Build a tiny universe, send a ship through it, and watch its clock tell a different story.
01Place a little possibility
Drag a body from the library, or click one and then click in space. For keyboard placement, focus a library item and press Enter: it appears in a free spot near the center.
02Give it a direction
Select an object and drag its arrow to adjust speed and heading. Change mass with the slider beneath it; size changes with mass. The nearby buttons hold it in place, set an orbit, toggle its clock, or remove it. Press N on the canvas to select the next object; arrow keys move it. Arrow keys on the velocity handle adjust motion.
03Visit another future
Press Space to play. Scrub any part of the four-minute timeline, including the future. Editing at that moment preserves the past and recalculates what happens next. Reset restores the preset.
04Bring a clock
Each object can carry a clock. The bars accumulate proper time across the full four-minute timeline. The muted track marks the predicted final total; the body icon leads the fill as its clock advances. The full bar represents 240 reference seconds. An 80% average-rate clock reaches 80% at the end. Changing a trajectory recalculates its target. A stopwatch hand takes one second per turn at 100%, independently of simulation playback speed. Pause freezes the animation; scrubbing restores each meter to its recorded total and resets the stopwatch hand. Numeric elapsed times still follow simulation history. Compare elapsed time, instantaneous rate, and the time lost relative to the far-away reference since the object was added. Stronger gravity and faster motion both slow its rate in this model.
The physics behind the playground
Motion uses softened Newtonian N-body gravity and fixed-step velocity-Verlet integration. Anchored bodies are held in place by an imagined external force. “Circular orbit” is an initial two-body estimate; other bodies can perturb it.
Clock rates multiply a gravitational factor √(max(0.0001, 1 + 2Φ/c²)) by a motion factor √(1 − v²/c²). Φ sums the Newtonian potentials, including an object’s own potential at its surface. This is a qualitative multi-body approximation, not a solution to Einstein’s equations. The speed of light is deliberately low (110 sandbox units/s) to make differences visible; speeds are capped at 0.85c.
The grid brightens toward white where its displayed surface bends most strongly. It is an artistic potential landscape, not a literal sheet or a spacetime metric. Sizes, masses, and seconds are sandbox units, not a scale model of the Solar System. Black-hole capture uses a radius of 2GM/c² plus the approaching object’s radius. Contact removes the smaller body from the simulation without a physical merger. Its recorded clock ends; this does not mean its proper time freezes. Black holes do not carry a surface clock. Ordinary body sizes scale gently with mass for readability; black-hole radii retain their mass relation. Strong-field results, collisions, and captures are illustrative; light bending, gravitational waves, and relativistic orbital precession are not simulated.
Learn more: Einstein Online: time dilation · NIST: putting Einstein to the test.