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There is something that always puzzles me and I would like if something could illuminate me. How are those kind of massive scale astrophysical calculations made
by Maken 9y ago
There is something that always puzzles me and I would like if something could illuminate me. How are those kind of massive scale astrophysical calculations made? Do they take into account every visible mass aggregate in the galaxy and simulate them, or just apply cinematic at scale assuming the galaxy centre is a point of certain mass?
I have played a little with N body simulations and I can't but be amazed about how numerically unstable physical simulations are, and how much the result can change depending on trivial simplifications.
- danielbarla 9y agoI'm not much of an expert on the topic, but have also played with writing N-body simulations. At some point, obviously N^2 is unfeasible, so most simulations seem to use some approximation, such as the Barnes-Hut approach [1]. That approach divides the simulation space into an octtree. One can then use said octtree to approximate the force of distant quadrants by simply using the center of mass of an entire region. Due to the rate at which gravity drops off over large distances, it's "accurate enough", and fortunately the majority of "N" is "distant". A good (but pretty shallow) overview exists on Wikipedia about other approaches [2] (under "Calculation optimizations"). [1] https://en.wikipedia.org/wiki/Barnes%E2%80%93Hut_simulation https://en.wikipedia.org/wiki/Barnes%E2%80%93Hut_simulation [2] https://en.wikipedia.org/wiki/N-body_simulation https://en.wikipedia.org/wiki/N-body_simulation EDIT: It's occurred to me that you were referring more to stability of the simulation, rather than what optimisations are used. I have less information to offer on that part, and I agree that it would be a huge problem, if complete accuracy was desired. I suspect, however, that in the kinds of simulations we're talking about, it's more about a general result such as "the whole galaxy should be divergent", not whether a single star went left or right when interacting with a black hole.
- deleted 9y ago[deleted]
- yorwba 9y agoYou may be interested in this paper: http://dl.acm.org/citation.cfm?doid=3028256.3015569 http://dl.acm.org/citation.cfm?doid=3028256.3015569
- lightsighter 9y agoActually in a lot of new HPC codes for doing physical simulation we are now starting to incorporate Uncertainty Quantification (UQ)[1] directly into the code such that we can determine where the instabilities and bifurcation points exist in non-linear systems. UQ is really expensive to compute, but it at least allows us to know when our simulations are accurate or are sensitive to perturbations. [1] https://en.wikipedia.org/wiki/Uncertainty_quantification https://en.wikipedia.org/wiki/Uncertainty_quantification