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Can you tell me what physical simulations have been implemented in the space engine?
I think Space Engine involves more emulation than simulation. For example it never simulates the geological processes behind terrain building or erosion. It instead emulates surface features by procedural algorithms. It didn't form the cosmic web structure of the universe by running an n-body simulation, but rather generated their distribution to look like the real universe on large scales.
The difference between simulation and emulation is important, because a simulation can only handle so many interactions before it's just not computationally feasible on a home computer. This is what makes Space Engine and Universe Sandbox so fundamentally different. Universe Sandbox is a powerful simulator and can portray things like planetary impacts that Space Engine cannot, but by the same token, it cannot simulate anywhere near as many objects as what Space Engine contains.
Space Engine is a 10x10x10 Gpc cube, and there are about 10 galaxies per cubic Megaparsec. That comes out to roughly 1013
galaxies in SE. If each one contains on average 100 million stars (I'm not sure the actual number), then that's 1021
star systems, and maybe about 1022
planets, not counting the rogue ones.