Tap to drop a body; drag to give it a velocity. G × turns the strength of gravity up or down: that's the "manipulation", here in a simulation. The integrator is leapfrog (kick-drift-kick), which conserves energy over long runs; the readout shows total energy E, its drift from the start (should stay tiny until bodies collide or you change G), and total momentum. Presets use real ratios: Earth–Moon (1000 km, Earth masses, hours), Solar System (AU, Sun masses, years, G = 4π²).
Embedding diagram: the grid sinks where gravity's potential is deepest, the "rubber sheet" picture of Einstein's curved spacetime. It's a picture of the math (the potential), not a literal fabric. The yellow ray shows light bending around the first mass, exaggerated so you can see it.
Deflection of light: δ = 4GM / (c² b). For starlight grazing the Sun this is 1.75 arc-seconds, measured during the 1919 eclipse, the first test of general relativity.
In 1798 Henry Cavendish hung a 1.86 m rod with two small lead balls from a thin wire, set two 158 kg lead balls next to them, and measured how far the rod twisted. The twist angle θ and the swing period T give G = 2π² L r² θ / (M T²), the first measurement of gravity's strength, from objects you could touch. Here the simulated rod twists by the angle real gravity would produce (with the noise you choose), and the "measurement" recovers G from it.
Build one at home: two small weights glued to the ends of a light rod (a wooden dowel), hung from its middle by a long thin fishing line inside a box with no drafts; a small mirror on the rod; a laser pointer bounced off the mirror onto a far wall. Put two heavy weights (lead, or big water jugs) next to the small ones. The laser dot on the wall moves by 2θ × (distance to the wall), so a few metres of throw turns a tiny twist into centimetres. It takes patience: hours of damped swinging. Background: Cavendish experiment.
The Moon and Sun pull a little less on your side of Earth than on its centre, so the gravity you feel rises and falls twice a day. This model computes the vertical tidal acceleration −(GMR/d³)(3cos²ψ − 1) from each body, ψ being the body's angle from overhead (simplified orbits, so the timing is approximate) and multiplies it by 1.16 because the solid Earth itself flexes. The swing is up to about 2–2.5 µm/s², about 0.00002% of g: a superconducting gravimeter sees it clearly, a phone cannot.
âš A visualization of the math, not a device. The Alcubierre warp metric (1994) is a valid solution of Einstein's equations on paper, but it needs negative energy density, matter that has never been observed in the amounts required. Nobody knows how to build it.
What's drawn: York time θ = v (x/r) df/dr, how fast space itself expands or contracts around the bubble. Space contracts in front (blue, down) and expands behind (pink, up); the ship sits in flat space in the middle and is carried along. f(r) = [tanh(σ(r+R)) − tanh(σ(r−R))] / [2 tanh(σR)].