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Universe Sandbox Wiki
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N-Body Simulation
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N-Body Simulation
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==Limitations== Errors are impossible to avoid in ''N''-body simulation. There must be compromises made when it comes to accuracy, as there are limitations defined by computational power. This is true even for ''N''-body simulations that are run by researchers, where the integration may take days and the output is in numerical data, not detailed graphics. Universe Sandbox is subject to even more constraints than research-level gravity simulators, as it displays a graphical representation of the simulation as it runs, and it runs on consumer-grade hardware rather than research-grade supercomputers. ===Time Step=== The main source of error in an ''N''-body simulation is the size of the simulation time step. Smaller time steps can more accurately simulate reality (in which time is continuous, rather than made up of discrete steps), but smaller time steps negatively affect performance. Universe Sandbox addresses this trade-off in accuracy vs. performance in several ways. First, the user can adjust the simulation speed, and thus the time step, of a simulation using the controls on the [[Interface#Bottom Bar|bottom bar]]. This allows the user to decide whether to prioritize accuracy or performance for any given simulation. Second, the maximum allowed simulation speed is limited by Universe Sandbox (unless the user turns off this option; see [[N-Body Simulation#Settings|Settings]] above). Third, Universe Sandbox using [[Sub-Stepping|sub-stepping]] to increase the accuracy of the ''N''-body integrator. ===Point Masses=== In reality, every particle that makes up a planet, moon, or star has mass and will exert a gravitational force on other particles. However, in an ''N''-body simulation, the number of objects in the simulation will have a significant negative effect on the performance of the simulation, so simulating every individual particle is unfeasible. Instead, Universe Sandbox's gravity model simulates each object as a [https://en.wikipedia.org/wiki/Point_particle#Physical_point_mass point mass]: a single point in space with all of the object's mass condensed into that point. This means that additional models are required to simulate effects related to the 3D size and shape of objects, like [[Fragmentation|fragmenting collisions]] or [[Tidal Heating|tidal heating]]. ===Non-Attracting Objects=== Because the number of objects simulated by an ''N''-body integrator is related to the performance of the integrator, Universe Sandbox improves the performance of the simulation by ignoring the gravitational effects of certain low-mass objects, such as [[Fragment|fragments]] or [[Volatile|volatiles]]. These [[Attracting|non-attracting]] objects may still be affected by the gravity of other objects, but they do not exert a gravitational force, and therefore do not slow the ''N''-body simulation. [[Galaxy|Galaxies]] in Universe Sandbox use non-attracting [[Galaxy Nebula|galaxy nebublae]] objects to approximate the structure and motion of real galaxies. See [[Galaxy Structure]] for more details about this approximation and its limitations. ===Relativity=== ''N''-body simulation in Universe Sandbox is simulated according to [https://en.wikipedia.org/wiki/Classical_mechanics classical mechanics]. This means that it does not account for [https://en.wikipedia.org/wiki/General_relativity general relativity]. This is largely due to computational limitations; simulating general relativity is incredibly complex and usually requires supercomputers. It is, however, often not necessary; using classical mechanics is extremely accurate for most simulations. There are a few limitations that the Universe Sandbox team would like to address in the future by implementing effects related to relativity, such as the speed of gravity and [http://universesandbox.com/blog/2016/02/gravitational-waves/ spinning black holes]. THE FONTTT [[Category:Simulation]] [[Category:Gravity]]
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