9 ms·

Gravity Simulator

467 points by evanb 12y ago

WoodenChair 12y ago
What's it built in? Is code available? Cool demo!
- sspiff 12y ago
  It's telling me to install Flash, so I'd say, Flash?
  - binocarlos 12y ago
    confirmed - right click -> About Adobe Flash Player
stepstep 12y ago
Be sure to click on the "Generate proto disk" button for instant action. :) > Particle radius is log of mass. Wouldn't it make more sense for the radius to be the cube root of mass (assuming uniform density)?
yoha 12y ago
I'm stuck with an empty loading bar and > Error #2046
- TilmanGriesel 12y ago
  This problem can be fixed by either increasing the Adobe Flash local storage space, (also known as “Flash cookies”) or deleting them.
  - snogglethorpe 12y ago
    How does one do that...? I did "rm -rf ~/.macromedia/Flash_Player/" but still get Error #2046....
shmerl 12y ago
It doesn't work with Mozilla Shumway.
- jgraham 12y ago
  Can you file a (shumway) bug? [1] [1] https://bugzilla.mozilla.org/enter_bug.cgi?product=Firefox&component=Shumway https://bugzilla.mozilla.org/enter_bug.cgi?product=Firefox&c...
  - shmerl 12y ago
    I reported the issue through Shumway UI, but I can file a separate bug.
p4bl0 12y ago
It's really nice. Be sure to activate paths, I'm having a lot of fun using that to draw cool stuff :). https://i.imgur.com/j3YwI5y.png https://i.imgur.com/j3YwI5y.png That was me trying to have a small particle making an 8-shaped path around two bigger ones that would be sufficiently far apart to not interact too strongly with each other. Of course, when the small particle went between the two bigger ones, it shifted their position a tiny bit, but sufficiently for those two to start very slowly moving towards each other, while the small particle was headed straight to the up-right direction. But when the two bigger particles became close enough to finally meet and become one single huge particle, it attracted back the small one, which since then goes back slowly to the huge particle only to shift it a bit and be "relaunched" by it for a new lap. The system seems to be totally stable that way. Update: https://i.imgur.com/heHekdc.png https://i.imgur.com/heHekdc.png
- deleted 12y ago
  [deleted]
- exDM69 12y ago
  > That was me trying to have a small particle making an 8-shaped path around two bigger ones that would be sufficiently far apart to not interact too strongly with each other. This kind of path is called a free return trajectory and was used in the Apollo program lunar missions. The trajectory is not periodic, ie. it doesn't repeat more than once. The trajectory around the earth takes less time than lunar period ("month") so when the craft goes back up, the moon has moved on. You might be able to construct a planet-moon system with an m:n resonance in the orbital periods of the moon and the craft so that the tracjetory is periodic. Free return trajectories are used in manned space flight to guarantee that the craft and crew return to earth if a failure prevents from entering lunar orbit.
zaptheimpaler 12y ago
Very cool! I was able to get 2 masses to oscillate around each other in a funny way[1]. Seems they keep going like that forever. What is this kind of equilibrium called? [1]http://imgur.com/FQYpo9V http://imgur.com/FQYpo9V
- xioxox 12y ago
  Isn't that a normal orbit around a common centre of mass? As the objects have a similar size the centre of mass appears to wobble as it moves along. In the frame of the centre of mass, the objects would just go round in standard orbits.
  - raverbashing 12y ago
    Yeah That's the thing with those kind of simulators (or, actually, with reality), your reference has no relation to the center of mass of the system. Of course, if you "were there" the reference would be the center of mass, like the reference of the solar system is "The Sun" (or a point inside it)
    - jessriedel 12y ago
      The center of mass of the solar system isn't always within the Sun. https://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy) https://en.wikipedia.org/wiki/Barycentric_coordinates_(astro...
      - deleted 12y ago
        [deleted]
  - lutusp 12y ago
    > Isn't that a normal orbit around a common centre of mass? If you limit yourself to looking at the position of one body with respect to another, the orbit will be an ellipse. A circular orbit is simply a special-case ellipse with an eccentricity of zero. This is true for any two orbiting masses -- from the perspective of either of them, the other body's orbit will be an ellipse. And yes, it's always with respect to the common center of mass. Interestingly, mostly because of Jupiter, the solar system's common center of mass can sometimes lie outside the sun.
qoobaa 12y ago
> To view this page ensure that Adobe Flash Player version 10.0.0 or greater is installed. What do you need Flash for?
lutusp 12y ago
Quote: "Particle mass is log of radius". Why not use real physics and make the mass proportional to the cube of the radius? It's also easier to compute. In fact, taking the log of the radius goes in the wrong direction -- the mass of a planet really does increase as the cube of its radius, which changes in a way opposite to log(). Oh, well. Here's my gravity simulator -- it uses JavaScript, no flash required: http://arachnoid.com/orbital_dynamics http://arachnoid.com/orbital_dynamics
- Udo 12y ago
  > "Particle mass is log of radius" They seem to have corrected the sentence now: "particle radius is log of mass". Wow, your simulator is much nicer, plus a great article about it to boot! You should add an option to play cosmic billiard, that's probably the main appeal of the flash simulator :)
  - lutusp 12y ago
    Thanks for your kind comments! > They seem to have corrected the sentence now: "particle radius is log of mass". It's still wrong. Here's a graph comparing mass = e^radius (the reciprocal of radius = log(mass) ) versus mass = radius^3: http://i.imgur.com/XkVFgIH.png http://i.imgur.com/XkVFgIH.png My point is that the two functions have a different behavior, the absolute values generated aren't very important compared to that. For solutions to f = G m1 m2 / r^2 where both bodies are computed, this will produce results wildly different than reality. (If only one body is computed, for example against a much larger parent body mass, the satellite mass stops making a difference.) > You should add an option to play cosmic billiard, that's probably the main appeal of the flash simulator :) Nice idea. I was more interested in portraying the solar system using real planetary masses and real physical constants. Still, it's a nice suggestion.
kyberias 12y ago
Similar simulation I've made earlier: http://newton.azurewebsites.net/ http://newton.azurewebsites.net/ Pure Javascript using Sylvester.js for Math.
- dj-wonk 12y ago
  Great! I wanted to say that the "add random" button tends to shoot particles light-years away, never to be seen again. Is that what you were intending?
shentheory 12y ago
So cool. The amount of time I just spent playing with that is going to screw up my whole day tomorrow!!
alokdhari 12y ago
Hey.. TBH its real fun to play with it. Question.. How big is the frame? I create two objects and they went out of frame. Then I create 2 more but a bit bigger ones and after 2-3 minutes I see those two objects coming back and it looks good too :D .. check it out http://imgur.com/IBYeysm http://imgur.com/IBYeysm
- fake-name 12y ago
  I don't think there is a "frame". It solves for all particles all the time. After all, if a particle leaves the viewport, it doesn't affect the particle count readout at all. I assume if things move far enough, there would probably be interesting floating-point precision errors, but I'd imagine that would take quite a while.
- nacs 12y ago
  The frame extends far beyond your the preset view. You can pan around with Control-click-drag.
carlob 12y ago
Why Euler? Isn't that terribly unstable?
- zamalek 12y ago
  Only in low FPS scenarios (it becomes even worse if the FPS is low AND wildly varies). For a toy like this there is nothing wrong with using Euler.
empressplay 12y ago
This is pretty cool! I wonder if there's a way I could implement that somehow into our turtle graphics app. Turtle gravity? As for this app it would be awesome if you could zoom out -- I find most of my stable(-ish) orbits are very long and at very large mass sizes, so it would be good to see their entire orbits.
Link- 12y ago
When I saw the mass OMFG I was expecting a black hole to pop up :(
andreif 12y ago
Next step: add dark matter
- lutusp 12y ago
  My simulator models dark energy (dark matter might be very difficult to model, since we don't understand it): http://arachnoid.com/orbital_dynamics/ http://arachnoid.com/orbital_dynamics/
  - deleted 12y ago
    [deleted]
kgabis 12y ago
I once wrote a similar simulation in C: https://github.com/kgabis/gravitysim https://github.com/kgabis/gravitysim It uses Barnes-Hut algorithm, so it runs in nlogn, instead of n^2.
ejfox 12y ago
Would love a setting to select the number of objects to drop on click, and the spread for them ie (20 objects, 50px radius) and also for objects to gain the mass of objects that hit them, so that if a bunch of huge objects collide, they create a super-object
- p4bl0 12y ago
  > for objects to gain the mass of objects that hit them, so that if a bunch of huge objects collide, they create a super-object That's already what happens here.
Zardoz84 12y ago
I did something similar on 3d but is not on realtime and generate outputfiles that could be ploted with gnuplot. It uses an third order Euler integration. Some day I should write a realtime mode for it: https://github.com/Zardoz89/nBodySim https://github.com/Zardoz89/nBodySim
simonswain 12y ago
I did something similar, but with combat - ships, lasers and missiles. There are a couple of variations, and some tweaks like the ships accelerating perpendicular to the planets so they don't crash land. http://codepen.io/simonswain/full/ftEjD/ http://codepen.io/simonswain/full/ftEjD/ http://codepen.io/simonswain/full/CeHmh/ http://codepen.io/simonswain/full/CeHmh/
- moron4hire 12y ago
  That is really cool.
- jonifico 12y ago
  That was great. COngrats!
- omikun 12y ago
  Amazing! I have a non-combat version in 3d. Now I know what mine is missing. https://www.youtube.com/watch?v=73O8aDN41sU https://www.youtube.com/watch?v=73O8aDN41sU
- NathanKP 12y ago
  Very cool. I notice that many ships tend to get stuck on the edge of the map though. Maybe make it so they loop back around to the other side of the map?
  - Terr_ 12y ago
    > Maybe make it so they loop back That's be a toroidal spacetime surface, wouldn't it? Would gravitational influence also have to "loop around" the borders? If not, you might get some very interesting effects if a mass is near the edge of the map.
  - simonswain 12y ago
    Thanks. Yeah, there are a couple of glitches in there. The ships bounce off the edge of the screen, and smaller ships run from bigger ones -- combined, that makes them crawl along the sides. Looping around is a good idea, or I'm wondering if there could be some kind of log scale to distance the further out from the centre the ships get. I did a variation a while back with a star in the middle that could be tweaked for that (click to add a ship). Mind you, the gravity from the star pretty much takes care of that problem. http://codepen.io/simonswain/full/cKejC/ http://codepen.io/simonswain/full/cKejC/
simonbarker87 12y ago
If you're interested in learning some of the basics behind this kind of simulation I highly recommend the Nature of Code by Daniel Shiffman - lots of physics and programming fun. http://natureofcode.com/ http://natureofcode.com/
alandarev 12y ago
Such an amazing simulator to play with. I can only imagine how breath-taking this can be for kids to play around, plus it teaches physics!
- TeMPOraL 12y ago
  Think about how breath-taking and educating must it be for a kid to write such a thing. As a person who actually learned basic mechanics by writing video games I do believe that "you ain't understood nothing until you know how to teach it to a computer". Trying to express knowledge in (working) code is a great exercise that mercilessly catches even tiny holes in one's understanding.
  - kator 12y ago
    Ditto that, I often tell people computers are very stupid. One proof of that is they let humans program them. The second proof is you have to teach them everything from scratch. :-)
quarterto 12y ago
Built something similar in C/GL a few years back. Quite a fun exercise, it only took a couple of hours to get the basics working. It has no collision detection, so particles that end up too close end up with infinite velocity, which is quite fun to watch. EDIT: oh hey, I found a binary. https://www.dropbox.com/s/ihfqnzjaxhtskwt/Screenshot%202014-09-08%2009.59.20.png?dl=0 https://www.dropbox.com/s/ihfqnzjaxhtskwt/Screenshot%202014-... https://github.com/quarterto/Yay-Physics https://github.com/quarterto/Yay-Physics
soci 12y ago
A friend of mine made something similar to this for iOS a while ago: http://gravityapp.info http://gravityapp.info His app is being used in schools.
dharma1 12y ago
this is awesome. had a look into doing this on the GPU a while ago, barnes-hut was the best speedup I could find There has to be a game in this somewhere...
- Zardoz84 12y ago
  Check NVIDIA papers of doing "n bodies problem" on CUDA. They got awesome results.
bdaver 12y ago
I've made touch interactive installation at 2008 with similar physics simulation http://vimeo.com/19361057 http://vimeo.com/19361057
dudeson 12y ago
Sun is the god.. (jk) http://s29.postimg.org/yb6ly7ybp/111.png http://s29.postimg.org/yb6ly7ybp/111.png
- vishnugupta 12y ago
  NSFW warning!!
spain 12y ago
Very fascinating! It reminds me of Conway's Game of Life, looking for different kinds of patterns that will make a stable construct and do something interesting.
csomar 12y ago
At first I thought it was a 2D simulation but my attempt showed otherwise. http://i.imgur.com/roHx3wi.png http://i.imgur.com/roHx3wi.png Really nice tool.
- oceanofsolaris 12y ago
  I think what you see there is not necessarily an indication of the simulation happening in 3-d space[1]. It is however evidence of [precession](http://en.wikipedia.org/wiki/Precession http://en.wikipedia.org/wiki/Precession), which should not happen in a 2-body problem. If no other heavy body was close to the ones shown in your screenshot, this should actually not happen. Since the simulation does, however, only have a finite precision, this could very well be an artifact of the simulation (happens often if you have near-collisions, since the change of velocity there happens rapidly and is hard to describe correctly with a fixed-length time step). [1] In fact if all initial positions and velocities lie in the same 2d plane, all trajectories would also lie in this plane. Since the interface only allows one to set such initial conditions, I am fairly confident that the simulation is 2d only.
pp19dd 12y ago
Reminds me of orbitalis - a Ludum dare game from last year that eventually ended up on steam. Object is simple, establish a stable orbit in the system for a span of one clock rotation (clock is on the very outside.) See http://www.alanzucconi.com/extra/0rbitalis/ld48/ http://www.alanzucconi.com/extra/0rbitalis/ld48/
bochi 12y ago
I did something similar in Javascript. My implementation uses the Runge-Kutta algorithm, that is more stable. My GUI does not allow planet creation, but it shows several stable configurations, including the 13 recently found three body planar orbit configurations. http://jbochi.github.io/planets/ http://jbochi.github.io/planets/ http://suki.ipb.ac.rs/3body/ http://suki.ipb.ac.rs/3body/
p4bl0 12y ago
Can anyone explain the graphical effects that appears here in the white lines https://i.imgur.com/sRCLAsn.png https://i.imgur.com/sRCLAsn.png https://i.imgur.com/81Jf56H.png https://i.imgur.com/81Jf56H.png? (To reproduce, use a really big stable mass (either enter it by hand or use the OMFG button and click several time at the same position without moving your mouse) and launch at fast speed a tiny particle in an approximately tangent direction of a circle centered on the big mass.)
- hughes 12y ago
  That's a Moiré pattern and is an interaction of the resolution of your display and the proximity of the lines. http://en.wikipedia.org/wiki/Moir%C3%A9_pattern http://en.wikipedia.org/wiki/Moir%C3%A9_pattern
  - p4bl0 12y ago
    That was quick! Thanks! I didn't know that Moiré pattern could appear on a screen "without" a second grid (or rather, I didn't realize that the pixels of my display could serve as the second grid, which is kind of obvious now that you said it). New screenshots: https://imgur.com/a/7W9FI https://imgur.com/a/7W9FI :)
Zeebrommer 12y ago
Ok, maybe I should finally buy Kerbal Space Programme
zalmoxes 12y ago
This is nice, but please rename the OMFG button. As someone who works in K-12, it would be a shame if a program like this is rejected because of a button.
- bengali3 12y ago
  agreed. This is really cool (and simple too) Also, how hard would it be to set an initial state of our current solar system? Teachers could set up scenarios for students to interact with, and allow students to collaborate. Maybe as simple as an output/input of textual data for saving & restoring state? i'd love to try model our current solar system, and see if i can get something to mars with just an initial trajectory. see: http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...
  - baddox 12y ago
    The scale of the Solar System would require you to visualize the bodies as much larger than their true size in order to see everything on one screen. If the Sun is 1 pixel, all other bodies are way smaller than a pixel, and Neptune is 3000 pixels away. And of course, you'd probably want to speed up the simulation speed.
- McDiesel 12y ago
  Its a shame that a simple button label would be the cause of an education tool to be rejected... sissy-state for the win!
  - golgappi 12y ago
    OMFG is not a standard abbreviation. If you are making a tool that can be used worldwide, best to stick with words and slangs that are universally known. Moreso, best to stick with terms that are acceptable to people of all cultures and backgrounds. The word 'fucking' may be a light-hearted slang to a 20 year old american, but it may just be offensive to someone older living in the UAE. Professionalism, though not perfect, does in fact help in making people from various background agree and understand good ideas by being austere, simple and well-defined.
jokoon 12y ago
did the same thing with box2d and sfml. not very hard to do... a funny thing to do is to try to put those into a stable orbit
jonifico 12y ago
All of a sudden, the universe makes a helluva lot more sense now.
eccstartup 12y ago
It is not easy to draw a circle.
arikrak 12y ago
Cool, it's a good way to teach people about gravity. Most people don't know the basic idea of how falling and orbiting are connected. I once made a simple 'game' for this purpose: https://www.learneroo.com/modules/2/nodes/42 https://www.learneroo.com/modules/2/nodes/42
- jheriko 12y ago
  really? i do remember the wow moment when i worked it out for myself but i was pretty young...
jheriko 12y ago
i like how the points merge, thats what makes this more than just a dumbass simulator imo. :) took a while for me to work out though.
DecoPerson 12y ago
I think I just learnt something about gravity and oscillations: http://i.imgur.com/OrUUFj7.png http://i.imgur.com/OrUUFj7.png Using the rapid-click feature on my mouse to place a dense circle outline: http://i.imgur.com/14IN5pi.png http://i.imgur.com/14IN5pi.png
- mkoryak 12y ago
  And I think I just learned something about all of your open tabs and bookmarks
  - DecoPerson 12y ago
    Hope it's useful :)
kator 12y ago
Very nice, wish it was HTML5 rather then flash.. It brings to light how hard it is to get a stable solar system. And of course I knew this but playing around I realized our sun is not stationary either... It's easy to forget the entire solar system we live in is this amazing mix of interactions between the planets, sun, other debris and outside mass from other parts of the universe.
- karol 12y ago
  The source is browsable if you modofy the URL. I expect someone with create a canvas driven clone within 48h:)
  - mcdevhammer 12y ago
    Done. Although I did this years ago: http://mobile.sheridanc.on.ca/~claassen/Gravity/Gravity.html http://mobile.sheridanc.on.ca/~claassen/Gravity/Gravity.html
    - dj-wonk 12y ago
      Great! I tried it out; when I combined a galaxy with a field , many particles went flying from the center at a high rate of speed. Why is that?
      - mcdevhammer 12y ago
        When a small particle gets accelerated to great speeds as it approaches a massive object it simply passes through because the collisions are off/aren't perfect even when they're on
- dj-wonk 12y ago
  It would be nice to arbitrarily choose a specific particle to be the center of the system (the frame of reference).
- mathu7 12y ago
  Can someone explain the math behind this? Which methods were used? I wasn't aware it was possible to run such a simulation on flash because each body results in 6x more calculation. Is this like plugging in the masses into a single equation? I want to learn more about how this is done .
eccstartup 12y ago
I think the simulation of the real gravity is better. Because we can find out what is the hidden magic of nature. Then there comes accuracy issue of the simulation. How to tell us that your simulation will be `correct`?
masswerk 12y ago
For historical interest: Here's a description of the humble beginnings of gravity put onto a computer's screen (think of PDP-1 Spacewar! code, 1962): http://www.masswerk.at/spacewar/inside/insidespacewar-pt6-gravity.html http://www.masswerk.at/spacewar/inside/insidespacewar-pt6-gr...
ErikRogneby 12y ago
This makes it easy to understand why there are so many binary systems.
tuzemec 12y ago
Relevant: http://codingmath.com/ http://codingmath.com/ Really nice series, showing how to do vectors, gravity and so on in JS.
blt 12y ago
I made this in CUDA a while back. Fun project - the naive n^2 algorithm is embarrassingly parallel so it's easy to write. Fun to hit the CPU/GPU switch and watch the huge performance boost, even on a dinky laptop GPU.
gdh73 12y ago
Love this. I managed to get a pretty stable orbit by using the right angle and velocity (by chance). Highlighted in yellow: http://i.imgur.com/OOaTLMP.png http://i.imgur.com/OOaTLMP.png
- guiomie 12y ago
  This is the coolest trajectory found on this thread for now!!!!
  - gdh73 12y ago
    It ran for several hours like that and never deviated from the original "circular" path. Unlike the other attempts, which were all much more elliptical. I guess that's kind of how planets would stay in a stable orbit though, having the right distance and speed for their mass.
exDM69 12y ago
Here is Universe Sandbox, a gravity simulation that can do massive scale simulation on home computers. Neat demo videos about colliding galaxies, etc. http://universesandbox.com/ http://universesandbox.com/
chops 12y ago
I, like many others on this thread, have dabbled in the Gravity Simulation game. I've done two of them. The first one was back in 1999, I wrote one with 13h[1] graphics for DOS (Borland Turbo C++), and to make it more fun, I used flat sprites to represent the objects (so the objects progressed from being rocks to asteroids to planets - with the largest sprite I used being an image of Jupiter). A few years later, I rewrote it in OpenGL (using the GLUT library) for a class. It worked much better (and of course the hardware was much faster too). I had always planned rewriting the physics engine to use Barnes-Hut, but never got around to it, so it always ran at N^2. Anyway, for the curious, the code is up on Github amongst my collection of other old garbage stuff from back in the day: https://github.com/choptastic/OldCode-Public/tree/master/GravSim https://github.com/choptastic/OldCode-Public/tree/master/Gra... [1] http://en.wikipedia.org/wiki/Mode_13h http://en.wikipedia.org/wiki/Mode_13h
Aardwolf 12y ago
Wow this is fun. But it has so much potential for improvements! -Saving your setting -Undo-ing -Auto generate small object in perfect circle orbit around larger object, and generate sun-earth-moon system -Export setting to other people -Allow to numerically finetune locations and velocities -Don't ruin the trails when dragging -Make it run faster, especially with trails enabled -etc... :) EDIT: Gee, when will Hacker News finally support newlines properly?
- mkoryak 12y ago
  well at least it supports negative mass ;)
- luckyno13 12y ago
  I would like to name individual "bodies," the ability to zoom in and out, and maybe even a mathematical readout of some sort. The values behind trajectory, etc.
mkoryak 12y ago
I have a question about a phenomenon I am seeing. say you add 3+ equal mass objects without a velocity and they all collide with each other without "missing or orbiting" one another. Sometimes the final mass will have a velocity. It would seem like it should not have a velocity because it would be canceled out. What am I missing? Where is this energy coming from?
- lutusp 12y ago
  In real life, the three colliding bodies would convert their kinetic energy into heat and combine into a very hot body. Sort of how we think the moon was created, from a Mars-sized body colliding with Earth long ago. But in a numerical simulation, usually what happens is that three bodies on a collision source approach one another but, because of the limitations of numerical differential equation solving, suddenly fly away from each other in a rather non-physical way, and with more energy than they had to begin with. The tl;dr: most of the funny effects in a simulator arise from the limitations of numerical simulation methods, and many of those aren't representative of real physics.
mkoryak 12y ago
here is an interesting one http://i.imgur.com/C835B8N.jpg http://i.imgur.com/C835B8N.jpg
programmarchy 12y ago
Can I play? I wrote a RK4 gravity simulator a while back in WebGL, but my main goal at the time was to find a way to visualize gravitational fields, so its nowhere near as interactive or user friendly as your implementation. I was trying for the effect you see in high school physics videos. My approach was to use a plane geometry mesh and apply a vertex displacement shader, but never quite got it to look or perform how I wanted. http://programmarchy.github.io/gravity http://programmarchy.github.io/gravity
nodesocket 12y ago
Somehow I ended up with a perfect orbit within my madness. http://i.imgur.com/wFReJps.png http://i.imgur.com/wFReJps.png
legohead 12y ago
create a few random fields turn on combine mass on collision add galaxy in middle watch the creation of a solar system :)
vhost- 12y ago
It's pretty sweet I got this to happen http://i.imgur.com/Sk4Rznu.png http://i.imgur.com/Sk4Rznu.png
junku901 12y ago
Does anyone know why perihelion moves in this simulation? I learned that perihelion movement can only be explained by general relativity, not newtonian mechanics. Is this just a limit of computer simulation? (which can't calculate the exact value of Gm1m2/r^2)
- vhffm 12y ago
  It is a shortcoming of the integrator (Euler), not in the calculation of the force (as you suspected). See [1]. Especially slides 11 and 12 may be of interest. [1] http://www.astro.sunysb.edu/phy277/lect30.pdf http://www.astro.sunysb.edu/phy277/lect30.pdf As a sidenote, there are better integration schemes, but all of them will have some error because we discretize the system.
  - junku901 12y ago
    Wow that is really interesting. Thanks. By the way, if there are always some error in all systems, then what kind of systems are used by NASA or other space exploration institutes?
    - vhffm 12y ago
      I'm not in the business of space mission engineering, so I don't know details. Sorry. But in general, you can control the error and make a trade-off between computing time and accuracy. You would select a timescale over which you want the error to be below some value, and then only consider the simulation valid until then.
escapologybb 12y ago
I would love to know if there's a name to this particular type of orbit, I spent a little bit too much time creating it and now need to know some more about it! :-) Anybody know how I can find out? http://imgur.com/4W7eqj9 http://imgur.com/4W7eqj9 Thanks
bipin-nag 12y ago
Most awesome thing ever seen on HackerNews !!!
prothid 12y ago
I lost my planet. http://i.imgur.com/EUK7kiE.jpg http://i.imgur.com/EUK7kiE.jpg
noisy_boy 12y ago
I love this but its a pity that paths vanish when I click-drag the frame.

Gravity Simulator

467 points by evanb 12y ago

WoodenChair 12y ago

What's it built in? Is code available? Cool demo!

sspiff 12y ago
It's telling me to install Flash, so I'd say, Flash?
- binocarlos 12y ago
  confirmed - right click -> About Adobe Flash Player

stepstep 12y ago

Be sure to click on the "Generate proto disk" button for instant action. :) > Particle radius is log of mass. Wouldn't it make more sense for the radius to be the cube root of mass (assuming uniform density)?

yoha 12y ago

I'm stuck with an empty loading bar and > Error #2046

TilmanGriesel 12y ago
This problem can be fixed by either increasing the Adobe Flash local storage space, (also known as “Flash cookies”) or deleting them.
- snogglethorpe 12y ago
  How does one do that...? I did "rm -rf ~/.macromedia/Flash_Player/" but still get Error #2046....

shmerl 12y ago

It doesn't work with Mozilla Shumway.

jgraham 12y ago
Can you file a (shumway) bug? [1] [1] https://bugzilla.mozilla.org/enter_bug.cgi?product=Firefox&component=Shumway https://bugzilla.mozilla.org/enter_bug.cgi?product=Firefox&c...
- shmerl 12y ago
  I reported the issue through Shumway UI, but I can file a separate bug.

p4bl0 12y ago

It's really nice. Be sure to activate paths, I'm having a lot of fun using that to draw cool stuff :). https://i.imgur.com/j3YwI5y.png https://i.imgur.com/j3YwI5y.png That was me trying to have a small particle making an 8-shaped path around two bigger ones that would be sufficiently far apart to not interact too strongly with each other. Of course, when the small particle went between the two bigger ones, it shifted their position a tiny bit, but sufficiently for those two to start very slowly moving towards each other, while the small particle was headed straight to the up-right direction. But when the two bigger particles became close enough to finally meet and become one single huge particle, it attracted back the small one, which since then goes back slowly to the huge particle only to shift it a bit and be "relaunched" by it for a new lap. The system seems to be totally stable that way. Update: https://i.imgur.com/heHekdc.png https://i.imgur.com/heHekdc.png

deleted 12y ago
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exDM69 12y ago
> That was me trying to have a small particle making an 8-shaped path around two bigger ones that would be sufficiently far apart to not interact too strongly with each other. This kind of path is called a free return trajectory and was used in the Apollo program lunar missions. The trajectory is not periodic, ie. it doesn't repeat more than once. The trajectory around the earth takes less time than lunar period ("month") so when the craft goes back up, the moon has moved on. You might be able to construct a planet-moon system with an m:n resonance in the orbital periods of the moon and the craft so that the tracjetory is periodic. Free return trajectories are used in manned space flight to guarantee that the craft and crew return to earth if a failure prevents from entering lunar orbit.

zaptheimpaler 12y ago

Very cool! I was able to get 2 masses to oscillate around each other in a funny way[1]. Seems they keep going like that forever. What is this kind of equilibrium called? [1]http://imgur.com/FQYpo9V http://imgur.com/FQYpo9V

xioxox 12y ago
Isn't that a normal orbit around a common centre of mass? As the objects have a similar size the centre of mass appears to wobble as it moves along. In the frame of the centre of mass, the objects would just go round in standard orbits.
- raverbashing 12y ago
  Yeah That's the thing with those kind of simulators (or, actually, with reality), your reference has no relation to the center of mass of the system. Of course, if you "were there" the reference would be the center of mass, like the reference of the solar system is "The Sun" (or a point inside it)
  - jessriedel 12y ago
    The center of mass of the solar system isn't always within the Sun. https://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy) https://en.wikipedia.org/wiki/Barycentric_coordinates_(astro...
    - deleted 12y ago
      [deleted]
- lutusp 12y ago
  > Isn't that a normal orbit around a common centre of mass? If you limit yourself to looking at the position of one body with respect to another, the orbit will be an ellipse. A circular orbit is simply a special-case ellipse with an eccentricity of zero. This is true for any two orbiting masses -- from the perspective of either of them, the other body's orbit will be an ellipse. And yes, it's always with respect to the common center of mass. Interestingly, mostly because of Jupiter, the solar system's common center of mass can sometimes lie outside the sun.

qoobaa 12y ago

> To view this page ensure that Adobe Flash Player version 10.0.0 or greater is installed. What do you need Flash for?

lutusp 12y ago

Quote: "Particle mass is log of radius". Why not use real physics and make the mass proportional to the cube of the radius? It's also easier to compute. In fact, taking the log of the radius goes in the wrong direction -- the mass of a planet really does increase as the cube of its radius, which changes in a way opposite to log(). Oh, well. Here's my gravity simulator -- it uses JavaScript, no flash required: http://arachnoid.com/orbital_dynamics http://arachnoid.com/orbital_dynamics

Udo 12y ago
> "Particle mass is log of radius" They seem to have corrected the sentence now: "particle radius is log of mass". Wow, your simulator is much nicer, plus a great article about it to boot! You should add an option to play cosmic billiard, that's probably the main appeal of the flash simulator :)
- lutusp 12y ago
  Thanks for your kind comments! > They seem to have corrected the sentence now: "particle radius is log of mass". It's still wrong. Here's a graph comparing mass = e^radius (the reciprocal of radius = log(mass) ) versus mass = radius^3: http://i.imgur.com/XkVFgIH.png http://i.imgur.com/XkVFgIH.png My point is that the two functions have a different behavior, the absolute values generated aren't very important compared to that. For solutions to f = G m1 m2 / r^2 where both bodies are computed, this will produce results wildly different than reality. (If only one body is computed, for example against a much larger parent body mass, the satellite mass stops making a difference.) > You should add an option to play cosmic billiard, that's probably the main appeal of the flash simulator :) Nice idea. I was more interested in portraying the solar system using real planetary masses and real physical constants. Still, it's a nice suggestion.

kyberias 12y ago

Similar simulation I've made earlier: http://newton.azurewebsites.net/ http://newton.azurewebsites.net/ Pure Javascript using Sylvester.js for Math.

dj-wonk 12y ago
Great! I wanted to say that the "add random" button tends to shoot particles light-years away, never to be seen again. Is that what you were intending?

shentheory 12y ago

So cool. The amount of time I just spent playing with that is going to screw up my whole day tomorrow!!

alokdhari 12y ago

Hey.. TBH its real fun to play with it. Question.. How big is the frame? I create two objects and they went out of frame. Then I create 2 more but a bit bigger ones and after 2-3 minutes I see those two objects coming back and it looks good too :D .. check it out http://imgur.com/IBYeysm http://imgur.com/IBYeysm

fake-name 12y ago
I don't think there is a "frame". It solves for all particles all the time. After all, if a particle leaves the viewport, it doesn't affect the particle count readout at all. I assume if things move far enough, there would probably be interesting floating-point precision errors, but I'd imagine that would take quite a while.
nacs 12y ago
The frame extends far beyond your the preset view. You can pan around with Control-click-drag.

carlob 12y ago

Why Euler? Isn't that terribly unstable?

zamalek 12y ago
Only in low FPS scenarios (it becomes even worse if the FPS is low AND wildly varies). For a toy like this there is nothing wrong with using Euler.

empressplay 12y ago

This is pretty cool! I wonder if there's a way I could implement that somehow into our turtle graphics app. Turtle gravity? As for this app it would be awesome if you could zoom out -- I find most of my stable(-ish) orbits are very long and at very large mass sizes, so it would be good to see their entire orbits.

Link- 12y ago

When I saw the mass OMFG I was expecting a black hole to pop up :(

andreif 12y ago

Next step: add dark matter

lutusp 12y ago
My simulator models dark energy (dark matter might be very difficult to model, since we don't understand it): http://arachnoid.com/orbital_dynamics/ http://arachnoid.com/orbital_dynamics/
- deleted 12y ago
  [deleted]

kgabis 12y ago

I once wrote a similar simulation in C: https://github.com/kgabis/gravitysim https://github.com/kgabis/gravitysim It uses Barnes-Hut algorithm, so it runs in nlogn, instead of n^2.

ejfox 12y ago

Would love a setting to select the number of objects to drop on click, and the spread for them ie (20 objects, 50px radius) and also for objects to gain the mass of objects that hit them, so that if a bunch of huge objects collide, they create a super-object

p4bl0 12y ago
> for objects to gain the mass of objects that hit them, so that if a bunch of huge objects collide, they create a super-object That's already what happens here.

Zardoz84 12y ago

I did something similar on 3d but is not on realtime and generate outputfiles that could be ploted with gnuplot. It uses an third order Euler integration. Some day I should write a realtime mode for it: https://github.com/Zardoz89/nBodySim https://github.com/Zardoz89/nBodySim

simonswain 12y ago

I did something similar, but with combat - ships, lasers and missiles. There are a couple of variations, and some tweaks like the ships accelerating perpendicular to the planets so they don't crash land. http://codepen.io/simonswain/full/ftEjD/ http://codepen.io/simonswain/full/ftEjD/ http://codepen.io/simonswain/full/CeHmh/ http://codepen.io/simonswain/full/CeHmh/

moron4hire 12y ago
That is really cool.
jonifico 12y ago
That was great. COngrats!
omikun 12y ago
Amazing! I have a non-combat version in 3d. Now I know what mine is missing. https://www.youtube.com/watch?v=73O8aDN41sU https://www.youtube.com/watch?v=73O8aDN41sU
NathanKP 12y ago
Very cool. I notice that many ships tend to get stuck on the edge of the map though. Maybe make it so they loop back around to the other side of the map?
- Terr_ 12y ago
  > Maybe make it so they loop back That's be a toroidal spacetime surface, wouldn't it? Would gravitational influence also have to "loop around" the borders? If not, you might get some very interesting effects if a mass is near the edge of the map.
- simonswain 12y ago
  Thanks. Yeah, there are a couple of glitches in there. The ships bounce off the edge of the screen, and smaller ships run from bigger ones -- combined, that makes them crawl along the sides. Looping around is a good idea, or I'm wondering if there could be some kind of log scale to distance the further out from the centre the ships get. I did a variation a while back with a star in the middle that could be tweaked for that (click to add a ship). Mind you, the gravity from the star pretty much takes care of that problem. http://codepen.io/simonswain/full/cKejC/ http://codepen.io/simonswain/full/cKejC/

simonbarker87 12y ago

If you're interested in learning some of the basics behind this kind of simulation I highly recommend the Nature of Code by Daniel Shiffman - lots of physics and programming fun. http://natureofcode.com/ http://natureofcode.com/

alandarev 12y ago

Such an amazing simulator to play with. I can only imagine how breath-taking this can be for kids to play around, plus it teaches physics!

TeMPOraL 12y ago
Think about how breath-taking and educating must it be for a kid to write such a thing. As a person who actually learned basic mechanics by writing video games I do believe that "you ain't understood nothing until you know how to teach it to a computer". Trying to express knowledge in (working) code is a great exercise that mercilessly catches even tiny holes in one's understanding.
- kator 12y ago
  Ditto that, I often tell people computers are very stupid. One proof of that is they let humans program them. The second proof is you have to teach them everything from scratch. :-)

quarterto 12y ago

Built something similar in C/GL a few years back. Quite a fun exercise, it only took a couple of hours to get the basics working. It has no collision detection, so particles that end up too close end up with infinite velocity, which is quite fun to watch. EDIT: oh hey, I found a binary. https://www.dropbox.com/s/ihfqnzjaxhtskwt/Screenshot%202014-09-08%2009.59.20.png?dl=0 https://www.dropbox.com/s/ihfqnzjaxhtskwt/Screenshot%202014-... https://github.com/quarterto/Yay-Physics https://github.com/quarterto/Yay-Physics

soci 12y ago

A friend of mine made something similar to this for iOS a while ago: http://gravityapp.info http://gravityapp.info His app is being used in schools.

dharma1 12y ago

this is awesome. had a look into doing this on the GPU a while ago, barnes-hut was the best speedup I could find There has to be a game in this somewhere...

Zardoz84 12y ago
Check NVIDIA papers of doing "n bodies problem" on CUDA. They got awesome results.

bdaver 12y ago

I've made touch interactive installation at 2008 with similar physics simulation http://vimeo.com/19361057 http://vimeo.com/19361057

dudeson 12y ago

Sun is the god.. (jk) http://s29.postimg.org/yb6ly7ybp/111.png http://s29.postimg.org/yb6ly7ybp/111.png

vishnugupta 12y ago
NSFW warning!!

spain 12y ago

Very fascinating! It reminds me of Conway's Game of Life, looking for different kinds of patterns that will make a stable construct and do something interesting.

csomar 12y ago

At first I thought it was a 2D simulation but my attempt showed otherwise. http://i.imgur.com/roHx3wi.png http://i.imgur.com/roHx3wi.png Really nice tool.

oceanofsolaris 12y ago
I think what you see there is not necessarily an indication of the simulation happening in 3-d space[1]. It is however evidence of [precession](http://en.wikipedia.org/wiki/Precession http://en.wikipedia.org/wiki/Precession), which should not happen in a 2-body problem. If no other heavy body was close to the ones shown in your screenshot, this should actually not happen. Since the simulation does, however, only have a finite precision, this could very well be an artifact of the simulation (happens often if you have near-collisions, since the change of velocity there happens rapidly and is hard to describe correctly with a fixed-length time step). [1] In fact if all initial positions and velocities lie in the same 2d plane, all trajectories would also lie in this plane. Since the interface only allows one to set such initial conditions, I am fairly confident that the simulation is 2d only.

pp19dd 12y ago

Reminds me of orbitalis - a Ludum dare game from last year that eventually ended up on steam. Object is simple, establish a stable orbit in the system for a span of one clock rotation (clock is on the very outside.) See http://www.alanzucconi.com/extra/0rbitalis/ld48/ http://www.alanzucconi.com/extra/0rbitalis/ld48/

bochi 12y ago

I did something similar in Javascript. My implementation uses the Runge-Kutta algorithm, that is more stable. My GUI does not allow planet creation, but it shows several stable configurations, including the 13 recently found three body planar orbit configurations. http://jbochi.github.io/planets/ http://jbochi.github.io/planets/ http://suki.ipb.ac.rs/3body/ http://suki.ipb.ac.rs/3body/

p4bl0 12y ago

Can anyone explain the graphical effects that appears here in the white lines https://i.imgur.com/sRCLAsn.png https://i.imgur.com/sRCLAsn.png https://i.imgur.com/81Jf56H.png https://i.imgur.com/81Jf56H.png? (To reproduce, use a really big stable mass (either enter it by hand or use the OMFG button and click several time at the same position without moving your mouse) and launch at fast speed a tiny particle in an approximately tangent direction of a circle centered on the big mass.)

hughes 12y ago
That's a Moiré pattern and is an interaction of the resolution of your display and the proximity of the lines. http://en.wikipedia.org/wiki/Moir%C3%A9_pattern http://en.wikipedia.org/wiki/Moir%C3%A9_pattern
- p4bl0 12y ago
  That was quick! Thanks! I didn't know that Moiré pattern could appear on a screen "without" a second grid (or rather, I didn't realize that the pixels of my display could serve as the second grid, which is kind of obvious now that you said it). New screenshots: https://imgur.com/a/7W9FI https://imgur.com/a/7W9FI :)

Zeebrommer 12y ago

Ok, maybe I should finally buy Kerbal Space Programme

zalmoxes 12y ago

This is nice, but please rename the OMFG button. As someone who works in K-12, it would be a shame if a program like this is rejected because of a button.

bengali3 12y ago
agreed. This is really cool (and simple too) Also, how hard would it be to set an initial state of our current solar system? Teachers could set up scenarios for students to interact with, and allow students to collaborate. Maybe as simple as an output/input of textual data for saving & restoring state? i'd love to try model our current solar system, and see if i can get something to mars with just an initial trajectory. see: http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...
- baddox 12y ago
  The scale of the Solar System would require you to visualize the bodies as much larger than their true size in order to see everything on one screen. If the Sun is 1 pixel, all other bodies are way smaller than a pixel, and Neptune is 3000 pixels away. And of course, you'd probably want to speed up the simulation speed.
McDiesel 12y ago
Its a shame that a simple button label would be the cause of an education tool to be rejected... sissy-state for the win!
- golgappi 12y ago
  OMFG is not a standard abbreviation. If you are making a tool that can be used worldwide, best to stick with words and slangs that are universally known. Moreso, best to stick with terms that are acceptable to people of all cultures and backgrounds. The word 'fucking' may be a light-hearted slang to a 20 year old american, but it may just be offensive to someone older living in the UAE. Professionalism, though not perfect, does in fact help in making people from various background agree and understand good ideas by being austere, simple and well-defined.

jokoon 12y ago

did the same thing with box2d and sfml. not very hard to do... a funny thing to do is to try to put those into a stable orbit

jonifico 12y ago

All of a sudden, the universe makes a helluva lot more sense now.

eccstartup 12y ago

It is not easy to draw a circle.

arikrak 12y ago

Cool, it's a good way to teach people about gravity. Most people don't know the basic idea of how falling and orbiting are connected. I once made a simple 'game' for this purpose: https://www.learneroo.com/modules/2/nodes/42 https://www.learneroo.com/modules/2/nodes/42

jheriko 12y ago
really? i do remember the wow moment when i worked it out for myself but i was pretty young...

jheriko 12y ago

i like how the points merge, thats what makes this more than just a dumbass simulator imo. :) took a while for me to work out though.

DecoPerson 12y ago

I think I just learnt something about gravity and oscillations: http://i.imgur.com/OrUUFj7.png http://i.imgur.com/OrUUFj7.png Using the rapid-click feature on my mouse to place a dense circle outline: http://i.imgur.com/14IN5pi.png http://i.imgur.com/14IN5pi.png

mkoryak 12y ago
And I think I just learned something about all of your open tabs and bookmarks
- DecoPerson 12y ago
  Hope it's useful :)

kator 12y ago

Very nice, wish it was HTML5 rather then flash.. It brings to light how hard it is to get a stable solar system. And of course I knew this but playing around I realized our sun is not stationary either... It's easy to forget the entire solar system we live in is this amazing mix of interactions between the planets, sun, other debris and outside mass from other parts of the universe.

karol 12y ago
The source is browsable if you modofy the URL. I expect someone with create a canvas driven clone within 48h:)
- mcdevhammer 12y ago
  Done. Although I did this years ago: http://mobile.sheridanc.on.ca/~claassen/Gravity/Gravity.html http://mobile.sheridanc.on.ca/~claassen/Gravity/Gravity.html
  - dj-wonk 12y ago
    Great! I tried it out; when I combined a galaxy with a field , many particles went flying from the center at a high rate of speed. Why is that?
    - mcdevhammer 12y ago
      When a small particle gets accelerated to great speeds as it approaches a massive object it simply passes through because the collisions are off/aren't perfect even when they're on
dj-wonk 12y ago
It would be nice to arbitrarily choose a specific particle to be the center of the system (the frame of reference).
mathu7 12y ago
Can someone explain the math behind this? Which methods were used? I wasn't aware it was possible to run such a simulation on flash because each body results in 6x more calculation. Is this like plugging in the masses into a single equation? I want to learn more about how this is done .

eccstartup 12y ago

I think the simulation of the real gravity is better. Because we can find out what is the hidden magic of nature. Then there comes accuracy issue of the simulation. How to tell us that your simulation will be `correct`?

masswerk 12y ago

For historical interest: Here's a description of the humble beginnings of gravity put onto a computer's screen (think of PDP-1 Spacewar! code, 1962): http://www.masswerk.at/spacewar/inside/insidespacewar-pt6-gravity.html http://www.masswerk.at/spacewar/inside/insidespacewar-pt6-gr...

ErikRogneby 12y ago

This makes it easy to understand why there are so many binary systems.

tuzemec 12y ago

Relevant: http://codingmath.com/ http://codingmath.com/ Really nice series, showing how to do vectors, gravity and so on in JS.

blt 12y ago

I made this in CUDA a while back. Fun project - the naive n^2 algorithm is embarrassingly parallel so it's easy to write. Fun to hit the CPU/GPU switch and watch the huge performance boost, even on a dinky laptop GPU.

gdh73 12y ago

Love this. I managed to get a pretty stable orbit by using the right angle and velocity (by chance). Highlighted in yellow: http://i.imgur.com/OOaTLMP.png http://i.imgur.com/OOaTLMP.png

guiomie 12y ago
This is the coolest trajectory found on this thread for now!!!!
- gdh73 12y ago
  It ran for several hours like that and never deviated from the original "circular" path. Unlike the other attempts, which were all much more elliptical. I guess that's kind of how planets would stay in a stable orbit though, having the right distance and speed for their mass.

exDM69 12y ago

Here is Universe Sandbox, a gravity simulation that can do massive scale simulation on home computers. Neat demo videos about colliding galaxies, etc. http://universesandbox.com/ http://universesandbox.com/

chops 12y ago

I, like many others on this thread, have dabbled in the Gravity Simulation game. I've done two of them. The first one was back in 1999, I wrote one with 13h[1] graphics for DOS (Borland Turbo C++), and to make it more fun, I used flat sprites to represent the objects (so the objects progressed from being rocks to asteroids to planets - with the largest sprite I used being an image of Jupiter). A few years later, I rewrote it in OpenGL (using the GLUT library) for a class. It worked much better (and of course the hardware was much faster too). I had always planned rewriting the physics engine to use Barnes-Hut, but never got around to it, so it always ran at N^2. Anyway, for the curious, the code is up on Github amongst my collection of other old garbage stuff from back in the day: https://github.com/choptastic/OldCode-Public/tree/master/GravSim https://github.com/choptastic/OldCode-Public/tree/master/Gra... [1] http://en.wikipedia.org/wiki/Mode_13h http://en.wikipedia.org/wiki/Mode_13h

Aardwolf 12y ago

Wow this is fun. But it has so much potential for improvements! -Saving your setting -Undo-ing -Auto generate small object in perfect circle orbit around larger object, and generate sun-earth-moon system -Export setting to other people -Allow to numerically finetune locations and velocities -Don't ruin the trails when dragging -Make it run faster, especially with trails enabled -etc... :) EDIT: Gee, when will Hacker News finally support newlines properly?

mkoryak 12y ago
well at least it supports negative mass ;)
luckyno13 12y ago
I would like to name individual "bodies," the ability to zoom in and out, and maybe even a mathematical readout of some sort. The values behind trajectory, etc.

mkoryak 12y ago

I have a question about a phenomenon I am seeing. say you add 3+ equal mass objects without a velocity and they all collide with each other without "missing or orbiting" one another. Sometimes the final mass will have a velocity. It would seem like it should not have a velocity because it would be canceled out. What am I missing? Where is this energy coming from?

lutusp 12y ago
In real life, the three colliding bodies would convert their kinetic energy into heat and combine into a very hot body. Sort of how we think the moon was created, from a Mars-sized body colliding with Earth long ago. But in a numerical simulation, usually what happens is that three bodies on a collision source approach one another but, because of the limitations of numerical differential equation solving, suddenly fly away from each other in a rather non-physical way, and with more energy than they had to begin with. The tl;dr: most of the funny effects in a simulator arise from the limitations of numerical simulation methods, and many of those aren't representative of real physics.

mkoryak 12y ago

here is an interesting one http://i.imgur.com/C835B8N.jpg http://i.imgur.com/C835B8N.jpg

programmarchy 12y ago

Can I play? I wrote a RK4 gravity simulator a while back in WebGL, but my main goal at the time was to find a way to visualize gravitational fields, so its nowhere near as interactive or user friendly as your implementation. I was trying for the effect you see in high school physics videos. My approach was to use a plane geometry mesh and apply a vertex displacement shader, but never quite got it to look or perform how I wanted. http://programmarchy.github.io/gravity http://programmarchy.github.io/gravity

nodesocket 12y ago

Somehow I ended up with a perfect orbit within my madness. http://i.imgur.com/wFReJps.png http://i.imgur.com/wFReJps.png

legohead 12y ago

create a few random fields turn on combine mass on collision add galaxy in middle watch the creation of a solar system :)

vhost- 12y ago

It's pretty sweet I got this to happen http://i.imgur.com/Sk4Rznu.png http://i.imgur.com/Sk4Rznu.png

junku901 12y ago

Does anyone know why perihelion moves in this simulation? I learned that perihelion movement can only be explained by general relativity, not newtonian mechanics. Is this just a limit of computer simulation? (which can't calculate the exact value of Gm1m2/r^2)

vhffm 12y ago
It is a shortcoming of the integrator (Euler), not in the calculation of the force (as you suspected). See [1]. Especially slides 11 and 12 may be of interest. [1] http://www.astro.sunysb.edu/phy277/lect30.pdf http://www.astro.sunysb.edu/phy277/lect30.pdf As a sidenote, there are better integration schemes, but all of them will have some error because we discretize the system.
- junku901 12y ago
  Wow that is really interesting. Thanks. By the way, if there are always some error in all systems, then what kind of systems are used by NASA or other space exploration institutes?
  - vhffm 12y ago
    I'm not in the business of space mission engineering, so I don't know details. Sorry. But in general, you can control the error and make a trade-off between computing time and accuracy. You would select a timescale over which you want the error to be below some value, and then only consider the simulation valid until then.

escapologybb 12y ago

I would love to know if there's a name to this particular type of orbit, I spent a little bit too much time creating it and now need to know some more about it! :-) Anybody know how I can find out? http://imgur.com/4W7eqj9 http://imgur.com/4W7eqj9 Thanks

bipin-nag 12y ago

Most awesome thing ever seen on HackerNews !!!

prothid 12y ago

I lost my planet. http://i.imgur.com/EUK7kiE.jpg http://i.imgur.com/EUK7kiE.jpg

noisy_boy 12y ago

I love this but its a pity that paths vanish when I click-drag the frame.