alt Hacker NewsShow HN: Gravity – interactive solar-system simulator, from Newton to Einstein
Just for fun and self education, I've built this over a weekend to teach myself why orbits exist, not just show planets going around. Something that was never clearly explain to me in school. It opens with a guided tour that builds the idea up step by step: two bodies and the equal/opposite force, inertia (the Sun is removed and Earth just drifts straight), then "an orbit is falling and continuously missing," cosmic velocities with a little rocket, Voyager 1 & 2's real gravity assists (the clock runs the actual 1977–1989 dates so the planets orbit into their grand-tour alignment and the slingshots line up), and it ends on Einstein — gravity as curved spacetime, the classic rubber-sheet well. What's real: every body uses its real radius/mass and J2000 orbital elements; positions come from solving Kepler's equation each frame. You can toggle to an N-body mode (symplectic leapfrog) that shows live energy drift (~1e-6%) so you can see the integrator is honest. The only thing faked is scale — at true scale you can't see anything — so there's a toggle between true scale and a log-remapped "visual" scale, with physics always running in real AU. Tech: TypeScript + Three.js + Vite, fully client-side, no backend, works offline (surface textures are generated procedurally from value-noise; only Earth uses a real image). Source: https://github.com/qunabu/Gravity
Happy to answer questions — and feedback on the physics or the explanations is very welcome. This project might be totally inaccurate in terms of real physics, this is how i do understand this on my own - i'm happy to confront this with reality
Comments
This is nice.
I did laugh at how the Gravity built the Earth, with a tiny North America and all, and then as more mass was accumulated, North America got to get bigger and bigger and bigger!
That doesn’t look right: in the 7th panel (too fast it escapes) the force and velocity of earth are constant? 0_o
My physics bias would like to see earth forming while it's constituents were orbiting around the sun.
In any case, nice visualization.
Great job! 14 is misleading though - while the context is one day, the animation depicts axial precession which takes place over ~26,000 years
Super fun! I might show it to my kids later today. Thanks for making it!
> Einstein
How are you handling relativistic effects in the N-body simulation?
Looks great but on mobile the popover covers a quarter of the screen, obscuring the sun
the way the original mathematicians figured all this out absolutely melts my brain
no computers, no calculators, barely working telescopes looking at the moons orbiting Jupiter
(don't be limited by episode title, lots of amazing astrophysics in there)
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Very nice, fairly efficient too.
I don't like the explicit split of Newtonian and relativistic gravity, this is often how it's presented in educational content, but it creates too much confusion; for instance it gives the illusion that they are somehow separate theories even though Newtonian gravity is a limiting case of Einsteinian gravity when v << c and gravitational fields are weak (see Poissons eq for Newtons gravitational potential.
Lastly, you should consider rendering spacetime similar to Alessandro Roussels spacetime visualization https://www.youtube.com/watch?v=wrwgIjBUYVc; probably the best and most innovative one I've seen.