The glowing dip. The sheet shows how a mass "warps" the space around it, deeper where the mass is greater. It's a helpful cartoon (real gravity also bends time, which a flat sheet can't show), so read it as a picture of where the pull is strong, not the literal thing.
Three is where it breaks. Two objects orbit predictably forever. Add a third and the motion turns chaotic, a tiny nudge sends the whole dance somewhere completely different. This "three-body problem" is still a famously hard problem in physics.
All of this is plain Newtonian gravity, the bedrock. Push the same force to its extreme and it bends light itself: see the black hole, and The Bubble for where the propulsion claims go from here.
The same honest line as the rest of the site, drawn in plain sight.
This is the Newtonian bedrock under the black hole: the ordinary law of gravity that holds a planet in orbit, before it's taken to the edge where it bends light. No relativity is included here, that's the black hole's job. The propulsion claims chase the other route entirely; see The Bubble.
| Feature | Tier | What that means |
|---|---|---|
| Orbital motion (inverse-square gravity) | T1 Established | Bodies attract as G·m₁·m₂ / r², Newton's exact law. Ellipses, flybys and barycentres all emerge from it, nothing is scripted. |
| Velocity-Verlet integration | T2 Numerical | A symplectic (energy-stable) scheme with sub-stepping. Faithful, but a finite time-step: the energy-drift readout shows the honest error live. |
| Close-encounter softening & merging | T3 Simplified | A small softening length avoids infinite forces; bodies that overlap merge while conserving mass and momentum. Real bodies wouldn't always merge. |
| The curved "spacetime well" grid | T4 Analogy | The dipping sheet is a teaching picture of the gravitational potential, not real spacetime. The famous flaw: it uses gravity (the sheet's own dip) to explain gravity. Motion is computed from the law, not from the sheet. |
| Star background | T4 Illustrative | Procedurally scattered points. Not a star catalogue. |
In one line: the motion is real Newtonian physics; the rubber-sheet is an honest analogy laid underneath it so you can see the shape of the field the bodies are actually moving through.