5 ms·

> Can somebody who knows about this tell me how much of the novelty in this is CS and how much physics? It's not so much a novelty as it is pushing the envelop

by adekok 10y ago

> Can somebody who knows about this tell me how much of the novelty in this is CS and how much physics? It's not so much a novelty as it is pushing the envelope. The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star. And then add in interstellar dust, which has non-neglible mass. And then run the simulation for 8 billion years. There are naturally some optimizations and short-cuts to be made. :)

privong 10y ago
> The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star. Summary of what's below: The type of simulations described in the article do not scale like O(N^2) but more like O(N log N). A small correction here.. The gravitational force calculation for N-body simulations are only O(N^2) if they're done using direct summation and force calculation. That's only necessary if you care about the motions of individual stars. The largest simulations I'm aware of that do that use a few 10s or 100s of millions of stars. They're typically used for studying individual star clusters, the regions around supermassive black holes, and perhaps dwarf galaxies. But the simulation in this article/press release has a few 10s of billions of particles, I believe. And those particles are all much more massive than individual stars. So while there are "star particles" in the simulations, it's more accurate to think of them as a set of particles which constitute a monte carlo sampling of the stellar mass distribution. One aspect of that is you now no longer care about the detailed trajectories of individual particles[0], instead caring about the overall representation of the mass distribution. So you don't need as high of accuracy in the force calculation. Going back to the 1980's, people have been using numerical tricks to reduce the scaling of the simulations with the particle number. This includes algorithms such as treecodes, particle-mesh codes, etc., which reduce the scaling to O(N log N) or even O(N). These are sufficient to run simulations of describing the overall gravitational field. The scaling for hydro codes is probably more complicated, particularly for simulations which use a moving mesh. There's a nice summary of numerical techniques for this type of work here: http://arxiv.org/abs/1412.5187 http://arxiv.org/abs/1412.5187 [0] One can determine if interactions between individual stars is important to a system's evolution by comparing the 2-body relaxation time to the age of the system. This is discussed on pages 8–9 of the preprint linked above.
- dr_zoidberg 10y ago
  Some simulations also use Smoothed Particle Hydrodinamics (see here[0] for a simplified example), like the Earth-Theia impact Moon simulations[1]. However I'm not in the field and can't really tell how much this kind of code has been used lately, but you did remind me of it when you talk about this simulations being monte carlo sampling of stellar mass distribution, because SPH has some concepts like that "inside" the particles. Also, for everyone to enjoy simulations results, check the simulations by Canup from the giant impact hypothesis[2]. Another thing to consider, the supercomputer used for this specific simulation had about the same computer power as a high end GPU from today. [0] https://benedikt-bitterli.me/bbw.html https://benedikt-bitterli.me/bbw.html [1] http://lasp.colorado.edu/~espoclass/ASTR_5835_2013_Files/2013_10_29_Canup.Moon.Icarus.2004.pdf http://lasp.colorado.edu/~espoclass/ASTR_5835_2013_Files/201... [2] http://www.boulder.swri.edu/~robin/moonimpact/ http://www.boulder.swri.edu/~robin/moonimpact/
  - privong 10y ago
    SPH has been fairly common in galaxy simulations over the past 20 years or so. But there's some glaring issues with "classic" SPH formulations, to do with fluid instabilities. As a result, lots of people are transitioning to moving mesh codes. I put a few links in my other comment (https://news.ycombinator.com/item?id=12461336 https://news.ycombinator.com/item?id=12461336). I'm pretty sure the simulation in the press release is also using a moving mesh for the hydrodynamics.
    - lutorm 10y ago
      No, this sim is done using GIZMO (a derivative of the GADGET-2 code) which uses a hybrid of SPH and AMR methods: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html
      - privong 10y ago
        GIZMO is the name of the code suite, not the numerical technique. I believe GIZMO contains multiple hydro solvers, including various formulations of SPH, moving mesh, and meshless techniques. But you're right. that the simulation in question wasn't a moving mesh, it was a meshless technique.

by adekok 10y ago

privong 10y ago

> The main problem with these kind of simulations is that they're O(N^2). You can't get away with modelling each star, you have to model the interaction of each star with every other star. Summary of what's below: The type of simulations described in the article do not scale like O(N^2) but more like O(N log N). A small correction here.. The gravitational force calculation for N-body simulations are only O(N^2) if they're done using direct summation and force calculation. That's only necessary if you care about the motions of individual stars. The largest simulations I'm aware of that do that use a few 10s or 100s of millions of stars. They're typically used for studying individual star clusters, the regions around supermassive black holes, and perhaps dwarf galaxies. But the simulation in this article/press release has a few 10s of billions of particles, I believe. And those particles are all much more massive than individual stars. So while there are "star particles" in the simulations, it's more accurate to think of them as a set of particles which constitute a monte carlo sampling of the stellar mass distribution. One aspect of that is you now no longer care about the detailed trajectories of individual particles[0], instead caring about the overall representation of the mass distribution. So you don't need as high of accuracy in the force calculation. Going back to the 1980's, people have been using numerical tricks to reduce the scaling of the simulations with the particle number. This includes algorithms such as treecodes, particle-mesh codes, etc., which reduce the scaling to O(N log N) or even O(N). These are sufficient to run simulations of describing the overall gravitational field. The scaling for hydro codes is probably more complicated, particularly for simulations which use a moving mesh. There's a nice summary of numerical techniques for this type of work here: http://arxiv.org/abs/1412.5187 http://arxiv.org/abs/1412.5187 [0] One can determine if interactions between individual stars is important to a system's evolution by comparing the 2-body relaxation time to the age of the system. This is discussed on pages 8–9 of the preprint linked above.

dr_zoidberg 10y ago
Some simulations also use Smoothed Particle Hydrodinamics (see here[0] for a simplified example), like the Earth-Theia impact Moon simulations[1]. However I'm not in the field and can't really tell how much this kind of code has been used lately, but you did remind me of it when you talk about this simulations being monte carlo sampling of stellar mass distribution, because SPH has some concepts like that "inside" the particles. Also, for everyone to enjoy simulations results, check the simulations by Canup from the giant impact hypothesis[2]. Another thing to consider, the supercomputer used for this specific simulation had about the same computer power as a high end GPU from today. [0] https://benedikt-bitterli.me/bbw.html https://benedikt-bitterli.me/bbw.html [1] http://lasp.colorado.edu/~espoclass/ASTR_5835_2013_Files/2013_10_29_Canup.Moon.Icarus.2004.pdf http://lasp.colorado.edu/~espoclass/ASTR_5835_2013_Files/201... [2] http://www.boulder.swri.edu/~robin/moonimpact/ http://www.boulder.swri.edu/~robin/moonimpact/
- privong 10y ago
  SPH has been fairly common in galaxy simulations over the past 20 years or so. But there's some glaring issues with "classic" SPH formulations, to do with fluid instabilities. As a result, lots of people are transitioning to moving mesh codes. I put a few links in my other comment (https://news.ycombinator.com/item?id=12461336 https://news.ycombinator.com/item?id=12461336). I'm pretty sure the simulation in the press release is also using a moving mesh for the hydrodynamics.
  - lutorm 10y ago
    No, this sim is done using GIZMO (a derivative of the GADGET-2 code) which uses a hybrid of SPH and AMR methods: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html
    - privong 10y ago
      GIZMO is the name of the code suite, not the numerical technique. I believe GIZMO contains multiple hydro solvers, including various formulations of SPH, moving mesh, and meshless techniques. But you're right. that the simulation in question wasn't a moving mesh, it was a meshless technique.